When many people hear the word 'shark' they imagine a very archetypal predator, with several sets of fins on the top and sides of a long, slender body that is tipped by a powerful tail. And whilst this stereotype does actually hold true for a majority of the 540+ different species of sharks, there are several oddballs whose body plans have completely broken the mold. Squashed to the point they look more like a ray, dorso-ventrally flattened body plans can be found in several different families of sharks. So which sharks are flat and are these creatures true sharks or actually rays? Why are they flat? And how did their body shape evolve?
We Are Family
Whilst the sharks and the "batoids" (aka rays) diverged away from each other some 300 million years ago, they are actually still very closely related (evolutionarily speaking). Therefore, we talk about sharks, skates and rays collectively as a group known as the "elasmobranchs". All these animals are surprisingly similar - complex predators with cartilaginous skeletons, that live in water, breathe via gills and reproduce sexually - yet they are remarkably diverse.
There are 13 different taxonomic orders of elasmobranchs (Heinicke et al, 2009; Stein et al, 2018). To learn more about the distinguishing features of these different groups you can check out The Magnificent 9.
Thanks to their flattened bodies and extended wings, all of the skates (order Rajiformes), electric rays (Torpediniformes) and stingrays (Myliobatiformes) are very obviously rays. However, the same flattened body shape also pops up in two different groups of sharks: the angel sharks (Squatiniformes) and the wobbegongs (order Orectolobiformes, family Orectolobidae) (Compagno, 2001; Stein et al, 2018).
Batoids in the order Rhinopristiformes almost look like an evolutionary stepping stone between the truly flat sharks (the rays) and the flattened true sharks (like the wobbegongs), as they have a similarly levelled body, but also have a strong, developed tail. To learn more about these unusual animals, you can check out The Extinction of the Rhino-Saws.
Flat as a Pancake
Batoids have evolved to have a flattened body shape because it allows for a completely different type of locomotion. Over many millions of years their "pectoral fins" (the pair of fins that stick out of the sides of most sharks) have extended and fused along the length of their body, to look more like wings. Therefore, instead of using their tail and "caudal fin" to create thrust, many rays undulate their wings in order to swim (Heinicke et al, 2009; Sternes & Shimada, 2020; Vullo et al, 2021).
The body plan has prevailed and become so common because it aids rays in finding food and in avoiding becoming prey themselves. Their compressed shape means that they can swim flat against the seafloor vacuuming up small animals, like shellfish and worms. It also means that when a bigger predator comes along, they can easily bury themselves in the sand to remain unseen, or swim into shallow water, where it is harder for a bigger predator to get at them (Egeberg et al, 2014: Sternes & Shimada, 2020).
Two Timing
There are two groups of sharks that have also developed a flattened body shape: the angel sharks and the wobbegongs. There are 23 different species of angel sharks, but new species are being discovered very regularly recently. They can be found from the Northeast Atlantic and Scandinavia, to north Africa and the Canary Islands. They prefer relatively shallow coastlines with sandy or gravelly "substrates" (Compagno, 2001; Weigman et al, 2023).
Comparatively, there are 12 different species of wobbegong, all living in the Pacific and Indian Oceans. They are most common around Australia and Indonesia, as they favour complex environments, especially coral reefs (Compagno, 2001).
Where these flat sharks differ from their flat batoids cousins is in the wings and the gills. Like all sharks, both these groups of sharks have their gills located on their flanks, where in all batoids, the gills are situated on the underside of their bodies. In sharks, whilst the pectoral fins are remarkably extended, making their bodies appear very flat, these fins are not fused all the way along their bodies and remain separate from their heads (Compagno, 2001).
Same Same, But Different
What these two groups have in common is how they make the most of their slender forms - both wobbegongs and angel sharks are "ambush predators". Lying in wait on the ocean floor, these different types of sharks equally rely on camouflage to remain hidden and will then lunge out incredibly quickly to snatch a passing fish as it swims by. Angel sharks can bury themselves in the sand so as not to be seen, where wobbegongs count on their remarkable skin markings allowing them to blend in against the background of rocks and corals (Compagno, 2001).
However, as they are separated by many millions of years of evolution and thus these two families are not at all closely related, why are these groups of sharks so similar? Both in how they look and how they behave?
When two species have developed a similar trait despite not being closely related, this is known as "convergent evolution". Both angel sharks and wobbegongs have adapted to have a flattened body shape, in order to become successful ambush predators (who are also very good at hiding away from any bigger fish in order to survive). So whilst they might seem similar, in fact, this adaptation has actually arisen multiple times over the course of modern elasmobranch evolution (Heinicke et al, 2009; Vullo et al, 2021).
References
Compagno LJV (2001). Sharks of the world: An annotated and illustrated catalogue of shark species known to date (Vol. 2). Food & Agriculture Organisation. Access online.
Egeberg CA, Kempster RM, Theiss SM, Hart NS & Collin SP (2014). The distribution and abundance of electrosensory pores in two benthic sharks: a comparison of the wobbegong shark, Orectolobus maculatus, and the angel shark, Squatina australis. Marine and Freshwater Research, 65:11. Access online.
Heinicke MP, Naylor GJP & Hedges SB (2009). Cartilaginous fishes (Chondrichthyes). The timetree of life, 9, 320-7. Access online.
Stein RW, Mull CG, Kuhn TS, Aschliman NC, Davidson LNK, Joy JB, Smith GJ, Dulvy NK & Mooers AO (2018). Global priorities for conserving the evolutionary history of sharks, rays and chimaeras, Nature Ecology & Evolution. Access online.
Sternes PC & Shimada K (2020). Body forms in sharks (Chondrichthyes: Elasmobranchii) and their functional, ecological, and evolutionary implications. Zoology, 140, 125799. Access online.
Vullo R, Frey E, Ifrim C, González MAG, Stinnesbeck ES & Stinnesbeck W (2021). Manta-like planktivorous sharks in Late Cretaceous oceans. Science, 371:6535, 1253-1256. Access online.
Weigmann S, Vaz DF, Akhilesh KV, Leeney RH & Naylor GJ (2023). Revision of the western Indian Ocean angel sharks, genus Squatina (Squatiniformes, Squatinidae), with description of a new species and redescription of the African Angel Shark Squatina africana Regan, 1908. Biology, 12:7, 975. Access online.
By Sophie A. Maycock for SharkSpeak
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