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).
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 , . Such force occurs with a single tail slap that gas is diffused out of the seawater and rises to the surface in bubbles . 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.
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.
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 . 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 .
The common and bigeye thresher are ‘Vulnerable’ to extinction under the IUCN Red List , , and the pelagic thresher is further threatened and classified as ‘Endangered’ . 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 , the latter commonly used in modern cosmetics and supplements. All species are usually retained if accidentally caught on commercial vessels –. In one study  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 , and threshers accounted for up to 3% of the total fin mass in the past .
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 .
The New Zealand commercial fishery is not exempt from thresher shark landings. A recent report by Fisheries New Zealand  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 . A 30 cm swordfish (Xiphiasgladius) 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.
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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.
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).
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).
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).
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.
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.
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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.
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 .
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.
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.
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)  and the critically endangered hawksbill turtle (Eretmochelys imbricata) , 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.
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Humans have begun to drastically alter the atmospheric environment since the Industrial Revolution of the 1700s, where the burning of fossil fuels released exponential amounts of carbon dioxide into the air. Along with increasing the global temperature, affecting life cycles of organisms, and disassembling natural species interactions, this release initiated a process known as ocean acidification, whereby the pH of the ocean has decreased from 8.2 to 8.1 (Feely, Doney, & Cooley, 2009). The effect that ocean acidification has on marine organisms starts at the base of the food chain with primary producers, such as phytoplankton, and repercussions can then be seen throughout entire food webs. Calcareous phytoplankton (i.e., coccolithophores) may be at high risk of impact from ocean acidification due to the calcium carbonate plates that they produce through calcification. Coccolithophores are key players in the global biogeochemical cycle, the pelagic food chain, and oxygen production through photosynthesis, so any negative impacts that ocean acidification may have on them need to be understood and mitigated for the health and wellbeing of humankind.
Climate change refers to global climatic shifts that were intensified in the mid-20th century through the burning of fossil fuels (Griffin, 2018). This anthropogenic influence has resulted in an increasing amount of carbon dioxide (CO2) in the atmosphere with a >90% probability that the observed average global temperature increase is due to human-induced greenhouse gas concentrations and around half of this increase occurring in the last three decades (Feely et al., 2009; Rosenzweig et al., 2008). Anderson, Hawkins, and Jones (2016) describe the greenhouse effect as infrared energy that has been re-emitted from solar radiation is absorbed by water vapour and CO2 to create a ‘blanket’ around planet Earth. As CO2 concentrations increase, this greenhouse effect is enhanced, and the planet warms further beyond its natural average temperature.
Regional warming exhibits observable biological changes in terrestrial systems. Included amongst these changes are increases in coastal erosion (Beaulieu & Allard, 2003; Forbes, Parkes, Manson, & Ketch; 2004; Orviku, Jaagus, Kont, Ratas, & Rivis, 2003), melting permafrost (Frauenfeld, Zhang, Barry, & Gilichinsky, 2004), and glaciers shrinking in all seven continents (Oerlemans, 2005). Natural increases and decreases in Earth’s temperature have been recorded over millions of years (Savin, 1977), which can add scepticism to the idea that current global heating is due to anthropogenic activity rather than part of a natural cycle. A study by Reichert, Bengtsson, and Oerlemans (2002) into Swiss and Norwegian glacial retreat showed that the retreat could not be due to natural climatic change because it exceeds glacial fluctuations derived from the general circulation model, meaning that another force must be acting on the glaciers.
Climate change’s effects also reach the ocean. Edwards and Richardson (2004) describe phenological changes that occur in the wake of a warming ocean. An increase in sea surface temperature (SST) is used as an indicator for climate change, and it affected the seasonal development and phenology of plankton species. Massive phenological changes occurred at a 0.9oC increase of SST. The extent of these changes was more significant than those of terrestrial studies (Root et al., 2003), which may indicate that marine communities have heightened sensitivity to climate change. Furthermore, Edwards and Richardson’s (2004) study showed there was a different rate of response in communities to ocean warming, creating a “mismatch between successive trophic levels and a change in the synchrony of timing between primary, secondary and tertiary production” (p. 883). This mismatch will directly affect populations at higher trophic levels, including commercial fish species, marine mammals, and seabirds, and adaption will need to take place to realign these trophic levels with primary production. Not only are higher trophic levels at risk in response to changing phenology of plankton due to climate change, but other essential ecosystem services will be impacted, including the production of oxygen that we breathe, the sequestration of CO2, and the biogeochemical cycling of nitrogen, phosphorus, and silica (Richardson & Schoeman, 2004). Beaugrand and Reid (2003) study provided evidence for climate change-induced, long-term changes to the three trophic levels of phytoplankton, zooplankton, and salmon. Various declines and increases of phytoplankton and zooplankton ensued due to regional temperature increases over time, which caused the ultimate decline of salmon stocks in 1988, and this decline is expected to continue as the climate proceeds to change.
The ocean has absorbed approximately one-quarter of anthropogenically emitted CO2 over the industrial era, causing chemical reactions that reduce oceanic pH, concentrations of carbonate ions (CO32-), and saturation states of two calcium carbonate (CaCO3) minerals calcite and aragonite; this is the process of ocean acidification (OA) (Feely et al., 2009). The ocean absorbs CO2 in two ways: through photosynthesis undertaken by marine phytoplankton (Rost, Riebesell, & Burkhardt, 2003) and through the dissolving of CO2 in seawater (Feely et al., 2009). When CO2 reacts with seawater, it creates carbonic acid (H2CO3), which dissociates to hydrogen ions (H+) that combine with carbonate to form bicarbonate ions (HCO3-). As atmospheric CO2 increases, the ocean continually absorbs greater amounts of CO2, and as temperature increases, CO2 leaks out of the ocean back into the atmosphere. As CO2 is absorbed, carbonate gets used up and must be replaced by stocks from the deeper ocean. Currents bring water with fresh carbonate to the surface and circulate water carrying the captured carbon into the ocean. As SST increases, this circulation process becomes more difficult, and the ocean stratifies. The surface water begins to saturate with CO2, decreasing support for phytoplankton, and photosynthetic CO2 uptake slows. Feely et al. (2009) reported the average pH of the ocean since the industrial era to have decreased by 0.11 pH units (29% acid concentration increase) and predicted a further decrease of 0.3 pH units (150% acid concentration increase) by 2100.
Marine organisms (e.g. coral, bivalves) extract bicarbonate ions from seawater to form skeletons or shells in a process called calcification:
Ca2+ + 2HCO3- → CaCO3 + CO2 + H2O
Calcification is a source of CO2 and is balanced by weathering, a process where rainwater reacts with carbonate rocks and consumes atmospheric CO2 on its way to rivers:
CaCO3 + CO2 + H2O → Ca2+ + 2HCO3-
A decline in the availability of carbonate ions will affect the degree of carbonate polymorph saturation in seawater, therefore compromising marine organisms’ ability to construct their skeletons/shells (Gledhill, Wanninkhof, & Eakin, 2009). A study by Gattuso et al. (2009) examined OA’s ability to affect the calcification of calcareous plankton, realising that any changes experienced by calcified taxa may alter the oceans’ ability to act as a global carbon sink. Calcareous, planktonic foraminifera observations from the Southern Ocean showed a decrease in shell weight compared to older records, with the implication that OA is the causing factor (Moy, Howard, Bray, & Trull, 2009). Suppose OA can change the morphology of a species and consequently affect multiple species up the trophic levels. In that case, the question is whether these species will cope with the effects of acidification through adaptation. This was highlighted in an experiment by Bibby, Cleall-Harding, Rundle, Widdicombe, and Spicer (2007) who observed the effects of acidification on the defence mechanisms of the periwinkle Littorina littorea toward its predator the green shore crab Carcinus meanus. L. littorea can reinforce their calcified shells when experiencing extensive predation pressure, with snails exposed to C. meanus for 15 days producing shells that were 30% thicker than those that were not under predation pressure. At reduced seawater pH, L. littorea could not thicken their shell and compensated by altering their behaviour to avoid the crabs. This shows that adaptation is possible, but at what cost? Behavioural alteration in L. littorea could mean they spend less time feeding, or it could affect their interactions with other species with unknown consequences.
Coccolithophores are unicellular, eukaryotic, calcareous phytoplankton that produces overlapping calcite platelets called coccoliths and are presently one of the main primary producers of the open ocean and significant contributors to carbonate sediments in the deep sea. (Hofmann et al., 2010; Renaud, Ziveri, & Broerse, 2002). Coccolithophores produce coccoliths through the uptake of dissolved inorganic carbon and calcium, and CaCO3 and CO2 are produced. Some of the CO2 released in calcification can be used in photosynthesis (Mackinder, Wheeler, Schroeder, Riebesell, & Brownlee, 2012). It is possible that over long periods, coccolithophores may contribute to decreased levels of atmospheric CO2. Even though one CO2 molecule is released in calcification, one carbon atom becomes trapped within the CaCO3 molecule used to make coccoliths 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, over the long-term coccolithophores provide a sink for CO2 (Mackie, McGraw, & Hunter, 2011). It is currently unknown as to the function of the coccolith, but many theories have been proposed. These include protection from predators or grazing zooplankton (Young, Andruleit, & Probert, 2009), ballasting the cell for vertical migration into deeper, more nutrient-rich water (Raven & Waite, 2004), protection from virus infection (Raven & Waite, 2004), or aiding in the filtering of non-photosynthetically active radiation at the surface to aid in photosynthesis or focussing light to the chloroplasts in deeper water (Raven & Crawfurd, 2012).
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). As the most abundant species, this also means that E. huxleyi is a key organism in the marine pelagic system, where the base of the food web is comprised of over 5,000 species of phytoplankton (Rost & Riebesell, 2004). Among this vast array of species, only a few select taxonomic groups are responsible for most of the pelagic systems primary production, one of these is coccolithophores and therefore reinforces their importance in the oceanic ecosystem. Coccolithophores are sensitive to nitrogen, phosphorus, and silicate ratios in the water, inducing competitive dominance between coccolithophores and other phytoplanktonic communities such as diatoms, microflagellates, and dinoflagellates. Anthropogenic interference with these ratios comes in the form of agricultural runoff leading to eutrophication and increasing nitrogen in seawater, causing coccolithophores to form blooms in these favourable environments and outcompete other species (Yunev et al., 2007). As agriculture and the use of nitrogen-based pesticides increases, more runoff makes its way to the ocean, which could see a tip in the balance of conditions to favour coccolithophore species of phytoplankton. From there, will other phytoplankton species be able to adapt in time, or will they be outcompeted and eventually become extinct?
As OA decreases carbonate saturation in seawater, coccolithophores ability to produce coccoliths may be inhibited as the increase in atmospheric CO2 may affect their calcification mechanisms. When environments change, some organisms adapt to suit their changing environment, so as ocean pH decreases, will coccolithophores have an evolutionary response? Beaufort et al. (2011) found that coccolithophores have channels to pump out H+ ions during calcification to avoid acidosis. Furthermore, a feedback loop is created because when the function of these channels is disrupted, calcification is halted. This study provided evidence that increased oceanic CO2 concentrations decreased coccolith mass as OA impairs the normal function of ion channels and places selection pressure on coccolithophore’s calcification rates (Tyrell, Holligan, & Mobley, 1999). Flynn, Clark, and Wheeler (2016) found that coccolithophores put under OA conditions showed selection for lower calcification rates to avoid the risk of acidosis at higher H+ concentrations. They predicted coccolithophore calcification would decrease by 25% as OA continued and atmospheric CO2 reached 750 ppm (an increase of 335.62 ppm from current atmospheric CO2 levels). Conversely, a long-term study of an E. huxleyi population that was allowed to reproduce for 700 generations under conditions similar to those predicted for the year 2100 showed their population could adapt and increase calcification and CaCO3 content (Benner et al., 2013). This contrasts with other short-term experiments and shows that long-term OA exposure could alter calcification responses in E. huxleyi, and potentially other calcareous phytoplankton as well. Similarly to this experiment, Smith et al. (2012) found naturally forming populations of highly calcified coccolithophores in water with low CaCO3 saturations.
The Paleocene-Eocene Thermal Maximum (PETM) occurred approximately 55.5 million years ago and saw a global temperature increase of 5–8 °C and ~12,000 gigatons of carbon released over 50,000 years (0.24 gigaton per year) (McInherney & Wing, 2011). Today, the anthropogenic release of carbon is equal to 10 gigatons of carbon per year; therefore, it will only take 1,200 years for 12,000 gigatons of carbon to be released as in the PETM. It has been shown that there was no change in coccolithophore distribution attributed to acidification in the PETM (Beaufort et al., 2011; Iglesias-Rodriguez et al., 2008), meaning that they were likely able to adapt over the 50,000 years of exposure to increasing carbon levels. Will 1,200 years be enough time for them to adapt again? Results from studies about the effect OA has on coccolithophores contradict one another. Different experiment environments show different outcomes, including short-term experiments vs long-term experiments, different coccolithophore species, and different regional populations of coccolithophores (Beaufort et al., 2011; Benner et al. 2013; Flynn et al., 2016; Smith et al., 2012). An alternative way to look at the discrepancies seen between experiments is that studies about the effects of OA are often hypothesised to have a negative outcome. If it is looked at that a decreased pH is a more favourable condition to coccolithophores, rather than the alternative, it can explain why highly calcified coccolithophores can be found in conditions of low CaCO3. In this scenario, they would have bioengineered their higher pH environment to a more favourable, lower one through calcification and the release of CO2, creating highly calcified individuals. Once they “created” an environment with favourable pH that now had low concentrations of CaCO3, they would no longer need the CaCO3. In some studies, at low pH environments, coccolith growth appeared to be inhibited. However, perhaps the coccolithophores were satisfied with the pH in their environment and therefore did not need to alter it further through calcification. But, I digress. What are your thoughts on the matter?
Anthropogenically induced climate change has a clear impact on natural, global processes, specifically the ocean’s role in balancing CO2 levels in the atmosphere. Coccolithophores play a part in this role by producing vast amounts of calcite and acting as a carbon sink. The effects of ocean acidification on coccolithophores have been explored through previous studies and experimentation, with multiple conclusions drawn. This makes it difficult to understand how catastrophic the effects of ocean acidification will be in the future as atmospheric CO2 continues to rise, primarily because any changes to coccolithophores will have direct and indirect consequences for other taxa and ecosystem processes. Alternatively, coccolithophores might be environmental bioengineers and use calcification to alter oceanic pH levels to suit their environment. Further research is essential to narrow down coccolithophores precise role in the ecosystem, determine the reason for their coccoliths, discover if they can adapt to a changing environment and how quickly they can do this, and establish the effect ocean acidification has had on them and can potentially have on them in the future, and what will this mean for the world.
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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.
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.
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.
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.
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.
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.
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.
Class Echinoidea, aka echinoids, is composed of sea urchins and sand dollars.
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.
There are around 1,500 species of sea star that make up the class Asteroidea.
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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.
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Wulff, L. (1995). Sponge-feeding by the Caribbean starfish Oreaster reticulatus. Marine Biology, 123(2), 313–325.
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.
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.
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).
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).
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.
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.
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 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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
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 Porifera, better known as the sponges, are an interesting group of animals. Yes, they are animals, albeit they are the most primitive of all multicellular creatures as they were the first to separate from the common ancestor of animals, but animals nonetheless (Feuda et al., 2017; Giribet, 2016). Porifera are the first group of marine invertebrates I will be chatting about over the next few weeks, so move over megafauna cause the little guys are in town!
Porifera is estimated to contain around 15,000 species of sponge, many of which have not even been described yet (Degnan et al., 2015). Sponges don’t exactly have any true tissues or organs; what they do have is a mesohyl (a gelatinous matrix that resembles a type of connective tissue) sandwiched between two thin layers of cells (more on the different types of cells soon). One of the craziest things about sponges is that they are an aggregation of cells in space and time; at that moment, they are part of that sponge, but they could be part of another sponge at a different time. Lavrov and Kosevich (2016) took this to the next level when they mechanically separated sponge cells and saw them reaggregate again before their very eyes!
Respiration, digestion, and excretion
Sponges have no nervous system, no digestive system, no excretory system, and no circulatory system, so how are they even… alive??? Sponges are sessile and are therefore attached to a substrate via their pinacocytes (contractile cells that line the outer wall). By orienting themselves perpendicular to the water flow to create low pressure at the excurrent opening (osculum), they then open and close incurrent pores (ostia) to regulate water flow; up to 20,000 times the volume of the sponge can be filtered through the sponge in 24 hours and up to 90% of the bacteria in the water may be filtered out. The structure of the sponge maximises the efficiency of water flowing into the ostia through the central cavity, where respiration and digestion occur (Hutchings, Kingsford, & Hoegh-Guldberg, 2019). As water flows through the body, cells absorb oxygen by diffusion and dump waste products into the outgoing current. This water flow also delivers food particles to the sponge. If the food is larger (>50μm), it cannot enter the ostia, so pinacocytes (remember these from earlier?) grab ‘em and digest ‘em. Usually, though, food particles are <0.5μm, so they can easily pass through the ostia to where the choanocytes are waiting. Choanocytes have a flagellum that beats, creating a unidirectional flow of water, drawing in food particles. Choanocytes have a collar of microvilli which filters nutrients from the water; the choanocytes then store the nutrients in vacuoles of adjacent cells, usually amoebocytes which distribute nutrients around the sponge. Some sponges, like Clarohizdae, are carnivorous and will passively capture small invertebrates via their sticky surface, where cells will migrate to and envelop the prey (Hestetun, Tompkins-Macdonald, & Rapp, 2017).
A peculiar characteristic of Porifera is their spicules, structures made from either calcium carbonate (CaCO3) or silica that vary in size and shape from rods to three-dimensional stars (Renard et al., 2013). They are held in place by collagen fibres (one of the places where “marine collagen” comes from) and produced in the mesohyl by sclerocyte cells. Spicules are thought to be a deterrent from predators or to provide skeletal structure or support.
These are calcareous sponges with CaCO3 spicules.
These are the glass sponges that mainly inhabit deep water. They have siliceous spicules that form stable lattices. You may remember these sponges from David Attenborough’s Blue Planet II, a romantic story of 2 shrimp larvae, male and female, that get swept into a Hexactinellid sponge and grow too large to be able to escape; a twist of fate that leaves them stuck together forever… awww.
Some Demospongiae don’t have spicules, but if they do, they are siliceous spicules that are held together by collagen.
Degnan, B. M., Adamska, M., Richards, G. S., Larroux, C., Leininger, S., Bergum, B., … & Degnan, S. M. (2015). Porifera. In Evolutionary developmental biology of invertebrates 1 (pp. 65–106). Springer, Vienna.
Feuda, R., Dohrmann, M., Pett, W., Philippe, H., Rota-Stabelli, O., Lartillot, N., … & Pisani, D. (2017). Improved modeling of compositional heterogeneity supports sponges as sister to all other animals. Current Biology, 27(24), 3864-3870.
Giribet, G. (2016). Genomics and the animal tree of life: conflicts and future prospects. Zoologica Scripta, 45, 14–21.
Hestetun, J. T., Tompkins-Macdonald, G., & Rapp, H. T. (2017). A review of carnivorous sponges (Porifera: Cladorhizidae) from the Boreal North Atlantic and Arctic. Zoological Journal of the Linnean Society, 181(1), 1–69.
Hutchings, P., Kingsford, M., & Hoegh-Guldberg, O. (Eds.). (2019). The Great Barrier Reef: biology, environment and management. Csiro publishing.
Lavrov, A. I., & Kosevich, I. A. (2016). Sponge cell reaggregation: Cellular structure and morphogenetic potencies of multicellular aggregates. Journal of Experimental Zoology Part A: Ecological Genetics and Physiology, 325(2), 158–177.
Renard, E., Gazave, E., Fierro‐Constain, L., Schenkelaars, Q., Ereskovsky, A., Vacelet, J., & Borchiellini, C. (2013). Porifera (sponges): recent knowledge and new perspectives. eLS.
The shortfin mako shark (Isurus oxyrinchus) is a pelagic, cartilaginous fish with a wide distribution range that covers most oceans and undergoes migrations that can be as large as 5,300km in just under 1.5 years (Barreto, de Farias, Andrade, Santana, & Lessa, 2016; Kohler, Turner, Hoey, Natanson, & Briggs, 2002). There is high demand for mako shark meat, and it is a prize game species in recreational fishing worldwide (Barreto et al., 2016).
As of 2019, I. oxyrinchus is classified as “Endangered” on the International Union for the Conservation of Nature’s (IUCN) Red List (Rigby et al., n.d.). Rigby et al. (n.d.) concluded that shortfin mako’s population trend is decreasing; there is an estimated decline everywhere except in the South Pacific and an overall estimated average reduction of 46.6% over 72-75 years. There is an estimated decline in biomass and abundance of 99.9% since the early 1800s, the main reason being overfishing (Ferretti et al., 2008).
Shortfin mako circulatory systems utilise a heat-exchanging technique that raises their internal temperature above that of the surrounding environment (Carey, Teal, & Kanwisher, 1981; Kohler et al., 2002). They have streamlined bodies, and aerobic muscles centred closer to their rear, which aids in thunniform swimming and increases power (Donley, Sepulveda, Konstantinidis, Gemballa, & Shadwick, 2004; Wegner, Sepulveda, Olson, Hyndman, & Graham, 2010). Emery and Szczepanski (1986) concluded that the gill area of I. oxyrinchus is 2-3 times larger than other pelagic shark species, which could aid in the mako’s speed, agility, and ability to swim long distances. Shortfin mako habitat extends globally in tropical and temperate oceans and they can be found inshore in coastal areas or at least 500m down in oceanic zones (Kohler et al., 2002).
Sexual dimorphism is prevalent in I. oxyrinchus,with females often occurring larger than males, with males reaching a maximum size of 2.6m and females reaching a maximum size of 3.4m (Barreto et al., 2016; Cema & Lincandeo, 2009; Chan, 2001; Doño, Montealegre-Quijano, Domingo, & Kinas, 2014; Hsu, 2001; Natanson et al., 2006; Semba, Nakana, & Aoki, 2009). Shortfin mako sharks are oophagous and ovoviviparous (Kohler et al., 2002), and a study by Mollet et al. (2000) predicts a gestation period of 15-18 months, although a study by Duffy and Francis (2001) puts makos in New Zealand waters at a 21-month gestation period. They have a 3-year reproductive cycle (Mollet & Cailliet, 2002). Bishop, Francis, Duffy, and Montgomery (2006) concluded that New Zealand shortfin mako births are concentrated in spring and gave a theoretical birth date of 1 October, with the average length of the shark at birth to be 61cm. Francis and Duffy’s (2005) study on sexual maturity of New Zealand shortfin makos concluded that maturity occurs between 7-9 years for males and 19-21 years for females (Bishop et al., 2006). Bishop et al. (2006) found evidence of New Zealand shortfin makos living to 29 years, although this number is probably higher because there is a lower chance of catching older sharks which make up a small percentage of the overall population. The same study found the sharks grow quickly within their first year after birth; this growth rate rapidly reduces in the subsequent years to steadier growth. Late maturity, moderately long longevity, the estimated low natural mortality rate, and low annual fecundity causes low productivity in the species (Bishop et al., 2006; Mollet et al., 2000).
Stillwell and Kohler (1982) analysed the stomach contents of shortfin mako sharks and found evidence of bony fish, swordfish, and cephalopods. As shark body length increased, so did the average volume of food, indicating that as makos grow larger, they may switch to larger prey items (Kohler et al., 2002).
Sharks have evolved for 400 million years (Donley et al., 2004), leading species such as I. oxyrinchus to be apex predators at the top of their food chain; they have no natural predators which results in low natural mortality. There is evidence of parasitic activity in shortfin makos, although it could not be determined if there was a negative effect on the shark (Borucinska & Hege, 1999). One example of anthropogenically influenced mortality is seen in 1966 when a longfin mako (Isurus paucus) died from a fishing hook retained in its flesh (Adams, Borucinska, Maillett, Whitburn, & Sander, 2015).
Sharks are keystone species and have a strong effect on multiple ecosystems due to their predatory role and wide dispersal range (Feretti, Worm, Britten, Heithaus, & Lotze, 2010). I. oxyrinchus is considered to be a large shark with “strong, top-down forces” (Feretti et al., 2010, p1055). Therefore, their removal from an ecosystem is highly likely to drastically alter communities, induce trophic cascades, release mesopredators such as smaller sharks and rays, and consequently cause a decline in commercial fish stocks (Fig. 1).
There is a high commercial demand for shortfin mako shark meat, and the species is exploited globally (Barreto et al., 2016). Peru is one of the biggest shark-fishing nations, and I. oxyrinchus is one of the top 2 most caught species with a rarely enforced catch size limit (Gonzalez-Pestana, Kouri, & Velez-Zuazo, 2016; Fischer, Erikstein, D’Offay, Guggisberg, & Barone, 2012). Brazilian fleets in the western and central South Atlantic exploit immature shortfin makos, specifically females (Barreto, 2016); it was rare for them to catch individuals greater than 2m. On a global scale, it is both a target and bycatch species in commercial and small-scale fisheries, including longline, gillnet, purse seine, trammel net, and trawls (Camhi, Pikitch, & Babcock, 2008; Rigby et al., 2019). There is likely underreporting of catch, and commercial post-release mortality from longlines alone is reported at 30-33% (Campana, Joyce, Fowler, & Showell, 2016; Rigby et al., 2019). Shortfin mako fins made up 1.2% of shark fin imported to Hong Kong in 2014 (Fields et al., 2017), and their skin, jaws, and liver oil are also used (Compagno, 2001). In New Zealand’s EEZ, I. oxyrinchus is a common bycatch species on tuna longlines and less commonly on pelagic longlines, trawls, and set nets (Bagley et al., 2000; Francis, 1998; Francis, Griggs, Baird, Murray, & Dean, 2000; Martinsohn & Muller, 1992). Since 1993, commercial catch averaged 60 tonnes annually, but observer reporting estimated 100-200 tonnes per year from the longline tuna fishery alone (Francis, 1998; Francis et al. 2000); the discrepancy between these numbers could be related to lack of accurate recording. From 1988-2015, the New Zealand tuna longline fishery total catch was comprised of 11.1% target species (southern bluefin tuna, bigeye tuna, and swordfish) and 88.9% bycatch (including albacore tuna, lancetfish, porbeagle shark, deepwater dogfish, dealfish, mako shark, moonfish, escolar, sunfish, and butterfly tuna) according to a report by Fisheries New Zealand [FNZ] (2018). Since October 2014, shark finning is illegal in New Zealand, yet over half of all makos caught by charter vessels in the 2014-15 tuna longline fishery year were kept for their flesh (FNZ, 2018). The low productivity of mako sharks causes them to be vulnerable to overfishing and makes it hard for the species to replace lost individuals. CITES (2019) reported that shortfin mako is in danger of population collapse due to the overfishing of most of the juveniles since the 1980s; this means that sharks dying of old age now will not be replaced with mature individuals leaving a 10-20-year gap.
The mining of precious metals increases the presence of toxic contaminants in the environment, especially in coastal environments, specifically mercury (Maz-Courrau et al., 2011). Sharks bioaccumulate mercury through their tissues and organs from the environment and through the food they eat; mercury travels up the trophic web and magnifies at each level, eventually accumulating in shark muscular tissue (Maz-Courrau et al., 2011). Maz-Courrau et al. (2011) found that 33% of shortfin mako sharks they studied off the Baja California coast had levels of mercury higher than was estimated to be fit for human consumption by Mexican Law and Watling et al. (1981) found mercury concentrations in mako sharks off the coast of Australia to be almost twice that. As Maz-Corrau et al. (2011) only sampled juveniles, it is likely that as sharks grow and eat more, more mercury is accumulated into their tissue over time. When people eat this contaminated tissue, it bioaccumulates up the food chain again and causes adverse health effects in humans.
Current/Proposed Management Actions
As of 2019, I. oxyrinchus officially met criteria to be listed on the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) Appendix II, which “includes species not necessarily threatened with extinction, but in which trade must be controlled in order to avoid utilisation incompatible with their survival” (CITES, 2019, p. 2).
Pressure from overfishing appears to be the most significant contributor to shortfin mako shark decline globally. Due to the widespread distribution of I. oxyrinchus and migratory nature, conservation effort and management is needed at both a local and international scale (Adams, Flores, Flores, Aarestrup, & Svendsen, 2016; Corrigan et al., 2018). Because of their “Endangered” status on the with a decreasing population trend, it is essential to increase their population trend and bring them to the next highest, more stable level on the Red List, “Vulnerable”, and take measures to ensure they do not go the other direction and become classified as “Critically Endangered”. The most effective way this can be achieved is through strict management of fisheries and the implementation of Marine Protected Areas (MPA) in habitats that are suitable for shortfin makos by means of international treaties (Birkmanis, Partridge, Simmons, Heupel, & Sequeira, 2020; Dulvy et al., 2008; Rigby et al., 2019).
Community Education & Awareness
Films, documentaries, and TV series are a great way of moving an audience and opening their eyes to what is going on in the world around them in an entertaining way. Saving Jaws by Ocean Ramsey, Shark Week by The Discovery Channel, and BBC’s Blue Planet, narrated by David Attenborough, are a few.
For the booklovers, don’t worry, we got you. Ocean Ramsey’s What You Should Know About Sharks: Shark Language, social behavior, human inter-actions, and life saving information educates readers on shark behaviours, how to swim with them safely, and debunks common shark myths. Sylvia A. Earle’s book The World is Blue: How Our Fate and the Ocean’s are One shows readers how every one of us literally breathes the ocean, and her other book, Sea Change: A Message of the Oceans, has been compared to Rachel Carson’s Silent Spring. For those who do not enjoy reading, Christian Vizl’s Silent Kingdom is a photography-based book that shows the raw beauty, power, and elegance of marine creatures, such as sharks, in stunning black and white photos.
NGO’s also play an essential role in community education and awareness. In New Zealand, groups such as Auckland Whale & Dolphin Safari and Whale Watch Kaikōura take their customers to visit sea life up close. SEA LIFE Kelly Tarlton’s Aquarium educates both adults and children alike and cares for sick and dying turtles.
Monitoring & Research
There is enough research on I. oxyrinchus populations to know something has to change, but what lacks is monitoring the factors contributing to their dismal fate. As of 2019, CITES began to monitor the commercial catch and trade of shortfin mako but agreed that it might not be enough (CITES, 2019). In 2013, MPI released a National Plan of Action for the Conservation and Management of Sharks (NPOA). In the plan, they stated one of their objectives was to “systematically review management categories and protection status to ensure they are appropriate to the status of individual shark species” (MPI, 2013, p. 19) among others, yet there is a lack of reporting available for public access to back this up. They also had intentions of issuing a revised NPOA in 2018, which seems not to exist altogether.
Bycatch records need to be scrutinised and fisheries must be forced to show accurate record-keeping. Data collected from accurate bycatch records of I. oxyrinchus can then be accumulated around the globe to highlight the fisheries that are exploiting shortfin makos.
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My name is Emily Jupp. I’m a studying Marine Biologist and Geospatial Scientist, and I’ve got some things I’d like to say. So, grab yourself a hot bevvy and buckle up because it’s all underwater from here.
There’s a plethora of information out there, and some of that information is about the ocean. The only real way to figure out the truth about something is to do your own research, which can be really hard when there’s so much research material to choose from. That’s where I come in. I hope to provide you with as much reliable, honest, and up-to-date oceanic information as possible, including references that will help you with your own study.
You don’t need to be a scientist to read about science; all you need is an inquisitive mind and a childlike curiosity about the world that surrounds you.