marine – The Ocean: Explained

An introduction to the thresher shark

Introduction

The thresher shark is an all-encompassing term referring to three surviving species from the family Alopiidae: the pelagic thresher (Alopias pelagicus), the bigeye thresher (Alopias superciliosus), and the common thresher (Alopias vulpinus).

Pelagic thresher shark (*Alopias pelagicus*). By Thomas Alexander – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50280277

Bigeye thresher shark (*Alopias superciliosus*). By PIRO-NOAA Observer Program – http://ias.pifsc.noaa.gov/lds/obs_training/SharkThresherNew.pdf, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534429

Common thresher shark (*Alopias vulpinus*). By NOAA/PIER – http://www.nmfs.noaa.gov/gallery/images/photos/6472384727.html, Public Domain, https://commons.wikimedia.org/w/index.php?curid=18920049

Comparison between the three thresher species. By FactZoo.com – https://www.factzoo.com/fish/pelagic-thresher-shark-longtail-smack.html

Morphology

Thresher jaws are small. They have clearly not evolved to attack prey like tuna and seals; if anything, they look like they should be eating small crabs or scavenging chunks of floating debris that someone else took the time to kill. They would seemingly struggle to catch a live fish because the long snout overhangs their tiny mouth so much. That’s where the notorious tail comes in. The thresher tail is designed to be used like a whip to strike and immobilise prey [1], [2]. Such force occurs with a single tail slap that gas is diffused out of the seawater and rises to the surface in bubbles [1]. Typically, pelagic sharks hunt and capture one fish at a time; this strategy enables the shark to catch on average 3 fish, and sometimes more, in one go.

A sequence of still images taken from an overhead tail-slap hunting event [1].

The easiest way to differentiate between the three species is the colouration. Common threshers are likely to be more grey and they lack any colouration above their pectoral fins; sometimes, they have white dots on the tips of their fins. Pelagic threshers have distinct colouration above their pectoral fins. The bigeye’s eye is visible from the top of the head and has characteristic groves above the eyes and gills.

A common thresher shark, identified by the lack of colouration above the pectoral fin and white dots on the tips of the pectoral and dorsal fins. By Paul E Ester at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=19628266

A pelagic thresher shark, identified by the colouration above its pectoral fin. By NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_pelagic_thresher_sharks/obs_pelagic_thresher_shark5.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5553329

A bigeye thresher shark, distinguished by the birds-eye view of the large eyes and the lateral grooves. By PIRO-NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_bigeye_thresher_sharks/obs_bigeye_thresher_shark5.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534392

A bigeye thresher shark is distinguished by its large eyes and lateral groove above the eye and gills. By PIRO-NOAA Observer Program – http://www.fpir.noaa.gov/Graphics/OBS/obs_sharks/obs_bigeye_thresher_sharks/obs_bigeye_thresher_shark4.jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=6534388

Distribution

Threshers are pelagic, meaning they live in the deep, open ocean. The only thresher known to inhabit New Zealand waters is the largest of the trio, the common thresher [3]. The bigeye may also be found in NZ, but due to its habit of staying hundreds of metres below the ocean surface during the day, it is unlikely to be encountered if it is there at all [4].

Threats

The common and bigeye thresher are ‘Vulnerable’ to extinction under the IUCN Red List [3], [5], and the pelagic thresher is further threatened and classified as ‘Endangered’ [6]. Estimated global population reductions are as follows:

Common thresher: 30–49% over 3 generations (76.5 years)
Bigeye thresher: 30–49% over 3 generations (55.5 years)
Pelagic thresher: 50–79% over 3 generations (55.5 years)

All three species are globally targeted for their meat, fins, skin, and liver oil [7], the latter commonly used in modern cosmetics and supplements. All species are usually retained if accidentally caught on commercial vessels [8]–[11]. In one study [15] conducted in the Indian Ocean, 13 out of 19 bigeye threshers caught on a commercial vessel’s longline were dead upon retrieval. In the same study, in the Atlantic Ocean, 412 out of 849 bigeye threshers caught on longlines were dead. On the same ships were deaths of stingrays and manta rays as well as blue, shortfin mako, silky, and smooth hammerhead sharks. One of the largest hubs for shark fin trading globally is Hong Kong [9], and threshers accounted for up to 3% of the total fin mass in the past [8].

Threshers are also popular among recreational, big game anglers. Although a tag and release method is more commonly practised nowadays, there is also a risk of post-release mortality. With threshers likely to be hooked from their tail due to their hunting style, a study of common threshers found 78% of tail-hooked sharks died after release [12].

The New Zealand commercial fishery is not exempt from thresher shark landings. A recent report by Fisheries New Zealand [13] saw that in 5 years from the 2014/15 to 2018/19 commercial fishing seasons, 149,916 tonnes of common thresher shark were caught by the core deep-water fleet. With the average 5-metre common thresher shark usually weighing in at 230 kg, we can infer that in the 5 years, the deep-water fleet landed approximately 651 individual thresher sharks.

Thresher vs swordfish

In April 2020, a dead, 4.5 metre, female bigeye thresher was found beached on a Libyan coast [14]. A 30 cm swordfish (Xiphias gladius) rostrum was found embedded in its head. It is understood that the rostrum severely injured some of the shark’s nerves, arteries, and gill arches. It was concluded that the impalement was what led to the ultimate death of the shark.

A young swordfish (*Xiphias* *gladius). By Michael Landress,* CC BY-NC-ND 2.0 – https://www.flickr.com/photos/myfwc/26802909040

*Taken from Jambura et al (2021) [14]. “*Female bigeye thresher *Alopias superciliosus* (TL = 445 cm) stranded on the Libyan coast (Mediterranean Sea), with a swordfish *Xiphias* *gladius* rostrum embedded deep in the branchial chamber. Scale bars indicate 50 cm (b) and 10 cm (c and d). Photo (a and b) and video content (c and d) courtesy of Faraj Habrisha and Abdalhakim Ahmed Al sebaihe”.

[1] S. P. Oliver, J. R. Turner, K. Gann, M. Silvosa, and T. D’Urban Jackson, “Thresher Sharks Use Tail-Slaps as a Hunting Strategy,” PLoS One, vol. 8, no. 7, 2013, doi: 10.1371/journal.pone.0067380.

[2] S. A. Aalbers, D. Bernal, and C. A. Sepulveda, “The functional role of the caudal fin in the feeding ecology of the common thresher shark Alopias vulpinus,” J. Fish Biol., vol. 76, no. 7, pp. 1863–1868, 2010, doi: 10.1111/j.1095-8649.2010.02616.x.

[3] C. L. Rigby et al., “Alopias vulpinus,” IUCN Red List Threat. Species 2019, vol. e.T39339A2, 2019.

[4] H. Nakano, H. Matsunaga, H. Okamoto, and M. Okazaki, “Acoustic tracking of bigeye thresher shark Alopias superciliosus in the eastern Pacific Ocean,” Mar. Ecol. Prog. Ser., vol. 265, pp. 255–261, 2003, doi: 10.3354/meps265255.

[5] C. L. Rigby et al., “Alopias superciliosus,” IUCN Red List Threat. Species 2019, vol. e.T161696A, 2019.

[6] C. L. Rigby et al., “Alopias pelagicus,” IUCN Red List Threat. Species 2019, vol. e.T161597A, 2019, doi: https://dx.doi.org/10.2305/IUCN.UK.2019-3.RLTS.T161597A68607857.en.

[7] R. W. Jabado et al., “The trade in sharks and their products in the United Arab Emirates,” Biol. Conserv., vol. 181, pp. 190–198, 2015, doi: 10.1016/j.biocon.2014.10.032.

[8] S. C. Clarke, J. E. Magnussen, D. L. Abercrombie, M. K. McAllister, and M. S. Shivji, “Identification of shark species composition and proportion in the Hong Kong shark fin market based on molecular genetics and trade records,” Conserv. Biol., vol. 20, no. 1, pp. 201–211, 2006, doi: 10.1111/j.1523-1739.2005.00247.x.

[9] A. T. Fields et al., “Species composition of the international shark fin trade assessed through a retail-market survey in Hong Kong,” Conserv. Biol., vol. 32, no. 2, pp. 376–389, 2018, doi: 10.1111/cobi.13043.

[10] S. C. Clarke et al., “Global estimates of shark catches using trade records from commercial markets,” Ecol. Lett., vol. 9, no. 10, pp. 1115–1126, 2006, doi: 10.1111/j.1461-0248.2006.00968.x.

[11] F. Dent and S. Clarke, “State of the global market for shark products,” Rome, Italy, 2015.

[12] C. A. Sepulveda et al., “Post-release survivorship studies on common thresher sharks (Alopias vulpinus) captured in the southern California recreational fishery,” Fish. Res., vol. 161, pp. 102–108, 2015, doi: 10.1016/j.fishres.2014.06.014.

[13] Fisheries New Zealand, “Annual Review Report for Deepwater Fisheries for 2018/19,” 2020.

[14] P. L. Jambura, J. Türtscher, J. Kriwet, and S. A. A. Al Mabruk, “Deadly interaction between a swordfish Xiphias gladius and a bigeye thresher shark Alopias superciliosus,” Ichthyol. Res., vol. 68, no. 2, pp. 317–321, 2021, doi: 10.1007/s10228-020-00787-x.

[15] R. Coelho, J. Fernandez-Carvalho, P. G. Lino, and M. N. Santos, “An overview of the hooking mortality of elasmobranchs caught in a swordfish pelagic longline fishery in the Atlantic Ocean,” Aquat. Living Resour., vol. 25, no. 4, pp. 311–319, 2012, doi: 10.1051/alr/2012030.

Estuaries: how habitats and food webs create transitional estuarine ecosystems of high productivity

Introduction

An estuary is a coastal passage of water where tides from the open sea meet river currents, and salty seawater mixes with fresh riverine water to create an environment of high productivity (Correll, 1978; Pritchard, 1967). The intermediate location of estuaries causes it to act as a transitional environment between terrestrial and marine ecosystems. Estuaries can be viewed as a continuum, whereby further upstream, the calmer, fresher water of the river progresses into the tidal, saline water of the ocean. This estuarine continuum – due to the range of its attributes, including salt content, depth, temperature, tidal energy, et cetera – allows for the diversification of the habitats within it. Three key estuarine habitats include salt marshes, mangroves, and seagrass beds. The organisms within these habitats interact with one another in numerous ways, creating not only food chains but food webs of complex interactions and energy exchanges.

Correll (1978) states that estuaries are areas of unusually high productivity because the conditions that create estuaries result in high levels of photosynthate from in situ primary production byphytoplankton and plants. Estuarine food webs are sustained by their habitats; therefore, it is important to analyse the relationship between habitats and food webs and how they contribute to the overall high productivity of estuaries. Furthermore, because estuaries are closely linked with terrestrial and marine ecosystems, estuarine productivity may be attributed to or contribute to adjacent environments.

In this essay, I will do the following: describe the features of estuarine habitats and food webs; determine how estuarine habitats and food webs are both linked with terrestrial and marine ecosystems, making estuaries a transitional environment; and critically analyse how habitats and food webs contribute to a highly productive estuarine system.

Estuarine habitats

In order to analyse the high productivity of estuaries, it is crucial first to explain the features of estuarine habitats and the organisms which inhabit them. Links to food webs from the habitats can then be made to understand better how energy and nutrients enters the estuarine system. Three important estuarine habitats exist, including salt marshes, mangroves, and seagrass beds; each possesses unique attributes and organisms living there.

Salt marshes are wetlands dominated by few species of halophytic vegetation and are alternately inundated and drained by the tides (Christiansen, Wiberg, & Milligan, 2000; Gedan, Altieri, & Bertness, 2011; Richardson, Swain, & Wong, 1997). The halophytic plants add stability to the marsh by trapping and binding sediment and can also withstand stressful conditions like the constant flooding and draining of the marsh as well as the salty, waterlogged, and, sometimes, hypoxic soil (Gedan et al., 2011; Pennings, Grant, & Bertness, 2005). Three types of fauna utilise the salt marsh habitat. (1) Specialist species inhabit marshes as their primary home, utilise the high vegetation biomass for energy, and are well adapted against desiccation; these specialist animals tend to be invertebrate species (Kon et al., 2012; Minello & Webb, 1997; Richardson et al., 1997). (2) Another type of fauna uses marshes as an extension of their home and are therefore regular visitors to the marsh. As a general example, a crab species uses the marsh for protection while their home in the mudflat is under heavy predation, then returns home once predation has eased. (3) Terrestrial or ariel species venture into the marsh for certain activities such as gathering food, courtship, or nesting (Burger, 1979; Hanson & Shriver, 2006; Teixeira, Duarte, & Caçador, 2014). Overall, the conditions that define a salt marsh create a habitat dominated by marine invertebrates (Daiber, 1982).

Restored Tomago salt marsh #marineexplorer | Coastal salt ma… | Flickr — Restored Tomago salt marsh.
By John Turnbull – Own Work, CC BY-NC-SA 2.0, https://www.flickr.com/photos/johnwturnbull/31538875578/

Mangroves are woody plants that can withstand excessive salt concentrations and water inundation, and “true” mangroves are from the genus Rhizophora. Mangroves are distributed globally in tropical and sub-tropical regions (Parida & Jha, 2010; Simard et al., 2019). Mangrove plants contend with adverse conditions like saline water and anoxic substrates, which are difficult, if not impossible, environments for terrestrial plants to live in. Mangroves have combatted this by developing salt glands, roots with low permeability, and adventitious roots with pneumatophores (Liang, Zhou, Dong, Shi, 2008; Medina, 1999; Parida & Jha, 2010; Yáñez-Espinosa, Terrazas, & Angeles, 2008). The foliage, trunks, and roots of mangroves all provide unique niches for organisms to inhabit (Nagelkerken et al., 2008). Above the water, the foliage provides a habitat for terrestrial fauna like birds, mammals, reptiles, and insects. The trunks and pneumatophores provide a rigid substrate for organisms to settle on. Under the water, the roots can support sponges, bivalves, and algae, and provide a labyrinth of microhabitats for aquatic species like fish. Due to the abundance of food and shelter, mangroves may function as a nursery for juvenile fish and invertebrates species (Nagelkerken et al., 2008).

Above and below water view of mangroves and roots.
Public Domain, https://commons.wikimedia.org/w/index.php?curid=142126

Seagrasses are the only flowering plants that have evolved to survive submersion in saltwater (Orth et al., 2006). The seagrasses, anchored to the sediment by underground runners called rhizomes, congregate to form vast beds that provide a large habitat area for organisms to inhabit. The stability of seagrass habitats means it is a popular environment for transient species to lay eggs and is one of the most productive and diverse marine ecosystems (Constanza et al., 1997; Duarte & Chiscano, 1999). Seagrass habitats provide shelter, food, and a nursery area for fish and marine invertebrates as well as food for waterfowl and marine mammals like dugongs and manatees (Heck, Hays, & Orth, 2003; Holm & Clausen, 2006; Rybick & Landwehr, 2007; Thayer, Bjorndal, Ogden, Williams, & Zieman, 1984; Weisner, Strand, & Sandsten, 1997).

Estuarine food webs

Habitats contain niches that make way for a variety of interactions between organisms. In estuaries, salt marshes, mangroves, and seagrass beds create unique environments that induce complex food webs. These food webs begin with an input to the first trophic group known as the primary producers, a group that includes phytoplankton or plants like the vegetation of salt marshes, algae, and seagrasses. Trophic transfers take place from one group to another as the energy that began with primary production is moved up the food web to other trophic groups like herbivores or carnivores. During trophic transfers, some energy may be transported out of the food web into a cycle (such as the nitrogen cycle) or be expelled as a respiratory end product (Day, Kemp, Yáñez-Arancibia, & Crump, 2012).

File:FoodWeb.svg — A freshwater, marine, and terrestrial food web.
By File:FoodWeb.jpg: Thompsmaderivative work: Pixelsquid – This file was derived from: FoodWeb.jpg:, CC0, https://commons.wikimedia.org/w/index.php?curid=102527971

Detritus is dead, particulate, organic material, and can be processed repeatedly by organisms at most levels of a food web until it is either buried in sediment or exported (Darnell, 1961); therefore, detritus is an important input to food webs. Salt marsh vegetation, mangrove litter, and seagrasses are likely to provide significant amounts of plant detritus to estuarine ecosystems (Teal, 1962; Odum & de la Cruz, 1963).

Estuaries as transitional environments of high productivity

Estuaries are a transitional environment between terrestrial and marine ecosystems. Terrestrial species will use estuarine salt marshes for specific activities because the marshes provide an environment unlike any terrestrial habitat. Furthermore, fishes may also enter salt marshes at high tide to feed before returning to the marine environment (Heck et al., 2008). Liang et al. (2008) determined mangroves as a buffer for the terrestrial environment against marine energies such as erosion and damage from waves and typhoons. Seagrasses provide an essential link from estuaries to marine ecosystems because live seagrass leaves not consumed enter the detrital pool (Heck et al., 2008). The detrital seagrass can be transported by currents and waves over extensive distances and be used by organisms for shelter, habitat, or consumed along the way (Ochieng & Erftemeijer, 1999; Thresher, Nichols, Gunn, Bruce, & Furlani, 1992). The detrital seagrasses journey may conclude in deep-sea canyons, where it accumulates and supports organisms in an environment with poor habitat structure and low primary production (Josselyn et al., 1983; Vetter, 1998). Thus, seagrass meadows link estuaries with an unlikely marine environment, the deep sea. Estuaries also receive significant detritus inputs from both terrestrial and marine ecosystems (Day et al., 2012).

Estuaries are thought to be highly productive per unit area. This high productivity can mean that the plants are very productive, that more organic matter is produced than used, or that estuaries sustain a substantial amount of fauna compared to other environments (Day et al., 2012). Schelske and Odum (1962) considered the different contributors to one estuary’s productivity, and these can be extended to all estuaries in a general sense:

Firstly, there is diversity in the sources of organic matter that enter an estuary. The three key estuarine habitats (salt marshes, mangroves, and seagrass beds) all provide organic matter to estuarine systems. These habitats provide substrata for epiphytic algae to grow on and shallow, photic waters for pelagic phytoplankton. Adjacent terrestrial environments also passively contribute organic matter into the estuarine system. More specifically, the many sources of organic matter in an estuary can be narrowed down to a diverse species abundance of photosynthetic organisms. This means that light can be utilised throughout all seasons, reducing the chance of a substantial drop in organic matter production by the photosynthesising species.
Secondly, physical energy, such as tides, currents, wind, and waves, produces considerable water movement in estuarine ecosystems, increasing the surface area by which phytoplanktonic photosynthesis can occur. For example, the flooding and draining of a salt marsh transports material and nutrients in and out of the marsh, and tides and currents transport unused seagrasses out of the seagrass beds to enter a different part of the food web.
Thirdly, an abundant supply of various nutrients is thought to be present in estuaries, especially when compared to marine and freshwater environments. Nutrient abundance in estuaries allows for a higher rate of photosynthesis (Nixon 1980, 1995; Rhyther & Dunstan, 1971).
Finally, because estuaries are relatively shallow and well-mixed, both food and nutrients are available to all organisms in the water column and benthic and pelagic food webs can be coupled, creating interactions that may be unique to estuaries alone (Day et al., 2012). Bivalves and microorganisms also play a role in biogeochemical cycling of nutrients. The mixing of salt and freshwater provides high nutrients in both the water and the sediment.

Habitats could potentially be viewed as the most important contributor to estuarine productivity because they give way to the environments which allow estuaries to be productive; the complexities of estuarine food webs would cease to exist if it were not for the availability of diverse habitats in which they can operate. Salt marsh, mangrove, and seagrass bed ecosystems have relationships with terrestrial and marine ecosystems, increasing estuaries’ productivity. The value of these habitats to certain trophic levels depends on their needs for shelter, food, or to carry out species-specific activities, showing that the way these habitats are structured affects trophic transfers in estuarine food webs (Walters, 2000). Some estuarine habitats may increase production in other estuarine habitats and adjacent ecosystems. The diversity of the mangrove ecosystems gives way to a rich microbial community, supported by nutrient cycling, which may promote faster sponge growth than in coral reefs (Kathiresan & Bingham, 2001). Odum and Helad (1972) suggest that the production of mangroves is exported to the marine environment, which provides food for secondary consumers and may lead to higher production in the marine ecosystem. Heck et al. (2008) describes how the highly productive nature of seagrass meadows can spill over into other ecosystems. For example, some fish live in mangroves but feed on seagrass, creating a trophic link between the two habitats. Also, some fish that inhabit coral reefs feed on seagrass, shifting organic matter and nutrients into the reef habitat and food web.

Conclusion

Productivity of any ecosystem is easily measured by the quantity of photosynthate it can produce because primary production essentially supports entire food chains. Estuaries have such a plethora of trophic interactions that it is necessary to extend their food chain to a food web that spans multiple ecosystems. This food web would cease to exist without the dynamic habitats that support them, with the key habitats being salt marshes, seagrass beds, and mangroves. The features that make these habitats unique from any other habitat globally, are also the features that sustain the species that live in them. Because estuaries are a transitional environment between terrestrial and marine ecosystems, it is likely that they benefit from the high productivity of estuaries, and this may be shown through the trophic interactions and nutrient cycling that occurs. Estuaries are a dynamic environment, and further research is needed to understand the global effect of their high productivity to learn how to better protect them.

Reducing pressure on Auckland's estuaries — An estuary in Auckland, New Zealand, showing how closely estuaries and terrestrial (specifically urban) environments are linked.
By Getty Images, https://www.newsroom.co.nz/ideasroom/reducing-pressure-on-aucklands-estuaries

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Rarotonga

What do you do when you’ve just finished a stressful semester at uni, and your tenancy application for a new house has been accepted? You ditch New Zealand and head to Rarotonga for 2 weeks! With the quarantine-free travel bubble opening between NZ and the Cook Islands and flights and accommodation being ridiculously affordable, trading in Auckland’s sub-zero temperatures for a tropical paradise seemed to be the only reasonable choice.

The climate was the first thing that got me about Raro. Snapchat was covered with 7am stories of frost-covered grass, and I’m getting a sweat on just walking to breakfast. I think the next thing I quickly noticed was the atmosphere. It’s a small island (67 km2; a population of 13,000), and the convivial ambience of the community was almost tangible. The way the bus driver greeted me, you’d think he was my uncle. After being in Raro for only a day, I felt the same feeling of being in Pukekohe; I felt like I was home.

Snorkelling

Obviously, the daily routine began with breakfast. Then, as soon as we could get our shit together, we went snorkelling. The water was so warm and so salty. One of the locals said that as kids, they all swam in the ocean with their eyes open to get used to the salt. I totally understand why they did that because water leaked into my mask one time and I thought I went blind. In NZ, in the height of summer, I can rarely go freediving in just a bikini for more than 30 minutes before the cold sets in; in Raro, I almost felt too hot when I was snorkelling in my 3 ml spring suit – crazy. The ocean would probably feel like a bath in summertime.

To me, the most interesting animal I saw while snorkelling in the lagoon was the octopuses. I feel like every time I go freediving and there’s an octopus, everyone but me manages to see it; not this time, suckers (pun intended). They inhabited the right-hand side of the lagoon, amongst an area of littered stones and rubble. They were day octopuses (Octopus cyanea) who, unlike their nocturnal counterparts, are active during the day, stealthily patrolling the reef under an impressive camouflage [1, 2]. One of them even attempted to steal my GoPro and take it into its den for further inspection.

Bluefin trevally (Caranx melampygus) are the top predators in the Rarotongan lagoon systems. I came across a school of them. When I first saw one of them in the distance, I honestly thought it was a juvenile reef shark trapped in the lagoon. Once I got closer and noticed more of them, I realised they were, unmistakeably, trevally. They outsized all the other reef fish by three-fold, and they proved their predator status as they confidently cruised past me at arms-length. The biggest members of this school appeared to be around 70cm, putting them at the age of approximately 8 years [3].

Moray eels would appear out of nowhere in crevices and holes in the reef. Sometimes, I’d sink to the sand on my knees to be face to face with them for a moment and appreciate their beautiful ugliness and imagine what it’s like to be a little fish caught in their pharyngeal jaws.

The coral was beautiful, and, like the octopus, I have never seen coral like that in the wild before. The variation in size and colour was magnificent. So was the fact that they live in a highly changing, intertidal environment; I couldn’t touch the sandy bottom at high tide compared to the exposed reef at dead low. There was an abundance of brain coral and table coral, as well as soft coral species.

Darting around were parrotfish, doing their crucial job of scraping and cleaning the coral to keep it healthy.

The coral themselves provide homes for thousands of reef fish and act as a barrier for incoming waves that intend to reach the island. It is said that even though coral reefs cover less than 1% of the ocean floor, they provide support for around 25% of all known marine species.

Burrowing urchin (*Echinometra mathaei*)

Threadfin butterflyfish (*Chaetodon auriga*) and black triggerfish (*Melichthys niger)*

Starry pufferfish (*Arothron stellatus*)

This environment was like nothing I had ever seen before. I wanted to spend every moment under that water; if a genie granted me only one wish, it would be to breathe underwater. Then again, my wish partly comes true every time I strap on some SCUBA gear and descend down 5 metres, 10 metres, 20 metres… which is exactly what I did.

Diving

Our first dive was the Papua Passage. Literally, a passage between two tall cliffs that felt like I was doing some sort of underwater, horizontal rock climbing to pass through. But, oh boy, was it worth it. Turtles everywhere you looked, sleeping on coral, drifting lazily past with their heavy eyes watching you meticulously. Two species exist in the Cook Islands, the endangered green turtle (Chelonia mydas) [4] and the critically endangered hawksbill turtle (Eretmochelys imbricata) [5], and I had the pleasure of meeting both.

Look closely at that photo. You can see the rear right quarter appears to be missing. If I had to take a guess, and I’ve stared at this photo forever trying to decipher it, then I would say that a shark, most likely a tiger, managed to get itself a quick on-the-go snack. This turtle is living proof that what doesn’t kill you makes you stronger.

The next dive was the Avaavaroa drop off. My god, did this blow my mind. An extensive coral reef that just drops off into the abyss.

Schools of fish, an eagle ray, and a couple of grey reef sharks cruised beneath us in front of a dark blue background that looked like it could engulf me at any moment. In a way, it was humbling to be on the edge of the vast, blue ocean like that.

The rest of our dives were on the northern and north-western sides of the island. Here, every time I slipped under the water, it felt like I was submerged in a woodland fairytopia.

The landscape was stunning. Here and there were crown of thorns starfish, a giant, impressive echinoderm with thorn-like ossicles containing venom. There appeared to be a healthy population here. Still, at another island within the Cook Islands called Aitutaki, their population is rampant. They are decimating the very coral they feed on.

Interestingly, on these northern dives was the amount of trash that seemed to be pulled along the floor by currents and would accumulate in hotspots around the reef and in caves. I grabbed all I could, but part of me wished to do a few dives whose sole purpose was to find as much trash as possible.

All in all, my trip to Rarotonga was fascinating. All I wanted to do once I got there was spend as much time as possible underwater, and that’s what I did. Rarotonga is a magical place, and it is so worthy of preservation. I hope to return back one day soon, maybe even to do my own research. But, for now, I’ll just keep harassing everyone’s insta feeds with Raro throwbacks.

References

1. Heukelem, W. V. (1973). Growth and life‐span of Octopus cyanea (Mollusca: Cephalopoda). Journal of Zoology, 169(3), 299–315.

2. Mather, J. A., & Mather, D. L. (2004). Apparent movement in a visual display: the ‘passing cloud’of Octopus cyanea (Mollusca: Cephalopoda). Journal of Zoology, 263(1), 89–94.

3. Sudekum, A. E., Parrish, J. D., Radtke, R. L., & Ralston, S. (1991). Life history and ecology of large jacks in undisturbed, shallow, oceanic communities. Fishery Bulletin, 89(3), 493–513.

4. Seminoff, J. A. (2004). Chelonia mydas. The IUCN Red List of Threatened Species 2004: e.T4615A11037468. http://dx.doi.org/10.2305/IUCN.UK.2004.RLTS.T4615A11037468.en

5. Mortimer, J. A. & Donnelly, M. (2008). Eretmochelys imbricata. The IUCN Red List of Threatened Species 2008: e.T8005A12881238. http://dx.doi.org/10.2305/IUCN.UK.2008.RLTS.T8005A12881238.en

The three carbon pumps of the ocean: biological, carbonate, and physical

Introduction

Carbon is the most critical component of all biological compounds and is exchanged around the Earth through a biogeochemical cycle (Archer, 2010). Although carbon is part of natural planetary systems, current concentrations of carbon dioxide (CO₂) are the highest they have been in 14 million years, and this increase is attributed to anthropogenic activity, specifically from the Industrial Revolution of the 1700s (Etheridge et al., 1996; Falkowski et al., 2000; Zhang, Pagani, Liu, Bohaty, & DeConto, 2013). The oceanic carbon cycle is comprised of processes that cycle carbon around different areas of the ocean, the seafloor, the Earth’s interior, and the atmosphere. In pre-Industrial Revolution times, the ocean provided a net source of CO₂ to the atmosphere, whereas now most of the carbon that enters the ocean comes from anthropogenic, atmospheric CO₂ (Raven et al., 2005). According to Falkowski et al. (2000), the ocean is a reservoir for ~38,400 gigatons (Gt) of carbon, a vast amount when compared to the terrestrial biosphere (~2,000 Gt) and the atmosphere (~720 Gt). CO₂ is diffused into the ocean’s surface waters and dissolves, now ready to enter the oceanic carbon cycle through three pumps: the biological pump, the carbonate pump, or the physical pump (Duan & Sun, 2003). The biological pump utilises autotrophy, such as photosynthesis by phytoplankton, to export carbon from the upper, sunlit ocean to the ocean interior or seafloor sediments and respire organic carbon into inorganic carbon (Emerson & Hedges, 2008). The carbonate pump is a process of ocean carbon sequestration driven by calcifying plankton, which releases CO₂ back into the atmosphere but sequesters it by sinking to the seafloor (Smooth & Key, 1975); this is why this process is also referred to as the carbonate counter pump. The physical pump is the physio-chemical process whereby carbon is transported from the ocean surface to its interior, where it can be stored for hundreds of years (Ito & Follows, 2003; Toggweiler, Murnane, Carson, Gnanadesikan, & Sarmiento, 2003).

The Biological Pump

The biological pump is a process of oceanic carbon sequestration that is driven mainly by autotrophic phytoplankton that inhabits the surface waters. This method of autotrophy, photosynthesis, converts CO₂ (dissolved inorganic carbon (DIC)) into organic biomass (particulate organic carbon (POC)) (Passow & Carlson, 2012; Sigman & Hain, 2012). Photosynthesis is the initial method of bringing carbon into the biological pump. It is further moved throughout the ocean by entering the food web after phytoplankton, which are primary producers at the lowest trophic level, are eaten by consumers. Carbon can then stay in the food web as higher trophic levels continuously consume organisms, or it can be released from the food web in the form of defecation or dead tissue (Passow & Carlson, 2012). This carbon sequestration process by primary production accounts for a vast majority of carbon fixation on Earth (Christina & Passow, 2007; De La Rocha, 2003).

Carbon is also moved into deep ocean currents or seafloor sediments by sinking organic matter. Organic material is formed by phytoplankton in the euphotic zone located at the surface level of the ocean. When plankton or other marine organisms eat, defecate, die, and decompose, this material, known as marine snow, begins to sink downwards (Passow & Carlson, 2012). One phytoplankton cell sinks at a rate of approximately 1 metre per day, meaning that, with an average depth of 4,000m, it can take over ten years for one carbon-carrying phytoplankton to reach the seafloor. Organic and inorganic matter, as well as expulsion of faecal matter from larger predators, aggregate to form marine snow that has a greater sinking velocity and can complete its journey to the seafloor in days (Heinze et al., 2015; Turner, 2015). Once this sinking organic biomass reaches deep-sea levels, it can enter the food web by becoming metabolic fuel for organisms that live there, including benthic organisms and deep-sea fish (Turner, 2015). Sinking matter transports an estimated 5–20 Gt of carbon to the deep ocean annually, where between 200 million–500 million tonnes of carbon is sequestered for thousands of years in seafloor sediment (Giering et al., 2020; Guidi et al., 2015; Henson et al., 2011). Any global warming-induced change on the integrity or function of phytoplanktonic populations will alter the efficiency at which POC is transported to ocean depths, with feedbacks on the rate of climate change.

With less than 0.5 Gt of sinking carbon reaching sequestration in seafloor sediment, between 44.5 Gt – 54.5 Gt of carbon is remineralised in the euphotic zone (Ducklow, Steinberg, & Buesseler, 2001) and between 5 Gt – 6 Gt of carbon is remineralised in midwater processes during particle sinking (Feely, Sabine, Schlitzer et al., 2004). Remineralisation occurs in the biological pump when heterotrophic organisms utilise organic matter produced by autotrophic organisms. They recycle the compounds from the organic form back to the inorganic form through respiration, making them available for reuse in primary production (Guidi et al., 2015). Remineralisation usually occurs with dissolved organic carbon (DOC) rather than POC because particles must typically be smaller than the organism taking it up for remineralisation (Lefevre, Denis, Lambert, & Miquel, 1996; Schulze & Mooney, 2012).

The particles that make it to the seafloor sediment may remain there for millions of years, trapping the carbon with them. Together, the processes that make up the biological pump ultimately remove carbon in its organic form from the ocean’s surface and return it to DIC in the deeper ocean. The thermohaline circulation (THC) returns deep-ocean DIC to the atmosphere on timescales that exceed millennia (the topic of THC will be explained further in “The Physical Pump”).

The Carbonate Pump

The carbonate pump is an extension of the biological pump but instead sequesters particulate inorganic carbon (PIC) and is driven by calcifying organisms (organisms that produce calcium carbonate (CaCO₃) shells). The leading contributor to the carbonate pump is the calcifying plankton known as coccolithophores due to the vast quantity of their global population. Coccolithophores are eukaryotic, unicellular phytoplankton that produces overlapping calcite platelets called coccoliths and are currently one of the most significant contributors to carbonate sediments in the deep sea (Hofmann et al., 2010; Renaud, Ziveri, & Broerse, 2002). Coccolithophore production of coccoliths through the uptake of dissolved inorganic carbon and calcium produces CaCO₃and CO₂, hence the alternate name of carbonate counter pump.

Ca²⁺ + 2HCO_3- → CaCO₃ + CO₂ + H₂O

However, some of the CO₂ released in calcification can be used in photosynthesis (Mackinder, Wheeler, Schroeder, Riebesell, & Brownlee, 2012), and over extended periods coccolithophores contribute to decreased levels of atmospheric CO₂. It is currently unknown as to the function of the coccolith. However, many theories have been proposed, including protection from predators or grazing zooplankton (Young, Andruleit, & Probert, 2009) or ballasting the cell for vertical migration into deeper water (Raven & Waite, 2004). The latter would be of considerable advantage to the carbonate pump in getting the carbon trapped in coccoliths to the seafloor. The most abundant species of coccolithophore is Emiliania huxleyi. It is likely to be the greatest global producer of calcite, meaning this species is an essential organism in transporting carbon from the ocean to be buried in marine sediment; they play a crucial role in the global biochemical carbon cycle (Balch, Holligan, & Kilpatrick, 1992).

The production of CaCO₃ shells in calcifying organisms such as molluscs, foraminifera, coccolithophores, crustaceans, echinoderms, and corals (Zondervan, Zeebe, Rost, & Riebesell, 2001) is the central part of the carbonate pump. When CO₂ dissolves in the surface layer of the ocean, it combines with water molecules. It enters into a series of chemical reactions that result in ions that calcifying organisms combine with calcium ions (Ca²⁺) to form CaCO₃ (Zeebe & Wolf-Gladrow, 2001). Even though one CO₂ molecule is released in calcification, one carbon atom becomes trapped within the CaCO₃ molecule used in calcification and becomes part of the sediment once it sinks to the bottom of the ocean. This means calcification takes two carbon atoms from the environment and only releases one back into it, even though the formation of calcium carbonate shells is a source of CO₂ (Mackie, McGraw, & Hunter, 2011) over the long-term calcifying organisms provide a sink for CO₂.

Calcifying organisms provide a large mechanism for the downward transport of CaCO₃ (Smith & Key, 1975). Dead organisms sink to the seafloor and dissolve on the way down and release carbon into deep-sea currents or reach the seafloor and build up to form CaCO₃ sediments stored for large timescales. The scale at which CaCO₃ makes its way down varies from species to species. For example, calcifying zooplankton (pteropods, ostracods and foraminifera) promote fast particulate inorganic carbon sequestration to the deep ocean due to the relatively large mass of their shells, which makes them sink rapidly. In comparison, calcifying phytoplankton such as coccolithophores will hardly sink individually, and even still, they have a broad range in sinking rates when they assimilate into biological aggregates. The burial of CaCO₃ in deep-ocean sediment is one of the primary mechanisms to reduce atmospheric CO₂ on geological timescales related to silicate weathering processes (Cartapanis, Galbraith, Bianchi, & Jaccard, 2018). Eventually, tectonic processes, including heat and pressure, transform seafloor sediments containing CaCO₃ into limestone; this process locks carbon away for millions of years (Folk, 1980). Over time, these sediment layers eventually return carbon to the oceans by weathering and erosion (Gibb, 1978).

The Physical Pump

The physical pump uses different processes to transport DIC from the ocean surface to its interior. Firstly, the solubility of CO₂ in water is the initial process of getting carbon into the ocean as DIC (Duan & Sun, 2003). Carbon dioxide dissolves in oceanic water and, unlike many other gases, it reacts with water to form a collective of ionic and non-ionic species (DIC), which include dissolved free carbon dioxide (CO₂ (aq)), carbonic acid (H₂CO₃), bicarbonate (HCO₃₋), and CO₃²⁻ (Weiss, 1974). There is a strong inverse function of seawater temperature on the solubility of CO2, as solubility is greater in colder water (Toggweiler et al., 2003). As sea surface temperature (SST) increases, less CO₂ can be taken up by the ocean; the progressive warming of the oceans releases CO₂ in the atmosphere because of its lower solubility in warmer seawater.

The THC is part of a global, oceanic conveyor belt, driven by heat and freshwater fluxes, where thermo refers to temperature, and haline refers to salinity, which together determines seawater’s density (Rahmstorf, 2003). The model of THC was first described by Stommel and Arons (1959), where they explored how temperature and salinity moved ocean water around the globe. Seawater with higher temperatures expands and is less dense than seawater at lower temperatures (Millero, Gonzalez, & Ward, 1976). Seawater with higher salinity is denser than seawater with lower salt content (Millero et al., 1976). Seawater with lower density floats over denser seawater; this is known as stable stratification (Maiti, Gupta, & Bhattacharyya, 2008). When dense seawater masses are initially formed, they are not stably stratified and will seek to correctly locate themselves vertically by their density and become stably stratified. This stratification process is the main driving force behind deep ocean currents, which carry carbon that has sunk from surface layers on the global conveyor belt, essentially sequestering it for hundreds of years.

The process of denser seawater joining the global conveyor belt is called downwelling. Higher density water accumulates and sinks below lower density water at places within the ocean where warm rings spin clockwise and create surface convergence, pushing the surface water downwards (Rao, Joshi, & Ravichandran, 2008; Yang, 2009). These areas of downwelling bring large amounts of carbon from the surface waters to be sequestered down below. Alternately, upwelling brings dense, cooler water to the surface to replace the warmer surface water. This water is usually nutrient-rich, including dissolved CO₂, meaning that these areas of upwelling provide an ideal location for an abundance of phytoplankton to carry out primary production and ultimately recycle the carbon brought to the surface from the deep ocean (Sarhan, Lafuente, Vargas, Vargas, & Plaza, 2000)

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Phylum Echinodermata: sea stars, sand dollars, urchins, and cucumbers

Introduction

Echinoderms are the largest marine-only phylum, and its ~7,000 species are found at every ocean depth from rock pools to the deep abyss (Dubois, 2014). Sea stars, sea urchins, sand dollars, and sea cucumbers comprise this phylum and let me tell you why they’re so darn cool.

Morphology

Echinoderms all possess radial symmetry, even though this can be quite hard to see in sea cucumbers, trust me, it’s there. Usually, the oral surface is on the underside of the animal, and the anus is located on top. The calcareous echinoderm endoskeleton is composed of ossicles which can be in the form of plates, spines, or lumps (Evamy & Shearman, 1965). The ossicles form a sponge-like structure called the stereom and are supported by a tough epidermis (Bottjer, Davidson, Peterson, & Cameron, 2006). Each ossicle develops from the deposition of a single cell which divides into more cells that deposit calcium carbonate in the original orientation. Ossicles are connected by collagen and may connect to muscles, allowing for flexibility and manoeuvrability. Spines are modified ossicles used for protection, locomotion, burrowing, gathering food and can also contain venom.

Pedicellaria

Pedicellariae are small, pincer-like structures used for protection against anything that may settle or graze on an animal’s body (Campbell, 2020; Coppard, Kroh, & Smith, 2012). Some pedicellariae are involved in capturing food, and some may be venomous. Each pedicellaria has its own muscles and sensory receptors, and therefore each pedicellaria is capable of reacting to a stimulus. There are four types of pedicellaria in urchins and two types for sea stars; one animal may have multiple types.

The incredibly dangerous flower urchin (*Toxopneustes pileolus*) with its long, venomous pedicellariae.
By Vincent C. Chen – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11721803.

Water-vascular system

Before we dive into this topic, take a look at this diagram, and feel free to refer to it as we dive into the incredibly unique water-vascular system.

Basic anatomy of an echinoderm. By CNX OpenStax, CC BY 4.0, https://commons.wikimedia.org/wiki/File:Figure_28_05_01.jpg.

The water vascular system is an elaborate closed system of canals used to help locomote the organism via its tube feet (Blake & Guensburg, 1988; Prusch & Whoriskey, 1976). The madreporite is a small sieve filter that creates an external link to the water-vascular system and is usually located to one side of the aboral surface and connects to a stone canal that extends vertically until it meets the ring canal. Radial canals emerge from the ring canal to the rays of the body in sea stars and around the body in sea urchins. From the radial canal stem lateral canals, which connect to ampullae and tube feet. Ampullae are bulb-like swellings that serve as reservoirs for water and fill the tube foot with water as they contract.

Tube feet have suction cup endings allowing for strong attachment to substrata and provide a general direction of movement. Pressure is exerted on the end of the sucker, and mucous functions as an adhesive. When the tissue inside the sucker contracts, it forms a cup that is secured to the substrate. In the case of sea stars living on soft substrates, tube feet are pointy so they can penetrate sediment and bury themselves.

Tube feet of the helmet urchin (*Colobocentrotus atratus*).
By Sébastien Vasquez – The uploader on Wikimedia Commons received this from the author/copyright holder., CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40284469.

Feeding

The feeding strategies differ significantly between species of Echinodermata. Some are filter feeders, most urchins are grazers, and most sea stars are carnivorous predators.

This predatory strategy of sea stars is perhaps the most interesting of the echinoderms (Melarange, Potton, Thorndyke, & Elphick, 1999; Semmens et al., 2013; Wulff, 1995). The oesophagus connects to a stomach with two sections called the cardiac stomach and the pyloric stomach. In evolutionarily advanced sea stars, the cardiac stomach can be everted out of the mouth and engulf and digest food. They can use their tube feet to create suction on bivalve shells and open them, where they will evert a section of their cardiac stomach inside the shell to release enzymes to digest the animal inside. The stomach retracts back inside and the partially digested prey can be passed to the pyloric stomach.

A starfish (*Circeaster pullus*) everting its cardiac stomach to feed on coral.
By NOAA – Flickr, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=47949152.

Regular vs Irregular Echinoids

Regular echinoids (e.g., sea urchin) have no front or back end, and the oral end is underneath and the aboral end is on top. From above, they are circular and radially symmetrical because regular echinoids roam the seafloor in search of food and need to move in any direction. This means they are exposed to predators and have evolved elaborate spines for defence and locomotion. Spines vary between species: needle-like, club-like, poisonous, or thorny. Regular echinoids are usually scavengers with a diet of plant matter, animal detritus, and other inverts, and can use their tube feet to grasp food. They have powerful, complex jaws called Aristotle’s Lanterns, which extend through the mouth to collect food and leave a distinctive star-shaped grazing trace.

Irregular echinoids (e.g., sand dollars) lead different lifestyles from the regulars. They burrow in the seafloor and extract nutrients from sediment and have one plane of symmetry – the oral end is at the front of the animal to collect food, and the aboral end is at the rear to leave waste behind. The spines have lost their defensive role and have become reduced and hair-like to help burrow, move through sediment, gather food, and generate currents in the burrow. Many have lost their jaws as they are unnecessary to their mode of life. The tube feet are modified into flanges for respiration and gathering food.

Classifications

Class Echinoidea

Class Echinoidea, aka echinoids, is composed of sea urchins and sand dollars.

Top – West Indian sea egg (*Tripneustes ventricosus*). Bottom – reef urchin (*Echinometra viridis*).
By Nhobgood, Nick Hobgood – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=11449176.

Lateral view of Aristotle’s Lantern of a sea urchin.
By Philippe Bourjon – The uploader on Wikimedia Commons received this from the author/copyright holder, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35659671.

Sand dollar (*Mellita* species) burying itself in the sand.
By John Tracy from Snellville, GA, USA – End of the line, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=26620303.

Class Holothuroidea

Sea cucumbers have leathery skin and an elongated body that is radially symmetrical along its longitudinal axis. They have no oral or aboral surface but instead, stand on one of their sides. Extraordinarily, they can loosen or tighten the collagen that forms their body wall and can essentially liquefy their body to squeeze through small gaps.

Brown sea cucumber (*Actinopyga echinites*) displaying its feeding tentacles and tube feet.
By François Michonneau – d2008-Kosrae-0084.jpg, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=20189778.

A giant sea cucumber (*Thelenota ananas*).
By Leonard Low from Australia – Flickr, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=1552175.

Class Asteroidea

There are around 1,500 species of sea star that make up the class Asteroidea.

Necklace sea star (*Fromia monilis*).
By Nhobgood Nick Hobgood – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6279893.

Crown-of-thorns sea star (*Acanthaster planci*) is one of the largest sea stars, and it gets its name from the venomous, thorny ossicles covering its surface.
By jon hanson on flickr. – https://www.flickr.com/photos/jonhanson/89930167/, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=665552.

References

Blake, D. B., & Guensburg, T. E. (1988). The water vascular system and functional morphology of Paleozoic asteroids. Lethaia, 21(3), 189–206.

Bottjer, D. J., Davidson, E. H., Peterson, K. J., & Cameron, R. A. (2006). Paleogenomics of echinoderms. Science, 314(5801), 956–960.

Campbell, A. C. (2020). Form and function of pedicellariae. In Echinoderm studies (pp. 139-167). CRC Press.

Coppard, S. E., Kroh, A., & Smith, A. B. (2012). The evolution of pedicellariae in echinoids: an arms race against pests and parasites. Acta Zoologica, 93(2), 125–148.

Dubois, P. (2014). The skeleton of postmetamorphic echinoderms in a changing world. The Biological Bulletin, 226(3), 223–236.

Evamy, B. D., & Shearman, D. J. (1965). The development of overgrowths from echinoderm fragments. Sedimentology, 5(3), 211–233.

Melarange, R., Potton, D. J., Thorndyke, M. C., & Elphick, M. R. (1999). SALMFamide neuropeptides cause relaxation and eversion of the cardiac stomach in starfish. Proceedings of the Royal Society of London. Series B: Biological Sciences, 266(1430), 1785–1789.

Prusch, R. D., & Whoriskey, F. (1976). Maintenance of fluid volume in the starfish water vascular system. Nature, 262(5569), 577–578.

Semmens, D. C., Dane, R. E., Pancholi, M. R., Slade, S. E., Scrivens, J. H., & Elphick, M. R. (2013). Discovery of a novel neurophysin-associated neuropeptide that triggers cardiac stomach contraction and retraction in starfish. Journal of Experimental Biology, 216(21), 4047–4053.

Wulff, L. (1995). Sponge-feeding by the Caribbean starfish Oreaster reticulatus. Marine Biology, 123(2), 313–325.

Phylum Arthropoda: the largest phylum in the animal kingdom

Introduction

Arthropoda: you may remember them from such fears as arachnophobia and your recent nightmare, “Help! I’m Locked in a Coffin of Cockroaches!” But, no fear, I won’t be burdening you with any terrestrial garbage because, as you know, it’s all underwater from here.

Morphology

To qualify for Phylum Arthropoda, you must be one of over 10 million species that lack a backbone, have an exoskeleton, segmentation, bilateral symmetry, a coelom, and paired, jointed appendages. Their segments are grouped into body divisions called tagmata, where segments and limbs have specialised functions; the three tagmata are the head, thorax, and abdomen, although some species have a combined head and thorax called a cephalothorax.

Exoskeleton

Arthropod exoskeletons are a cuticle that is secreted by the epidermis and is composed of two layers which aid in support and protection (Chen, Lin, McKittrick, & Meyers, 2008). The thin, waxy outer layer is called the epicuticle and is used in waterproofing. The thick, inner layer is called the procuticle and is the central structural part composing the majority of the exoskeleton. The exoskeleton is attached to the soft body by muscles and the animal uses those muscles to flex their joints (although some use hydraulic pressure to extend them).

Moulting

The exoskeleton is not flexible and, therefore, restricts arthropod growth. In order to grow, arthropods moult and shed the old exoskeleton in an almost continuous cycle until they reach their full size. First, the epidermis secretes a moulting enzyme that separates the old cuticle from the body. While the old cuticle is detaching, the epidermis secretes a new layer that will form part of the procuticle. After this is complete, the animal will take on seawater to split the old cuticle along predetermined weaknesses, and the animal will crawl out of its old exoskeleton. The new cuticle is exceptionally soft, and the animal is highly vulnerable as it continues to pump itself up with seawater to stretch the soft cuticle out. The cuticle will harden, and the animal can relax and eat its old exoskeleton to get back some nutrients (this gives me big Goldmember vibes iykyk).

Subphylum Crustacea

Crustaceans are what I like to call the insects of the ocean, and incudes isopods, copepods, barnacles, shrimp, krill, crabs, lobsters… the list goes on.

Appendages

The head region contains two pairs of sensory antennae, mandibles for crushing food, and first and second maxillae to sort and deliver food to the mandibles. The thoracic regions appendages are called thoracopods and may be specialised into maxillipeds which are specialised for feeding, and pereiopods, specialised for walking and swimming. The abdominal region contains pleopods which can be specialised for swimming, jumping, respiration, egg brooding, or copulation. The final pleopods may modify into a tail called a uropod. The abdomen terminates at the telson, which usually sits above the uropod and contains excretory organs. The number and diversity of appendages vary from amongst crustacean species.

File:Anatomy of a shrimp 3.jpg - Wikimedia Commons — The appendages of a shrimp: A: antennae. R: rostrum. C: carapace. Mx: maxilliped. U: uropod. T: telson. P: pereiopod. Pl: pleopod. 1–9: abdominal segments.

Crustaceans usually have biramous appendages that branch into two, where each branch consists of a series of segments attached end-to-end. The branching takes place on the second article. The external branch of the appendages is known as the exopodite, while the internal branch is known as the endopodite. Crustacean appendages have adapted to function in sensing their environment, defending against predators, swimming, walking, grasping, transferring sperm, generating water movement, and gas exchange. Some crustaceans have uniramous appendages thought to result from evolutionary loss of the second branch.

The difference between biramous and uniramous appendages within the phylum Arthropoda.

Classifications

Class Cirripedia

The most famous cirripeds are the acorn and gooseneck barnacles, and they live attached to hard substrates (Doyle, Mather, Bennett, & Bussell, 1996). They have a hard carapace made from calcareous plates that enclose the soft body parts. Their thoracic appendages are called cirri, which are biramous. The endopodites and exopodites are covered with setae to filter food particles from the water; they can also respire through the cirri. Most barnacles are hermaphroditic, and the penis extends into neighbouring barnacles to deposit spermatophores (Charnov, 2018). The larvae are planktonic and moult until they find a suitable substrate in which they settle on their “back”, the carapace, which adheres permanently to the substrate.

Gooseneck barnacles (order Pedunculata) growing in a tidal cave.

Northern acorn barnacles (*Semibalanus balanoides*).

Order Amphipoda and Order Isopoda

Amphipods are the most annoying crustacean. They’re the ones that bite you at the beach, aka sandflies. Isopods are similar in some ways but are lice. Let me break them both down for you.

Amphipods are scavengers and consume smaller invertebrates and plant matter; that’s why you often find them around driftwood or decaying seaweed at the beach. They are frequently consumed therefore making them an integral part of coastal food webs. Their bodies are laterally compressed (flattened from side to side) with no carapace and have the three main arthropod tagmata. They have strong uropods which aid them in jumping all over your lovely picnic.

A freshwater amphipod species (*Gammarus roeseli*).

Isopods have a range of feeding strategies from scavengers to carnivores and parasites to filter feeders. Their bodies are dorsoventrally flattened (flattened top to bottom, creating a wide, flat profile), and they lack a prominent carapace; it’s more of a helmet, if anything. Like amphipods, they contain all three main body parts and have a pleotelson where the last abdominal segment is fused with the telson.

A carnivorous isopod called the speckled sea louse (*Eurydice pulchra*).

A giant, marine isopod (*Bathynomus giganteus*).

Order Decapoda

Decapods, meaning “ten-footed”, are your supermarket crustaceans, e.g., crabs, lobsters, prawns, and shrimps, although I’m sure you’ll agree they look a lot better in the ocean! One of their thoracic appendages may be specialised into large pincers called chelae (think lobster claws), used to crush shells, tear up food, and pass pieces to the maxillipeds. The maxillipeds are the first three pairs of thoracic appendages and are modified for feeding. The abdominal appendages function to carry eggs, brood young, or transfer spermatophores. They usually have a uropod and telson that serve as a strong tail. Although, some decapods, e.g., crabs, have short abdomens, which are typically folded under the thorax. In males, this fold is triangular, and in females, it is broader so it can hold the eggs. Their carapace extends low enough to cover their gills.

Many decapod species can demonstrate the ability to autotomise, whereby they can regenerate an appendage after it has been dropped (Juanes & Smith, 1995; Shinji, Miyanishi, Gotoh, & Kaneko, 2016). They usually drop their limbs when threatened by a predator as a deterrent; the predator will be distracted by the limb, and the decapod can escape. A blot clot will prevent bleeding, and regeneration of the new limb will start immediately and can usually be seen after the successive moult. Growing a new appendage is extremely energy taxing, so dropping it in the first place is usually a last resort.

Purple rock crab (*Leptograpsus variegatus*).

Mantis shrimp (*Odontodactylus scyllarus*).

References

Charnov, E. L. (2018). Sexuality and hermaphroditism in barnacles: a natural selection approach. In Barnacle biology (pp. 89–103). Routledge.

Chen, P. Y., Lin, A. Y. M., McKittrick, J., & Meyers, M. A. (2008). Structure and mechanical properties of crab exoskeletons. Acta biomaterialia, 4(3), 587–596.

Doyle, P., Mather, A. E., Bennett, M. R., & Bussell, M. A. (1996). Miocene barnacle assemblages from southern Spain and their palaeoenvironmental significance. Lethaia, 29(3), 267–274.

Juanes, F., & Smith, L. D. (1995). The ecological consequences of limb damage and loss in decapod crustaceans: a review and prospectus. Journal of Experimental Marine Biology and Ecology, 193(1-2), 197–223.

Shinji, J., Miyanishi, H., Gotoh, H., & Kaneko, T. (2016). Appendage regeneration after autotomy is mediated by Baboon in the crayfish Procambarus fallax f. virginalis Martin, Dorn, Kawai, Heiden and Scholtz, 2010 (Decapoda: Astacoidea: Cambaridae). Journal of Crustacean Biology, 36(5), 649–657.

Worms: Phylum Platyhelminthes, Nemertea, Nematoda, and Annelida

Introduction

Why is it that every terrestrial creepy-crawly seems to have a marine counterpart? Slaters? How about isopods. Spiders? Try crabs. Worms? Well, let me tell you. I’ve got flatworms, I’ve got ribbon worms, I’ve even got roundworms, and you can bet your ass I’ve got ringed worms.

Phylum Platyhelminthes

Flatworms are simple folk; they are acoelomates (have no body cavity) and are restricted to a flattened body shape due to a lack of circulatory and respiratory organs. They do, however, have nervous ganglia and longitudinal nerve trunks running along their bodies. They are bilaterians and have three cell layers (endoderm, mesoderm, ectoderm) and have protonephridia which functions similarly to a kidney. Flatworms can be colourful or dull.

Dawn flatworm (*Pseudobiceros hancockanus*).

Class Turbellaria

Turbellarians are the more traditional class of flatworms and are represented by around 4,500 species. Most species are externally ciliated, and some have a duo-gland, an adhesive system that excretes mucous and other sticky materials to repeatedly attach and release the animal to substrates (Jennings, 1957).

The most interesting thing about Turbellarians is their reproductive strategy. All are hermaphrodites, and many of them asexually reproduce, but some species engage in a delicate, lovemaking episode known as penis fencing (Chim, Ong, & Gan, 2015; Collins III, 2017). Two flatworms will rear up, exposing two penises, and attempt to inseminate one another in a battle that can last up to an hour. They are fighting because the winner will inseminate the other and essentially become the father, free of all paternal duties and can swim away and continue with his life; the one who is inseminated must now work harder to gain the extra energy required to produce offspring.

Two flatworms (*Pseudobiceros bedfordi*) show off their penises in the penis fencing ritual.

Phylum Nemertea

These are the ribbon worms, and they move slowly with either their external cilia to glide on a trail of slime, muscular crawling, or undulated swimming. Similarly to flatworms, they are acoelomates and have a similar reproductive system, yet they differ in that they have a complete gut, circulatory system, and a proboscis.

The proboscis is an infolding of the body wall, and hydrostatic pressure “fires” the proboscis inside out to attack prey (McDermott, 1985). One type of proboscis exits from a pore that is separate from the mouth and entangles and immobilises prey with sticky, venomous secretions. A different kind of proboscis exits from the mouth and typically has a calcareous barb called a stylet to stab prey and inject it with venom and digestive fluids. Prey can then be swallowed whole, or its tissues may be sucked into the mouth.

Five-lined ribbon worm (*Baseodiscus quinquelineatus*).

Pink ribbon worm, possibly *Gorgonorhynchus* species.

Phylum Nematoda

Nematodes, aka the roundworms, are estimated at around 25,00 species (Hodda, 2011), although others estimate this number to be over the 1 million mark (Lambshead, 1993). They are regarded as pseudocoelomates as they have a fluid-filled cavity between the digestive tract and the body wall, although it is not lined with tissue, and there is no membrane-like tissue supporting the organs and, therefore, is not a true coelom. They have a complete digestive system and an external, collagenous cuticle, which is shed usually four times before reaching adulthood; just before the moult, the old cuticle is softened by enzymes that accumulate between the old and new cuticle until the old one is shed.

Microscopic, transparent, terrestrial roundworms (*Caenorhabditis elegans*), usually around 1mm in length and found in temperate, soil environments.

Phylum Annelida

I like to refer to annelids as your classics. They’re what you imagine when you hear the word “worm”, but they also include over 22,000 species of ragworms, earthworms, and even leeches. Between the lot of them, they have occupied a variety of niches, including tidal zones, hydrothermal vents, freshwater, and your backyard! Marine annelids have a range of habitats, life histories, and feeding strategies, making them a critical component of the oceanic ecosystem (Capa & Hutchings, 2021).

Annelids are pretty advanced. They have a complete gut, a closed circulatory system with blood vessels, bilateral symmetry, cephalisation (somewhat), and usually a pair of coelomata in each segment. Annelid segmentation facilitates the specialisation of body parts into different functions. A collagenous cuticle covers their bodies but, unlike nematodes, does not moult and has setae to provide traction.

Your mate from the backyard: the humble earthworm (Lumbricina species).

A marine leech (*Pontobdella muricata*).

References

Capa, M., & Hutchings, P. (2021). Annelid diversity: Historical overview and future perspectives. Diversity, 13(3), 129.

Chim, C. K., Ong, R. S., & Gan, B. Q. (2015). Penis fencing, spawning, parental care and embryonic development in the cotylean flatworm Pseudoceros indicus (Platyhelminthes: Polycladida: Pseudocerotidae) from Singapore. Raffles Bulletin of Zoology, 31, 60–67.

Collins III, J. J. (2017). Platyhelminthes. Current Biology, 27(7), R252–R256.

Hodda, M. (2011). “Phylum Nematoda Cobb 1932. In: Zhang, Z.-Q.(Ed.) Animal biodiversity: An outline of higher-level classification and survey of taxonomic richness. Zootaxa, 3148(1), 63–95.

Jennings, J. B. (1957). Studies on feeding, digestion, and food storage in free-living flatworms (Platyhelminthes: Turbellaria). The Biological Bulletin, 112(1), 63–80.

Lambshead, P. J. D. (1993). Recent developments in marine benthic biodiversity reserch. Oceanis, 19, 5–24.

McDermott, J. J., & Roe, P. (1985). Food, feeding behavior and feeding ecology of nemerteans. American Zoologist, 25(1), 113–125.

Lophophorata: Phylum Phoronida, Brachiopoda, and Bryozoa

Introduction

Lophophorata is a clade composed of three Phyla: Phoronida, Brachiopoda, and Bryozoa. They are grouped like this due to one uniting structure, the lophophore (Jang & Hwang, 2009; Temereva & Kuzmina, 2017).

Morphology

Let’s begin with the pièce de resistance: the lophophore. It is a feeding structure, similar to a bunch of ciliated tentacles, that surround the mouth; therefore, lophophorates are suspension feeders. The cilia create strong water circulation, allowing for gas exchange, the exportation of gametes, and food particle delivery. The lophophore is thought to be the result of convergent evolution (Halanych, 1996). The gut of a lophophorate is U-shaped, so the direction of water flow prevents the mixing of food and waste products.

Phylum Phoronida

Phoronids are also known as horseshoe worms and build chitinous tubes to protect and support their soft bodies (Abele, Gilmour, & Gilchrist, 1983). Phoronids can retract and extend their lophophore, and cilia manipulate food into their mouth. Phoronids actively assess the flow of the water current and can reorient themselves as water flow changes to maximise their food-capturing ability. Their diet includes zooplankton, detritus, and invertebrate larvae.

Phylum Brachiopoda

Please don’t be a pleb and confused them with Bivalvia. Brachiopoda has upper and lower valves, as opposed to left and right valves of bivalves. Anyway, they don’t even share the same ancestry.

The lophophore is connected to the lower brachial valve and is supported by cartilage and, sometimes, a brachidium (calcareous support attached to the brachial valve). Most brachiopods attach themselves to hard substrates by a stalk called the pedicle, a connective tissue that is part of the upper pedicle valve.

Inarticulate brachiopods (*Lingula anatine*).

Class Articulata

This class is defined by its “tooth-and-socket” hinge arrangement and a simple muscle strategy that opens and closes the hinges.

Class Inarticulata

This class has untoothed hinges and a more complex muscular strategy for aligning the valves.

Phylum Bryozoa

Bryozoans resemble phoronids, except bryozoans are microscopic, typically about 0.5mm in length. All genera, except one, form colonies that resemble moss. Each individual is called a zooid and has a hard casing called a cystid and a polypide that holds the organs in place. Within the colony, individual zooids may share resources through internal connections, and some zooids may specialise in a function. Vibracula zooids have a long bristle thought to function as defence or vibrate to keep the colony from becoming covered with sediment.

References

Abele, L. G., Gilmour, T., & Gilchrist, S. (1983). Size and shape in the phylum Phoronida. Journal of Zoology, 200(3), 317–323.

Halanych, K. M. (1996). Convergence in the feeding apparatuses of lophophorates and pterobranch hemichordates revealed by 18S rDNA: an interpretation. The Biological Bulletin, 190(1), 1–5.

Jang, K. H., & Hwang, U. W. (2009). Complete mitochondrial genome of Bugula neritina (Bryozoa, Gymnolaemata, Cheilostomata): phylogenetic position of Bryozoa and phylogeny of lophophorates within the Lophotrochozoa. Bmc Genomics, 10(1), 1–18.

Temereva, E. N., & Kuzmina, T. V. (2017). The first data on the innervation of the lophophore in the rhynchonelliform brachiopod Hemithiris psittacea: what is the ground pattern of the lophophore in lophophorates?. BMC evolutionary biology, 17(1), 1–19.

Phylum Mollusca: send nudes (nudibranchs)

Introduction

There’s a mollusc, see? And he walks, well he doesn’t walk up, he swims up. Well, actually, the mollusc isn’t moving, he’s in one place and then the sea cucumber… I mixed up. There was a mollusc and a sea cucumber. None of them were walking. Normally, they don’t talk, but in a joke, everyone talks. So, the sea mollusc says to the cucumber, “with fronds like these, who needs anemones!”

Mollusca forms the second largest invertebrate phylum and has a diverse evolutionary history and wide range of feeding and life-history strategies that have led to its success on land, freshwater, and the ocean (Rosenberg, 2014). Molluscs can be herbivores, carnivores, scavengers, and filter feeders, and they are responsible for the consumption of large amounts of organic matter. They themselves serve as food for a range of predators, and thus, are essential links in the food chain.

Morphology

All species of Mollusca have bilateral symmetry, lack of segmentation, a structured nervous system, and a mantle. When discussing mollusc morphology, a generalised overall structure is usually referred to, though many species are an exception to this structure.

Their head contains the mouth and feeding structures and a ganglion of nervous systems. The visceral mass is the metabolic region containing the stomach, heart, intestines, gonads, etc. Covering the visceral mass is the mantle which secretes the shell, and a fold on the mantle known as the mantle cavity contains excretory and respiratory organs. The shell thickens with age as it is secreted by the mantle and consists of three layers:

Outer layer – aka periostracum is composed of a durable organic material and may develop as a thin smooth coating, into hairs or into flexible spine-like outgrowths.
Middle layer – aka prismatic is made of columnar calcite.
Inner layer – is often nacreous (think iridescent and pearl-like) and laid down in thin sheets by the epithelial parts of the mantle.

Torsion

Torsion is evident in gastropods (snails) during larval development. Two torsion events of 90° result in 180° rotation of the mantle cavity and the organs it contains to an anterior position above the head (Page, 2006). A possible disadvantage of torsion is that the anus excreting above the head could cause fouling of the mouth and sensory organs. However, the success of the class Gastropoda suggests this may not be an issue. Possible advantages of torsion are that it allows the animal to retract its vulnerable head into the shell efficiently and in marine species, the anterior positioning may prevent sediment from entering the mantle cavity.

Feeding

The radula is unique to molluscs and is found in every class except bivalves (Steneck & Watling, 1982). It is a chitinous ribbon studded with small, hard teeth, used for scraping or cutting food before it enters the oesophagus. The radula protrudes from the floor of the buccal cavity, where the odontophore underlies the radula membrane and controls its protrusion and return. As the radula retracts into the buccal cavity, the teeth rasp food particles from the substrate and food is deposited into the pharynx. As teeth wear, new teeth are continuously being secreted, shaped, and added to the cuticle ribbon inside the radula sac.

Food particles pass from the pharynx to the oesophagus and then to the stomach, where digestion occurs intracellularly and extracellularly within folds of the stomach called diverticula. The stomach may have several functions: sorting, grinding, and digesting food particles.

Systems

The circulatory system is usually open, and blood flows through the haemocoel cavity. The respiratory pigment is called hemocyanin, which is pale blue when oxygenated and clear when deoxygenated. This blue is from the copper contained in the oxygen-binding molecules, as opposed to the red blood of mammal’s iron found in haemoglobin.

The nervous system is typically a mass of nerve cell bodies that associate with the sensory organs. The sensory organs often include eyes, statocysts (the sensory organ that orients animal to gravity, located in the foot), osphradia (sensory epithelium which act as chemoreceptors), and tentacles.

Classifications

Neogastropoda

Neogastropoda includes sea snails and is primarily carnivorous. They have a proboscis that extends out and can drill through shells of bivalves or is used to suck up nutrients from its prey (much like a butterfly feeding on nectar). Some species have a siphon to draw water into the mantle cavity to oxygenate the gill.

Mud whelks (*Nassarius jacksoniania*) eating a dead fish.

Bivalvia

Bivalvia includes your classic shellfish, e.g., clams, mussels, oysters, scallops, and cockles. They lack a radula, an odontophore, and a head. The name bivalve means “two shells”, which is exactly what they have, and these shells are connected by a hinge and are left and right, as opposed to top and bottom as with Brachiopoda (don’t worry, we’ll get there). The lack of head is made up for with their foot. It is usually well-developed and excellent for digging and ploughing through sediment. Some bivalves, e.g., mussels, have a byssus thread used to attach to hard substrates, and I guarantee you will notice it the next time you eat a mussel.

Little black mussel (*Xenostrobus pulex*).

Nudibranchia

Nudibranchia contains the crazy, colourful nudibranchs: a group of around 3,000 species of soft-bodied molluscs. But how can they be molluscs if they don’t have a shell? They shed their shell in the larval stage (Thompson, 1959). Along with a naked body, they also lack a mantle cavity, meaning that the nudibranchs probably like the term mollusc to be used loosely! To add to their peculiar nature, they are all carnivorous mostly feeding with a radula, and some store nematocysts from their Cniadrian prey and use them as a defence mechanism (Frick, 2003). They are hermaphroditic, but cannot fertilise themselves, and mate after a courting dance takes place.

Cephalopoda

And if you thought nudibranchs didn’t fit into the Mollusca mould, then don’t even bother reading about cephalopods. With their name meaning “head-feet”, Cephalopoda contains over 800 living species of octopus, squid, cuttlefish, and nautilus. They have bilateral symmetry and, as their name suggests, a prominent head atop a set of arms or tentacles that have evolved from the molluscan foot. And before you ask, “arms” are the suction cup ones, and “tentacles” only have suction cups at the end – some species have one or the other or both.

When you think of squid, you don’t immediately associate them with their bivalve or gastropod relatives mainly because, well, the latter two have a shell and a squid does not, right? Well, the answer is tricky, and, like with every marine invertebrate, I will forgive you for thinking something is something that it is not. Nautiluses have an external shell that is visible to the naked eye, still the case of cuttlefish, octopuses, and squid is slightly more complex. Some cephalopods have a vestigial shell, some have organic, internal, calcium carbonate structures, and some may have just evolved to lose their shell entirely (Baratte, Andouche, & Bonnaud, 2007; Furuhashi, Schwarzinger, Miksik, Smrz, & Beran, 2009; Warnke & Keupp, 2005).

Cephalopods are often regarded as extremely intelligent, with complex nervous systems and the ability to use tools and problem solve (Budelmann, 1995; Finn, Tregenza, & Norman, 2009; Richter, Hochner, & Kuba, 2016; Schnell, Amodio, Boeckle, & Clayton, 2021).

The genus Hapalochlaena contains four extremely venomous octopus species, more commonly known as the blue-ringed octopuses. They are tiny, reaching maximum sizes of 20cm, but deadly, with one animal containing enough tetrodotoxin to kill 26 adult humans with a painless bite that can paralyse within minutes.

The greater blue-ringed octopus (*Hapalochlaena lunulata.*).

The Palau nautilus (*Nautilus belauensis*).

The common cuttlefish (*Sepia officinalis*) – note its “W” shaped pupil thought to be useful in improving horizontal vision (Mäthger, Hanlon, Håkansson, & Nilsson, 2013).

File:Euprymna scolopes - image.pbio.v12.i02.g001.png - Wikimedia Commons — The Hawaiian bobtail squid (*Euprymna scolopes*) reaches a max of 3cm.

References

Baratte, S., Andouche, A., & Bonnaud, L. (2007). Engrailed in cephalopods: a key gene related to the emergence of morphological novelties. Development genes and evolution, 217(5), 353–362.

Budelmann, B. U. (1995). The cephalopod nervous system: what evolution has made of the molluscan design. In The nervous systems of invertebrates: An evolutionary and comparative approach (pp. 115–138). Birkhäuser Basel.

Finn, J. K., Tregenza, T., & Norman, M. D. (2009). Defensive tool use in a coconut-carrying octopus. Current biology, 19(23), R1069–R1070.

Frick, K. (2003). Predator suites and flabellinid nudibranch nematocyst complements in the Gulf of Maine. DIVING FOR SCIENCE… 2003, 37.

Furuhashi, T., Schwarzinger, C., Miksik, I., Smrz, M., & Beran, A. (2009). Molluscan shell evolution with review of shell calcification hypothesis. Comparative biochemistry and physiology Part B: Biochemistry and molecular biology, 154(3), 351–371.

Mäthger, L. M., Hanlon, R. T., Håkansson, J., & Nilsson, D. E. (2013). The W-shaped pupil in cuttlefish (Sepia officinalis): functions for improving horizontal vision. Vision research, 83, 19-24.Page, L. R. (2006). Modern insights on gastropod development: reevaluation of the evolution of a novel body plan. Integrative and Comparative Biology, 46(2), 134–143.

Richter, J. N., Hochner, B., & Kuba, M. J. (2016). Pull or push? Octopuses solve a puzzle problem. PloS one, 11(3), e0152048.

Rosenberg, G. (2014). A new critical estimate of named species-level diversity of the recent Mollusca. American Malacological Bulletin, 32(2), 308–322.

Schnell, A. K., Amodio, P., Boeckle, M., & Clayton, N. S. (2021). How intelligent is a cephalopod? Lessons from comparative cognition. Biological Reviews, 96(1), 162–178.

Steneck, R. S., & Watling, L. (1982). Feeding capabilities and limitation of herbivorous molluscs: a functional group approach. Marine Biology, 68(3), 299–319.

Thompson, T. E. (1959). Feeding in nudibranch larvae. Journal of the Marine Biological Association of the United Kingdom, 38(2), 239–248.

Warnke, K., & Keupp, H. (2005). Spirula—a window to the embryonic development of ammonoids? Morphological and molecular indications for a palaeontological hypothesis. Facies, 51(1), 60–65.

Phylum Ctenophora: is it a bird? Is it a plane? Is it a jellyfish?

Introduction

There are less than 200 known species of ctenophores, all of which are found exclusively in marine habitats. Ctenophores, more commonly known as the comb jellies, resemble cnidarian medusa (I will forgive you for confusing them with jellyfish), but ctenophores have a few specific features that make them unique.

Morphology

Opposed to jellyfish, who have radial symmetry, ctenophores have bilateral symmetry (Pang & Martindale, 2008). They don’t use jet propulsion like our Scyphozoan friends, but rather are the largest animals to swim with the help of cilia, with adults range from a few millimetres to 1.5 metres (Tamm, 2015). The cilia are packed in the thousands into ctenes, or comb plates, which are organised into comb rows around the body (Tamm, 2014).

Ctenophores are known to be bioluminescent, where the photoproteins, located in canals under the comb rows, are activated (Haddock & Case, 1999; Pang & Martindale, 2008). This is not to be confused with the rainbow effect of the comb rows produced as their cilia beat and scatter light (Welch, Vigneron, Lousse, & Parker, 2006).

A brief annotation of the anatomy of a ctenophore.

Feeding

Ctenophores have sticky cells in the epidermis of their tentacles called colloblasts which capture food. The tentacles expand when ready to capture food and the branches on the tentacles separate. The stickiness of the colloblasts allows the organism to “fire” a fibre with adhesive granules to capture food (e.g., copepods).

Colloblasts are linked to a nervous system that triggers the retraction of the tentacles towards the mouth when prey is captured. Prey is then wiped inside the mouth, swallowed, and liquified into a slurry by enzymes and muscular contractions in the pharynx. Cilia beat and distribute the slurry through the canal system where digestion occurs intra and extracellularly. Some waste is released through the anal pores but usually they regurgitate waste from the mouth.

Colloblasts are unique to ctenophores and are found in the epidermis of the tentacles and, similar to nematocysts of Cnidaria, the colloblasts are discharged from the tentacles and capture prey (Franc, 1978). However, colloblasts are not venomous but rather they are adhesive and stick to their prey. The rare ctenophore, Haeckelia rubra, has rid itself of colloblasts completely and instead collects nematocysts from their cnidarian prey (Mills & Miller, 1984).

Nervous system and navigation

Ctenophores have a structure, know as the statocyst, that aids in their navigation and orientation through gravitational sensitivity (Pang & Martindale, 2008; Tamm, 2015). If a ctenophore is pulled off balance, its statocyst will direct beating of specific comb rows in order to right itself. For movement over long distances, ctenophores mainly rely on ocean currents (Pang & Martindale, 2008). Instead of a nervous system, ctenophores have a complex nerve net that works closely with the statocyst and ctenes (Pang & Martindale, 2008; Tamm, 2014; Tamm, 2015).

Reproduction

Floating somewhat aimlessly around in the ocean as a relatively tiny and hard-to-see individual means the likelihood of you meeting someone you can have babies with is quite low. To combat this, almost all ctenophores are self-fertile hermaphrodites (Pang & Martindale, 2008).

Classifications

Class Tentaculata

Tentaculata have, you guessed it, tentacles. Commonly, they have long, feathery tentacles which are equipped with colloblasts.

Class Nuda

If Class Tentaculata have tentacles, then Class Nuda must have no tentacles. Organisms from Nuda are known as beroids (from the monophyletic order Beroida), and they feed by using their large mouths to engulf prey. Alternatively, some species spread their lips over prey whilst a sword-like structure chops the prey up (Tamm & Tamm, 1991). Beroids actively hunt their prey, which is usually soft-bodied organisms such as ctenophores – yes, they eat their own kind.

References

Franc, J. M. (1978). Organization and function of ctenophore colloblasts: an ultrastructural study. The Biological Bulletin, 155(3), 527–541.

Haddock, S. H., & Case, J. F. (1999). Bioluminescence spectra of shallow and deep-sea gelatinous zooplankton: ctenophores, medusae and siphonophores. Marine Biology, 133(3), 571–582.

Mills, C. E., & Miller, R. L. (1984). Ingestion of a medusa (Aegina citrea) by the nematocyst-containing ctenophore Haeckelia rubra (formerly Euchlora rubra): phylogenetic implications. Marine Biology, 78(2), 215–221.

Pang, K., & Martindale, M. Q. (2008). Ctenophores. Current Biology, 18(24), R1119–R1120.

Tamm, S. L. (2014). Cilia and the life of ctenophores. Invertebrate Biology, 133(1), 1–46.

Tamm, S. L. (2015). Functional consequences of the asymmetric architecture of the ctenophore statocyst. The Biological Bulletin, 229(2), 173–184.

Tamm, S. L., & Tamm, S. (1991). Reversible epithelial adhesion closes the mouth of Beroe, a carnivorous marine jelly. The Biological Bulletin, 181(3), 463–473.

Welch, V., Vigneron, J. P., Lousse, V., & Parker, A. (2006). Optical properties of the iridescent organ of the comb-jellyfish Beroë cucumis (Ctenophora). Physical Review E, 73(4), 041916.