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Walking Sharks

Evolution allows animals to adapt to their environment over the course of many generations. Much like dogs evolved from wolves to the pug, chihuahua, and golden retriever, sharks have grown to fit every ecosystem the oceans have to offer. From the icy waters the Greenland shark inhabits to the shallow coastal waters that the Blacktip shark is found in, even reaching to depths of 3.5 km where the Portuguese Dogfish dwells, these ancient animals have stretched across the globe. One unique group of sharks, those of the Linnaean genus Hemiscyllium (more commonly known as the epaulette sharks), has evolved a unique ability: to walk on land.

Now before you start worrying about land sharks greeting you at your door, they don’t really leave the water. Rather, they can walk across reef flats when the tide goes down in order to hunt for prey or escape back to deeper waters. Since tidepools receive both high and low levels of oxygen as the water rushes in and out through the day, epaulette sharks have grown to tolerate these changes, meaning they are anoxia (low oxygen) and hypoxia (high oxygen) tolerant. Further, while most sharks can only last outside the water for a few minutes, epaulette sharks can survive for around two hours. So I suppose if any sharks were going to walk up to your house, it would be these guys; luckily, they’re a small species usually less than a meter in length.

Epaulette sharks use their pectoral fins (the big set of side fins) and pelvic fins (the smaller set of side fins near their tail) to create this walking motion. The physical structure of these paired fins, both internal and external, is shared by all the Hemiscyllium species but is unique to only them. Additionally, some internal structures (such as the intermediate radial series and the levator pectoralis inferior) haven’t been found in any species outside of this genus. In general terms, epaulette sharks have more control of their paired fins than any other shark species, which allows them to rotate their fins in a way that supports walking.

Within the class Chondrichthyes, which includes all sharks, skates, rays, and chimaeras (or ghost sharks), we can see a handful of other species that exhibit similar walking techniques. While epaulette sharks are the only ones known to venture across tidepools while mostly out of water, some bottom-dwelling species such as nurse sharks can use their pelvic and pectoral fins to move. As they hunt for prey along the sea floor, rather than slowly swimming, they sink their fins into the sediment and slowly push themselves forward.

Similarly, many batoid species (including little skates, yellow stingrays, clearnose skates, and electric rays) have been seen “punting”. This term is reserved for species that use their pectoral fins to propel forward but rather than our usual walking motion (left foot, right foot, left foot, etc.), they jump with both fins simultaneously. Beyond the Chondrichthyans, some fish species are also known to stroll along the sea floor (such as the flatfish and red-lipped batfish), but not to the same extent.

The muscular adaptations of the walking sharks allow them to access prey that no other sharks can, but that’s not the only evolutionary advantage they have. This small species can be identified by the big black spot and surrounding thin white ring on their side, also called false eyespots. Scientists believe that the sharks camouflage into the sand with the false eyespots sticking out, so when larger predators swim by, they believe they are approaching a much bigger animal than they are. Epaulette sharks are believed to have split from their nearest common ancestor approximately nine million years ago, and it’s clear they’ve made good use of this time. To watch this magnificent shark go for a stroll, check out the video below.



References

Cover photo credit: jdobson via Australian.Museum

Dudgeon, C. L., Corrigan, S., Yang, L., Allen, G. R., Erdmann, M. V., Fahmi, Sugeha, H. Y., White, W. T., & Naylor, G. J. (2020). Walking, swimming or hitching a ride? phylogenetics and biogeography of the walking shark genus hemiscyllium. Marine and Freshwater Research, 71(9), 1107. https://doi.org/10.1071/mf19163

Goto, T., Nishida, K., & Nakaya, K. (1999). Internal morphology and function of paired fins in the epaulette shark, hemiscyllium ocellatum. Ichthyological Research, 46(3), 281–287. https://doi.org/10.1007/bf02678514

Macesic, L. J., & Kajiura, S. M. (2010). Comparative punting kinematics and pelvic fin musculature of benthic batoids. Journal of Morphology, 271(10), 1219–1228. https://doi.org/10.1002/jmor.10865

Parton, K. J., Doherty, P. D., Parrish, M., Shearer, P., Myrick, K., Shipley, O. N., & Gallagher, A. J. (2022). Opportunistic camera surveys provide insight into discrete foraging behaviours in nurse sharks (Ginglymostoma cirratum). Environmental Biology of Fishes, 106(1), 19–30. https://doi.org/10.1007/s10641-022-01366-x

Porter, M. E., Hernandez, A. V., Gervais, C. R., & Rummer, J. L. (2022). Aquatic walking and swimming kinematics of neonate and juvenile epaulette sharks. Integrative And Comparative Biology, 62(6), 1710–1724. https://doi.org/10.1093/icb/icac127

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