Lab notebook: the great hammerhead (Sphyrna mokarran)

Lab 1

Specific objectives:

  • Document the external anatomy of the specimen
  • Identify the specimen using taxonomic features

The great hammerhead (Sphyrna mokarran) is from the family Sphyrnidae of the “ground sharks” or Carcharhiniformes [1] (Fig. 1).

Figure 1 External morphology of the great hammerhead shark, (Sphyrna mokarran)

The sheer size of S. mokarran hints at its predatory nature. Its serrated, pointed teeth are excellent for tearing apart flesh from prey like fish and rays, or perhaps crushing shells of crustaceans too [2], [3]. The ampullae of Lorenzini, located on the underside of its snout, are electroreceptors in the form of a network of mucus-filled pores helping in prey detection [4]. Its head’s unique “hammer” shape is called a cephalofoil, a unique evolution to enhance the shark’s vision. The position of the eyes on the ends of the cephalofoil allow the shark to always see above and below itself [5]. The cephalofoil also increases surface area for distribution of the ampullae of Lorenzini, thereby allowing the shark to locate prey more effectively [6]. The cephalofoil acts as a lifting surface, similar to the hydrofoil of a boat, and assists with sharp turns to attack prey [3], [7]. The two nares under the snout act as nostrils where water can pass through, allowing the shark to “smell” and assist with prey detection. The shape of the head means the nares are located further apart which may enhance olfaction.

The two dorsal, pelvic, and anal fins act as vertical stabilisers (in males, the pelvic fin is modified into a reproductive organ known as a clasper). The two pectoral fins help to steer while swimming and, like a plane, provide lift in the water. The caudal fin provides thrust, and the caudal fin of S. mokarran has a smaller lower lobe than the upper lobe, indicating that it may spend much time swimming close to the seabed and that speed is not as essential for this species. S. mokarran likely has carangiform or thunniform propulsion because of the large, muscular tail.

The colouration of S. mokarran is known as countershading, whereby the top surface is darker than the lighter underside. This means that the shark is camouflaged from prey above it (looking down to the deep, dark ocean) and from prey below it (looking up at the light, sunlight, surface waters). This will help the shark ambush its prey effectively and, for younger/smaller individuals, avoid detection from carnivores looking for a hammerhead-shaped snack. Its skin is comprised of dermal denticles: rough, teeth-like scales that would help reduce swimming-induced drag and protect them from predators and parasites. The lateral line extends the length of the shark sides and helps orient the shark to movement and sound. As a sensory organ, it works in conjunction with the ampullae of Lorenzini to assist in prey detection.

Lab 2

Specific objective:

  • Describe the functions of its organs as it relates to the ecology/biology of the specimen

Sharks lack an operculum and instead have gill slits that act as openings to the gills [8], allowing seawater to flow over them and oxygen to be extracted for respiration. From viewing photographs of S. mokarran, it does not appear to have a spiracle, like other sharks do (except requiem sharks). The spiracle is a small hole behind the eye that opens to the buccal cavity, assisting sharks to take in water over their gills whilst stationary, suitable for sharks that spend much time on the seafloor [9]. The lack of a spiracle coincides with the pelagic nature of S. mokarran and infers that they must be ram ventilators who must continuously swim forward to encourage water flow over the gill filaments through the mouth or gill slits [9].

Elasmobranchs are unique in that the morphological structure of their jaw means that it is suspended by a musculoskeletal sling [10]. S. mokarran are even more unique in that their head is dorsoventrally compressed and laterally expanded to form the cephalofoil. Because the evolution of the cephalofoil is such a drastic, morphological difference among Carcharhiniforms, trade-offs have been made to the pharyngeal apparatus (as well as other systems that occupy the head) [11]. Additionally, the teeth are triangular and serrated (Fig. 2).

One observation [7] of S. mokarran predation on a stingray saw the shark use its cephalofoil to pin the ray to the seafloor on two occasions, each time taking a bite from the wings of the ray, rendering it incapacitated. The hammerhead was then able to consume the immobile ray easily.

Figure 2 Teeth in dried jaws of great hammerhead shark, Sphyrna mokarran. Doug Perrine. (2013). received from on 29 September 2021.

There is limited research on S. mokarran digestion specifically, so it is helpful to infer similar characteristics from other species with a similar diet. The digestive tract begins at the mouth, where a strong jaw and sharp, replaceable teeth can rip flesh or crush prey into swallowable sizes. This is where the first type of digestion takes place – mechanical.

Food travels through the oesophagus from the mouth, where striated muscles and secreted mucous assist food into the stomach [12]. The stomach of most sharks is J-shaped [13], although there are some exceptions. The bonnethead shark (Sphyrna tiburo) exhibits a straight (I-shaped) stomach [14]. Because S. tiburo is from the same genus as S. mokarran, it may seem wise to assume that S. mokarran has an I-shaped stomach too, but S. tiburo is omnivorous and digests seagrass, whereas S. mokarran is strictly carnivorous. Like other vertebrates, the cells on the stomach walls of an elasmobranch secrete mucous to protect it from the acidic gastric juices that biochemically digest food stored in the stomach [12]. Some sharks are known to undergo gastric evacuation, whereby undigestible contents of the stomach are regurgitated out of the mouth [11], [12], although it is unknown is S. mokarran can do this. Furthermore, some elasmobranch species can regulate gastric acid secretion, likely in times of fasting due to low prey availability [15].

Nutrients are transported to the intestine from the stomach, which is relatively shorter than most vertebrate intestines. Leigh et al. (2017) recommend separating elasmobranch intestines into three sections: proximal, spiral, and distal. The proximal region gives way to the spiral region or spiral valve. The anatomy of the spiral valve varies among species but can have between 2–50 turns and is thought to increase surface area for absorption of nutrients and/or slow the rate at which food travels through the intestine, therefore, increasing time available for digestion [14], [16]. Furthermore, the spiral valve ensures that oversized items (e.g., bones) cannot pass through their lower intestine and allows them to be sufficiently broken down first or regurgitated.

After the spiral valve is the distal intestine, characterised by thicker and more muscular walls to accommodate the accumulation of faeces [12]. As pressure increases on the rectal walls, nerve impulses are sent to the brain for muscles to relax, and faeces are passed through the cloaca, which serves as the anus as well as the genitals and urinary duct [12]. Another species from the same genus as S. mokarran, the scalloped hammerhead (Sphyrna lewini), demonstrates an increased gastric evacuation rate with increased meal size [17], so this may be the case for S. mokarran as well. The amount of speculation I’m having to undergo for S. mokarran based off closely related species is evidence that S. mokarran needs more research and investigation to better understand it.

The small, relative size of the elasmobranch digestive tract may make room for the large liver. Baldridge (1970) found that the liver accounted for 3.83% and 9.5% of total body weight in two S. mokarran individuals (imagine a 100kg man having a 3.8 kg or 9.5 kg liver!!). Elasmobranchs do not have swim bladders like bony fish and instead rely on the liver, which is saturated in oil, to maintain buoyancy [19].  The liver contains lightweight oils, increasing its buoyancy and, along with its fins, gives it the lift it needs to prevent sinking.

Overall, not much is known about the reproductive biology of S. mokarran. Male specimens of S. mokarran have two claspers inside each pelvic fin [20] that deposit sperm into the female’s cloaca [21]. Based on evidence from other shark species, it is apparent that females can store sperm in their shell gland for up to 16 months [21]. S. mokarran are viviparous (birthing live young), and the ova has a yolk sac that, once depleted, turns into a structure similar to a placenta [1], [21]. They usually litter between 6–42 pups after a gestation of ~11 months [1], although one female was known to have littered a record 55 pups [22].

Lab 3

Specific objective:

  • Provide information about the age and growth of the specimen

The maximum reported age for S. mokarran is 30 years [1], although one specimen was estimated to be around 40–50 years old [22].

S. mokarran are the largest hammerhead species, and a male can reach a maximum total length (TL) of up to 610 cm, although typically, they will average 370 cm TL [1]. Due to their lack of otoliths, cartilaginous fish are aged by counting banding patterns on their sagittal vertebrae (Fig. 3). The von Bertalanffy growth function (VBGF) (Fig. 4) shows that, at birth, S. mokarran is around 50-70 cm.

Figure 3 Taken from [23]: “Sagittal vertebral section from a 4-year-old great hammerhead, Sphyrna mokarran, illustrating the banding pattern and annuli used to assign age. Scale bar = 2 mm.”
Figure 4 Taken from [23]: “The best fit von Bertalanffy growth model for male and female great hammerhead sharks, Sphyrna mokarran, collected in the northwestern Atlantic Ocean and the eastern Gulf of Mexico.”

The VBGF (Fig. 4) shows that males grow faster than females but reach a smaller asymptotic size than females. This growth is likely due to different energy requirements for the sexes for somatic growth and reproductive development [23].

S. mokarran reaches sexual maturity between 210–300 cm total length, with males tending to reach maturity at a smaller size than females [1].

Lab 4

Specific objective:

  • Provide information about the reproductive dynamics and life history of your chosen specimen

The embryonic sex ratio of S. mokarran is close to 1:1 [24]. It is gonochoric, with no interesting or outstanding sexual dynamics to note [1].

Rigby et al. (2019) describe S. mokarran as aplacental viviparous, whereas Froese and Pauly (2021) describe the species as viviparous with a yolk-sac placenta. Either way, they birth live young that have hatched from an egg in-utero.

There is not much research into the spawning behaviour of S. mokarran, which is perhaps reflective of their naturally elusive lifestyle. Stevens and Lyle (1989) note mating scars on females, which indicates, like many shark species, the courtship process can be seemingly violent with the male holding onto the females with his teeth during copulation.

S. lewini were observed in a large group off the Galapagos Islands and were thought to be amidst a courtship ritual where the largest females dominated the centre of the group, and the males attempted to access them to mate [26]. This could provide insight into the mating rituals of S. mokarran, although S. mokarran populations are substantially smaller than S. lewini, and they have yet to be observed in such large numbers.

One account of mating S. mokarran in the Bahamas reported two individuals ascending in 21m of water as they spiraled around one another and copulated at the surface [27].

S. mokarran birth between 6–42 pups every two years [25]. Their parental mode is not well researched. Many elasmobranchs offer no maternal care once the pup is born, so it can be assumed that this is the same for S. mokarran. However, a study on the scalloped hammerhead (S. lewini) and the Carolina hammerhead (S. gilberti) illustrated that neonatal hammerheads are likely to rely on maternal provisioning in the first few weeks after birth [28]. Therefore, an increased maternal investment may be a part of the life history strategy of S. mokarran. Again, further research is crucial to understand their reproductive and life histories further.

There is regional variation in the size and age range of S. mokarran sexual maturity. As before mentioned, this species reaches sexual maturity between 210–300 cm total length, with males maturing from 225–269 cm and females maturing from 210–300 cm [1], [25]. Age-at-maturity for females is estimated to be 5.5–8.3 years in Atlantic and Pacific populations [25], [29].

S. mokarran eggs hatch in-utero and embryonic individuals spend 11 months in their mother’s uterus, and newborns are around 50–70 cm total length and are then known as pups [25], [29]. At birth, they resemble S. mokarran adults in external morphology (Fig. 5). They grow rapidly until ten years of age, where their growth rate reduces [23], likely because they have reached sexual maturity and fitness rather than size becomes more critical for courtship and survival.

Figure 5 Neonatal great hammerhead, Sphyrna mokarran, pups. By Apex Predators Program, NOAA/NEFSC –, Public Domain,

[1]      R. Froese and D. Pauly, “Sphyrna mokarran,” Fishbase, 2021. (accessed Sep. 21, 2021).

[2]      V. Raoult, M. K. Broadhurst, V. M. Peddemors, J. E. Williamson, and T. F. Gaston, “Resource use of great hammerhead sharks (Sphyrna mokarran) off eastern Australia,” J. Fish Biol., vol. 95, no. 6, pp. 1430–1440, 2019, doi: 10.1111/jfb.14160.

[3]      D. D. Chapman and S. H. Gruber, “A further observation of the prey-handling behavior of the great hammerhead shark, Sphyrna mokarran: Predation upon the spotted eagle ray, Aetobatus narinari,” Bull. Mar. Sci., vol. 70, no. 3, pp. 947–952, 2002.

[4]      E. E. Josberger et al., “Proton conductivity in ampullae of Lorenzini jelly,” Sci. Adv., vol. 2, no. 5, pp. 1–7, 2016, doi: 10.1126/sciadv.1600112.

[5]      K. R. Mara, “Evolution of the Hammerhead Cephalofoil: Shape Change, Space Utilization, and Feeding Biomechanics in Hammerhead Sharks (Sphyrnidae),” University of South Florida, 2010.

[6]      S. M. Kajiura, J. B. Forni, and A. P. Summers, “Olfactory morphology of carcharhinid and sphyrnid sharks: Does the cephalofoil confer a sensory advantage?,” J. Morphol., vol. 264, no. 3, pp. 253–263, 2005, doi: 10.1002/jmor.10208.

[7]      W. R. Strong, F. F. Snelson, and S. H. Gruber, “Hammerhead Shark Predation on Stingrays: An Observation of Prey Handling by Sphyrna mokarran,” Copeia, vol. 1990, no. 3, p. 836, 1990, doi: 10.2307/1446449.

[8]      W. J. Vanderwright, J. S. Bigman, C. F. Elcombe, and N. K. Dulvy, “Gill slits provide a window into the respiratory physiology of sharks,” Conserv. Physiol., vol. 8, no. 1, pp. 1–10, 2020, doi: 10.1093/conphys/coaa102.

[9]      J. L. Dolce and C. D. Wilga, “Evolutionary and Ecological Relationships of Gill Slit Morphology in Extant Sharks,” Bull. Museum Comp. Zool., vol. 161, no. 3, p. 79, 2013, doi: 10.3099/mcz2.1.

[10]    P. J. Motta, “Prey Capture Behavior and Feeding Mechanics of Elasmobranchs,” in Biology of Sharks and Their Relatives, J. C. Carrier, J. A. Musick, and M. R. Heithaus, Eds. Boca Raton: CRC Press, 2004, pp. 165–202.

[11]    J. M. Brunnschweiler, P. L. R. Andrews, E. J. Southall, M. Pickering, and D. W. Sims, “Rapid voluntary stomach eversion in a free-living shark,” J. Mar. Biol. Assoc. United Kingdom, vol. 85, no. 5, pp. 1141–1144, 2005, doi: 10.1017/S0025315405012208.

[12]    S. C. Leigh, Y. Papastamatiou, and D. P. German, “The nutritional physiology of sharks,” Rev. Fish Biol. Fish., vol. 27, no. 3, pp. 561–585, 2017, doi: 10.1007/s11160-017-9481-2.

[13]    W. C. Hamlett, Sharks, skates, and rays: the biology of shark fishes. Baltimore: The Johns Hopkins University Press, 1999.

[14]    P. Jhaveri, Y. P. Papastamatiou, and D. P. German, “Comparative Biochemistry and Physiology , Part A Digestive enzyme activities in the guts of bonnethead sharks ( Sphyrna tiburo ) provide insight into their digestive strategy and evidence for microbial digestion in their hindguts,” Comp. Biochem. Physiol. Part A, vol. 189, pp. 76–83, 2015, doi: 10.1016/j.cbpa.2015.07.013.

[15]    R. D. Day, I. R. Tibbetts, and S. M. Secor, “Physiological responses to short-term fasting among herbivorous, omnivorous, and carnivorous fishes,” J. Comp. Physiol. B Biochem. Syst. Environ. Physiol., vol. 184, no. 4, pp. 497–512, 2014, doi: 10.1007/s00360-014-0813-4.

[16]    C. Bucking, “Feeding and Digestion in Elasmobranchs: Tying Diet and Physiology Together,” in Fish Physiology, vol. 34, Academic Press, 2015, pp. 347–394.

[17]    A. Bush and K. Holland, “Food limitation in a nursery area estimates of daily ration in juvenile scalloped hammerheads,” J. Exp. Biol. Ecol., vol. 278, pp. 157–178, 2002.

[18]    H. D. Baldridge, “Sinking Factors and Average Densities of Florida Sharks as Functions of Liver Buoyancy Published by : American Society of Ichthyologists and Herpetologists ( ASIH ) Stable URL : REFERENCES Linked references are availabl,” Copeia, vol. 1970, no. 4, pp. 744–754, 1970.

[19]    M. Aidan, “Does Liver Size Limit Shark Body Size?,” Biology of Sharks and Rays, 2021. (accessed Sep. 27, 2021).

[20]    M. Aidan, “Why Do Sharks Have Two Penises?,” Biology of Sharks and Rays, 2021. (accessed Sep. 27, 2021).

[21]    M. Aidan, “From Here to Maternity,” Biology of Sharks and Rays, 2021. (accessed Sep. 27, 2021).

[22]    “Record Hammerhead Pregnant With 55 Pups,” Discovery Channel, 2006. (accessed Sep. 27, 2021).

[23]    A. N. Piercy, J. K. Carlson, and M. S. Passerotti, “Age and growth of the great hammerhead shark, Sphyrna mokarran, in the north-western Atlantic Ocean and Gulf of Mexico,” Mar. Freshw. Res., vol. 61, no. 9, pp. 992–998, 2010, doi: 10.1071/MF09227.

[24]    J. D. Stevens and J. M. Lyle, “Biology of three hammerhead sharks (Eusphyra blochii, sphyrna mokarran and s. lewini) from northern australia,” Mar. Freshw. Res., vol. 40, no. 2, pp. 46–129, 1989, doi: 10.1071/MF9890129.

[25]    C. L. Rigby et al., “Sphyrna mokarran, Great Hammerhead,” IUCN Red List Threat. Species, vol. e.T39386A2, p. 16, 2019, [Online]. Available:

[26]    BBC Earth, “Hammerhead Sharks’ Complex Mating Rituals | BBC Earth,” 2019. (accessed Oct. 06, 2021).

[27]    “Great hammerhead shark – Sphyrna mokarran,” Shark Research Institute, 2021. (accessed Sep. 27, 2021).

[28]    K. Lyons et al., “Maternal provisioning gives young-of-the-year Hammerheads a head start in early life,” Mar. Biol., vol. 167, no. 11, pp. 1–13, 2020, doi: 10.1007/s00227-020-03766-y.

[29]    H. H. Hsu et al., “Biological aspects of juvenile great hammerhead sharks Sphyrna mokarran from the Arabian Gulf,” Mar. Freshw. Res., vol. 72, no. 1, pp. 110–117, 2020, doi: 10.1071/MF19368.


The species-area relationship and Bergmann’s rule: attempts to explain global patterns in biodiversity


Humans have noticed the differences between species worldwide from the time they evolved up until the present, with the term “biodiversity” first being used by Tangley (1985). A natural progression from these discoveries of global biodiversity was to explain them using such rules as the species-area relationship (SAR) and Bergmann’s rule. SARs describe the link between the area of a habitat and the number of species found within that area (Losos & Schulter, 2000; Preston, 1962). Bergmann’s rule states that species of larger size are found in colder regions, and species of smaller size are found in warmer regions (Ashton, Tracey, & de Queiroz, 2000; Mayr, 1956). Both rules are utilised in the field of biogeography, and I will be looking at evidence that both supports and opposes SARs and Bergmann’s rule to critique the validity of each.

The SAR model has ideas such as island biogeography theory (IBT) and spatial scales and studies that support it as an ecological law (Franzén, Scweiger, & Betzholtz, 2012; Losos, 1996; Losos & Schluter, 2000; Rand, 1969). IBT states that larger islands have greater numbers of species than smaller islands (MacArthur & Wilson, 2001; MacGuinness, 1984) and spatial scales keeps each SAR study relevant to the size at which the study was undertaken (Drakare, Lennon, & Hillebrand, 2006; Turner, 2005; Tylianakis, Klein, Lozada, & Tscharntke, 2006). Criticism of IBT says that its equilibrium theory is too simplistic, which raises questions about it being successfully used to support SARs (Heaney, 2000; Lomolino, 2000; Tangney, Wilson, & Mark, 1990; Toft & Schoener, 1983; Webb & Vermaat, 1990; Wu, 1995). A lack of real-life SAR studies poses the question: is SARs more valuable as a tool for biogeography rather than an ecological rule (Diamond, 1984; Zimmerman, 2000)? 

Bergmann’s rule is supported by real-life studies that not only expressed the rule but followed a logical reason as to why they adhered to it (Ashton, Tracy, & de Queiroz, 2000; Fujita, 1986; Gigliotti et al., 2019; James, 1983; Rhymer, 1992; Sand, Cederlund, & Danell, 1995; Weaver & Ingram, 1969). Conversely, there are some exceptions to Bergmann’s rule that also carry their own rational explanation (Gigliotti et al., 2019; Oishi, 2010; Takeuchi, 1995; Kaneko, 1988). Geist (1987, 1990) disagrees with Bergmann’s rule and pointed out that not only is body mass a poor conserver of heat, but that body size is also due to a multitude of different factors. 

Species-area relationship

Supportive evidence for species-area relationship

SARs depict the number of species found in an area of habitat and are plotted on a graph to show a species-area curve where x = habitat area and y = species richness (Rosenzweig, 1995). The typical outcome of plotting SARs on a graph is a sharp increase in species richness over the initial area that begins to taper off as distance increases; it shows that, over a habitat area, species richness increases. Franzén et al. (2012) regarded SARs as an ecological tool used to make predictions for areas that have little or no data behind them. This means SAR predictions can be used as a powerful instrument in conservation for remote habitats or when there is little time to collect data in the field. 

The theory of island biogeography was coined by MacArthur and Wilson in 1967, and it states that species richness increases with island size and decreases with island distance from the mainland (MacArthur & Wilson, 2001). These islands receive new colonising species from the mainland, and, eventually, established populations of species become extinct; this creates an equilibrium point where immigration equals extinction. A study by Riebesell (1982) found that the species richness of plants on 13 peaks in the Adirondack Mountains is due to the area of the alpine ecosystem and immigration between summits. This reflects IBT as the plant seeds have been spread by birds and dropped on mountaintops (more seeds dropped on a bigger area). Extinction was evident because the plants on the island were not genetically identified as relict populations, meaning that an equilibrium has been met between seed dispersal (immigration) and plants dying (extinction). 

Spatial scales are essential in verifying the use of SARs in the context of the study being undertaken (C. Bishop, personal communications, May 7, 2020). This amplifies SAR effectiveness because it gives a guideline as to how SARs should be studied and therefore reduces the likelihood of error; when spatial scales are considered during SAR research, it increases the integrity of the research. Turner and Tjørve (2005) discuss spatial bias, where smaller areas are likely to be surveyed more intensely than larger ones for practical reasons, meaning that the number of species being sampled can potentially decrease with an increase in area. In a scenario such as this, the data will less likely represent the actual species richness of the area, reducing the validity of the species-area rule. A study of anthropogenic influences on bees and wasps by Tylianakis et al. (2006) found that estimation of species diversity of a larger area based on evidence gathered at a smaller spatial scale underestimated the actual result; had this gone unnoticed, it would have meant that greater conservation attention would have been given to the wrong area. This verifies the importance of using spatial scales to enhance SARs’ effectiveness.

Studies, such as those undertaken by Losos (1996) of the Anolis lizard and Franzén et al. (2012) of butterflies and moths on true islands show SARs as real examples. Losos (1996) found in his study of Anolis lizards on 147 Caribbean islands that a significant relationship exists between the number of species and area of the island. Larger islands were found to have more species present than smaller islands that were more geographically isolated and did not support more than 2 Anolis species at a time. This supports IBT and shows that Anolis lizards on smaller, more isolated islands have reached an equilibrium point, such as the one discussed by MacArthur and Wilson (2001). A positive relationship between area and species richness of 1016 moth and butterfly species was recorded by Franzén et al. (2012), who also noted species with a smaller geographic range were more sensitive to area change than species with a larger range; this emphasises the importance of spatial scales within SAR studies.

Counter-evidence for species-area relationship

There is a discussion about MacArthur and Wilson’s (2001) IBT to be used as more of a framework than a theory. It needs to be modernised, especially as its equilibrium model appears too simplistic (Heaney, 2000; Lomolino, 2000). As IBT is a SAR, any fragility found within IBT raises questions about the reliability of SARs as a law. IBT’s equilibrium model’s simplicity lies in the fact that it only accounts for species richness due to immigration and extinction and no other factors. Even so, the idea of equilibrium being reached on an island seems far-fetched as ecological stochasticity and geographical and geological fluctuations can affect a perfectly balanced model. Tangney et al. (1990) listed five other possible explanations for any studies that showed results consistent with IBT. These include random placement hypothesis (Arrhenius, 1921), habitat diversity hypothesis (Williams, 1943), incidence function hypothesis (Diamond, 1975), small island effect hypothesis (Whitehead & Jones, 1969), and small island habitat hypothesis (Kelly, Wilson, & Mark, 1989).

Assumptions of IBT (MacArthur & Wilson, 2001), and therefore SARs, can be challenged:

  1. Species richness balances immigration rates and extinction rates, excluding speciation over evolutionary time. For example, the adaptive radiation resulting in different finch species on the Galapagos Islands (Grant & Grant, 2008).
  2. The assumption that immigration rates to an island result from island size and distance from the mainland fail to account for other variables. These variables include the role ocean and air currents play in transporting species to islands (Payn, Dvorak, & Myburg, 2007), anthropogenic introduction of species, or immigration from other islands or emigration from the island.
  3. IBT ignores stochastic events and assumes that all species have an equal ability to immigrate or become extinct.
  4. Natural disasters can cause extinction on islands; natural disasters can also function independently of island size or distance to the mainland. 
  5. Climate can affect the species’ likelihood to immigrate to or leave an island. For example, a species that functions in colder weather on the mainland is unlikely to leave its niche to travel to a warmer island, and vice versa, regardless of the islands size or proximity to the mainland.

As mentioned before, SAR’s equations and models are used in conservation. Still, Zimmerman and Bierregaard (1986) introduce the thought of SAR data acting as a “blindman’s cane [to] show us roughly the way” (p. 141) after stating that predictions using IBT and SARs for conservation were nonbeneficial. Diamond (1984) mentions that the speed at which environments are being dismantled should cause ecologists to erect conservation plans faster for them to be preserved. A study of Amazonian frog conservation (Zimmerman and Bierregaard, 1986) was used as a case for the misuse of SARs and IBT and showed that these models may underestimate species richness. Therefore, mean areas receive less conservation than they need. Zimmerman and Bierregaard (1986) propose that time spent studying theory would be better used to understand the relationships that species have with their environment, which would, in turn, produce a better estimate of species richness in an area than using SAR equations. There were many articles available on SARs, yet it was all theory with little in-field evidence; lack of evidence challenges the integrity of the law. The questions here are: is the SAR model an excuse to use maths and models instead of physical evidence which could say otherwise? Is there too much theory and not enough tested hypotheses? Is the consequence of a lack of conservation, where it is needed, taxonomic extinction? In-field experimentation is necessary to answer these questions.

Bergmann’s rule

Supportive evidence for Bergmann’s rule

Bergmann’s rule says species of larger size are found in colder environments, whilst their smaller counterparts are found in warmer environments (Ashton, Tracey, & de Queiroz, 2000; Mayr, 1956). Many arguments have arisen about the validity of Bergmann’s rule (Geist, 1987, 1991; Paterson, 1990), yet, animals are not set aside from the laws of physics (Cambell, 1977), and Paterson (1988) states that Bergmann’s rule obeys mass and surface-area heat-transfer rules. (It is important to note that this refers to their mass or surface area when referring to smaller and larger individuals, rather than length unless noted (Paterson, 1988).)

Evidence of geographic variation in body size of birds has a solid environmental link, including that of the mallard ducks (Anas platyrhynchos) of America and Canada (Rhymer, 1992). It was found that the growth of the mallard ducklings was constrained by environmental conditions experienced during development; tarsus length was 3% longer in the colder Canadian population, and it was shown that this variation was due to the environment they were exposed to during maturation. James (1983) conducted a study with 12 avian species in the central and eastern United States. He found intraspecific size variation is related to a combination of temperature, moisture, and other climatic variables, rather than just temperate ones. A modification of Bergmann’s rule to include this will maintain the integrity and framework of the rule whilst adding more information and specificity to it.

Mammals in both controlled and natural environments displayed evidence of adhering to Bergmann’s rule. Pigs (Sus scrofa domesticus) raised at 5oC were bigger than pigs of the same species raised at 35oC (Weaver & Ingram, 1969). Swedish moose (Alces alces) not only had a 15-20% larger body mass in colder, northern areas than in the south, but also had higher growth rates and matured later than in warmer areas (Sand et al., 1995), indicating a slower metabolic rate for moose in colder environments. Bats (Myotic lucifugus) also showed a similar metabolic correlation to latitude and conforming to Bergmann’s rule in that the colder population grew slower, showed delayed aviation development, and were born larger (Fujita, 1986).

Northern American populations of snowshoe hare (Lepus americanus) conform to Bergmann’s rule to mitigate heat loss (Gigliotti et al., 2019). They can withstand the colder winter months by conserving heat and energy due to increased body mass. The strong selection pressure for larger female snowshoe hares is because females have higher energy demands due to gestation and lactation; the hares breed in winter months, so conserving energy during the cold period whilst also having maternal demands is essential for the hare. 

Counter evidence for Bergmann’s rule 

Contrarily, southern American snowshoe hare populations do not follow Bergmann’s rule as strongly as their northern counterparts (Gigliotti et al., 2019). This is an example of different selection forces acting on individuals, even within a species. In the northern population, a big driver for body mass was to conserve energy and mitigate heat loss, whilst, in the southern population, their large body mass was a result of an extended growing season. Gigliotti et al. (2019) found that the milder, southern winter meant heat conservation wasn’t an essential factor in determining hare body mass. Furthermore, southern populations experienced a long summer growing season and a shorter winter, meaning they had more time to accumulate body mass and less time to lose it.

Two subspecies of red fox (Vulpes vulpes) in Japan show a negative correlation to Bergmann’s rule as skull size is bigger in the subspecies inhabiting the warmer, southern regions than the subspecies inhabiting the colder, northern region (Oishi, 2010; Takeuchi, 1995). Oishi (2010) found the southern subspecies showed increasing skull size with decreasing latitude across its populations. Although the body length of the northern subspecies was larger than the southern subspecies, the male body weight was the same between both, and the female body weight was lighter in the northern species, countering Bergmann’s rule as colder populations had less body mass.

Geist (1987, 1990) opposes Bergmann’s rule with solid reasoning. Firstly, because Bergmann’s (1847) article is written in German, translation has become an issue, with many people following Mayr’s (1956) translation, which Geist (1989) claims is wrong. Secondly, using formulas derived from Bergmann’s rule, Geist (1987, 1990) predicts an exponential size increase with latitude, anticipating higher latitudinal populations of Calgarian sparrows to be the size of chickens, which they are not. Thirdly, Geist (1987) concludes that hair is a superior adaption than size to cold, predicting a hairless beast at different temperate intervals would finally have to weigh 15,000kg at a temperature of –32oC, whereas, for the same beast, 1cm of hair coat is worth a 5-fold increase in mass. Consequently, if it is cheaper to evolve a denser coat, evolution would favour this over body mass increase when subjected to cold stress. Finally, Geist (1990) concludes that body size depends not only on heat retention but also on other factors. These include: individuals surviving longer on larger fat stores; larger females bearing more and producing larger young; larger females producer richer or more milk, therefore sustaining healthier or more young respectively; large size favouring males in fighting and sexual selection; larger individuals can run for more extended periods (helpful in prey species were speed and endurance are necessary for survival). When considering these, it seems far-fetched that body size is solely due to selection for heat conservation at any latitude.    

Significance of ideas

In biogeography, it is essential to scrutinise these rules because it not only helps argue their place in science but uncovers more and more about the distribution of species in space and time. For SARs, it seems that more real-life evidence is needed to properly analyse this rule. For now, caution should be exercised when using it as a conservation tool. For Bergmann’s rule, it is clear there is a difference between and amongst species of different latitudes. Yet, it is naive and bold to assume these differences come down to something as simple as energy conservation. It seems far-fetched that Bergmann’s rule will hold true to species worldwide when you take in the complexities and specialised adaptations of the natural world.

I believe it is a scientist’s job to doubt rules (if only to end up strengthening their integrity in the end). Both SARs and Bergmann’s rule are a stepping-stone to the truth that nature hides. Any disparities should be indications of necessary modifications rather than a chance to disregard the rule entirely. 


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


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 CaCO3 and CO2, hence the alternate name of carbonate counter pump.

Ca2+ + 2HCO3- → 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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Turner, J. T. (2015). Zooplankton fecal pellets, marine snow, phytodetritus and the ocean’s biological pump. Progress in Oceanography, 130, 205–248.

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Zondervan, I., Zeebe, R. E., Rost, B., & Riebesell, U. (2001). Decreasing marine biogenic calcification: A negative feedback on rising atmospheric pCO2. Global Biogeochemical Cycles, 15(2), 507–516.

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


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.

Pedicellaria of a crown-of-thorns sea star (Acanthaster planci).
By Philippe Bourjon – The uploader on Wikimedia Commons received this from the author/copyright holder., CC BY 3.0,
The incredibly dangerous flower urchin (Toxopneustes pileolus) with its long, venomous pedicellariae.
By Vincent C. Chen – Own work, CC BY-SA 3.0,

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,

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,


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,

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

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,
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,
Sand dollar (Mellita species) burying itself in the sand.
By John Tracy from Snellville, GA, USA – End of the line, CC BY 2.0,

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,
A giant sea cucumber (Thelenota ananas).
By Leonard Low from Australia – Flickr, CC BY 2.0,

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,
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. –, CC BY-SA 2.0,


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 Cnidaria: jellyfish, corals, and anemnem… amenome… we give up.


Phylum Cnidaria contains all your favourite ocean stingers from jellyfish to corals to anenomes, and we can’t forget our favourite Hydrozoan, the Portuguese man o’ war.



The body wall is composed of three layers:

  1. Epidermis tissue – outer layer.
  2. Mesoglea – “jelly in the middle” composed of mucopolysaccharides & collagen; is not true tissue but provides support, buoyancy, and locomotion.
  3. Gastrodermis tissue – inner layer which lines the gastrovascular cavity.

A distinguishing feature of cnidaria is their simple gastrovascular cavity, present in only one other primitive phylum. It is a two-way system, where food enters through an opening that serves as a mouth and an anus and is extracellularly digested within the gastrovascular cavity, then waste exits back through the same hole (Shostak, 2001).

Medusa & polyp form

Cnidaria can exist in two forms: medusa or polyp (Piraino, Boero, Aeschbach, & Schmid, 1996; Seipel & Schmid, 2004). Some cnidarians only exhibit a polyp or medusa form, or have one predominantly over the other. Some may pass through both forms throughout their life histories.

Polyp form:

  • Sessile
  • Cylindrical tubes point upwards
  • The oral end is on top
  • The aboral end usually attached to the substrate
  • Tentacles point upward
  • E.g., sea anemone
  • Some polyps form colonies (e.g., coral)

Medusa form:

  • Swims
  • Is the inverse of the polyp
  • The oral end is below a bell-shaped body
  • Th aboral end is to top of the umbrella-like structure (velum)
  • Tentacles point downward
  • E.g., jellyfish
Difference between polyp and medusa. ( Copyright 2001 by Sinauer Associates Inc.

Nematocyte and nematocyst

Yikes. Let’s get this clear:

NematoCYTE is the CELL.

NematoCYST is the ORGANELLE.

The nematocyte is a specialised, ectodermal stinging cell involved in defence and prey capture. It contains the nematocyst (Beckmann & Özbek, 2012; David et al., 2008; Östman, 2000).

The nematocyst is an organelle inside the nematocyte consisting of an ejectable thread that causes a sting and injects toxins into predators or prey. The nematocyst capsule is firm and made from a type of collagen, and it holds a coiled thread that can be barbed, smooth, or hold toxins. A chemical or physical stimulant stimulates the ejection of this thread, and once stimulated, will uncoil and extrude out, penetrating or wrapping around the prey. Once paralysed, captured, or killed (who knew sea anemones are so gruesome??), the tentacles will move the prey to the oral cavity to be digested by the gastrovascular cavity.

File:Nematocyst discharge.png - Wikimedia Commons
Showing the discharge of a barbed nematocyst from a nematocyte.

There are three types of nematocysts:

  1. Penetrant – barbed thread with open tip; when discharged, it pierces the skin/exoskeleton and injects venom to paralyse or kill.
  2. Glutenant – smooth or bristled thread with an open tip that is sticky and has toxins.
  3. Volvent – smooth, lasso-like thread with closed tip entangles prey.


Cnidaria are capable of both sexual and asexual reproduction (Shostak, 2001). Sexual reproduction involves gametes, usually produced in separate individuals, and are fertilised in the gut, ovary, or water after being released by the mouth, tentacles, or breaks in the epidermal layer. Female gametes may produce a substance that attracts male gametes. Asexual reproduction usually happens in warmer months, where a bud develops via evagination from the adult body wall and contains an extension of the gastrovascular cavity. Once fully developed, it detaches from the parent.


Coral have a unique symbiotic relationship with zooxanthellae which gives the coral a range of different colours (Shostak, 2001). Coral produce carbon dioxide (CO2) and ammonium (NH4+) as a by-product of cellular respiration, and zooxanthellae use the CO2 and NH4+ to conduct photosynthesis which, in turn, supplies the coral with sugars, lipids, and oxygen.

Coral bleaching is a phenomenon where the coral consumes or expels their symbiotic inhabitants to ensure short-term survival when exposed to stressful conditions such as rising water temperature, leading to a white “bleached” appearance (Hoegh-Guldberg, 1999; Lesser, 2011; Nir, Gruber, Shemesh, Glasser, & Tchernov, 2014). The coral continues to live after bleaching, but under a prolonged, stressful environment, they will die from starvation.


Class Anthozoa

This group includes sea anemones, stony corals, and soft corals.

Apple anemone (Stomphia didemon).
Rare, long-lived, deep-sea Hawaiian gold coral (Kulamanamana haumeaae).

Class Scyphozoa

These are the true jellyfish.

Chrysaora melanaster, commonly known as the northern sea nettle or brown jellyfish, is a species of jellyfish native to the northern Pacific Ocean and adjacent parts of the Arctic Ocean.
A species of jellyfish from the genus Cephea, or the cauliflower jellyfish.

Class Cubozoan

Box jellyfish are distinguishable by their cube-shaped medusae. Some species have potent venom that can be extremely painful and potentially fatal to humans.

Chironex fleckeri, or the sea wasp, is thought to be the most lethal jellyfish in the world and is responsible for sixty-four deaths in Australia from 1884-2021 (Fenner & Williamson, 1996). C. fleckeri is said to contain enough venom to kill sixty adult humans, and stings are typically excruciatingly painful and, if left untreated, can kill within two to five minutes.

Chironex fleckeri.

Malo kingi, or the common kingslayer, is a species of Irukandji jellyfish named after one of its victims, Robert King (Gershwin, 2007). The Irukandji jellyfish are any of several box jellies that cause Irukandji syndrome after stinging their victims; Irukandji syndrome is characterised by severe pain, vomiting, and rapid rise in blood pressure. M. kingi are very small and inconspicuous in the water, making it hard for victims to see them.

Malo kingi.

Class Hydrozoa

To end on a fun note, let’s finish with Hydrozoa. They are a group of very small, predatory individuals that can live in solitude or within a colony. The colonial species can be large and sometimes the specialised individuals cannot survive outside of their colony. So, the next time you hear someone calling a Portuguese man o’ war (Physalia physalis) a jellyfish, feel free to roll your eyes and let them know they are actually a Hydrozoan.

Portuguese man o’ war (Physalia physalis) washed up on a beach.


Beckmann, A., & Özbek, S. (2012). The nematocyst: a molecular map of the cnidarian stinging organelle. International Journal of Developmental Biology, 56(6–7–8), 577–582.

David, C. N., Özbek, S., Adamczyk, P., Meier, S., Pauly, B., Chapman, J., … & Holstein, T. W. (2008). Evolution of complex structures: minicollagens shape the cnidarian nematocyst. Trends in genetics, 24(9), 431–438.

Difference between polyp and medusa. From Difference Copyright 2001 by Sinauer Associates Inc.

Hoegh-Guldberg, O. (1999). Climate change, coral bleaching and the future of the world’s coral reefs. Marine and freshwater research, 50(8), 839–866.

Lesser, M. P. (2011). Coral bleaching: causes and mechanisms. In Coral reefs: an ecosystem in transition (pp. 405–419). Springer, Dordrecht.

Nir, O., Gruber, D. F., Shemesh, E., Glasser, E., & Tchernov, D. (2014). Seasonal mesophotic coral bleaching of Stylophora pistillata in the Northern Red Sea. PLoS One, 9(1), e84968.

Östman, C. (2000). A guideline to nematocyst nomenclature and classification, and some notes on the systematic value of nematocysts. Scientia Marina, 64(S1), 31–46.

Piraino, S., Boero, F., Aeschbach, B., & Schmid, V. (1996). Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa). The Biological Bulletin, 190(3), 302–312.

Seipel, K., & Schmid, V. (2004). Mesodermal anatomies in cnidarian polyps and medusae. International Journal of Developmental Biology, 50(7), 589–599.

Shostak, S. (2001). Cnidaria (Coelenterates). e LS.

Phylum Porifera: aka Spongebob Squarepants and co.


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).

Morphology of a sponge, showing the osculua and ostia.


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.

A six-pointed star spicule of a sponge.
Three-pointed star and rod spicules of a sponge.



These are calcareous sponges with CaCO3 spicules.

A calcareous sponge (Leucetta primigenia).


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.

White hexactinellida glass sponge known as a venus flower basket (Euplectella aspergillum).


Some Demospongiae don’t have spicules, but if they do, they are siliceous spicules that are held together by collagen.

A purple encrusting sponge from the Strongylacidon genus.


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.

What to expect from The Ocean: Explained

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.