Walk this Way
Updated: May 17, 2021
The geography of a habitat has a significant effect on the evolution of animals that live there. Whilst you might assume that there are no barriers in the marine environment, sub-populations of a marine species can be isolated from each other in different areas; geographical changes, hostile environments barring the way and/or a limited ability for an animal to disperse. This means that species begin to evolve separately, which can sometimes give rise to whole new species, with remarkable adaptations. This process is known as "allopatric speciation".
"allopatric"- From (English): allo = other, (Greek) patra = fatherland
For example, a recent study used DNA analysis to determine how the a group of sharks known as the Hemiscyllium (or bamboo sharks) evolved. The bamboo sharks are all small (<1 m total length), benthic (aka living on the ocean floor) sharks, living in shallow, coastal waters of reefs and seagrasses, where they feed on small bony fishes and crustaceans (Dudgeon et al, 2020).
Multiple new species of bamboo sharks have been discovered quite recently. To date, a total of nine different species have been found over a 12 year study in waters around northern Australia and South-East Asia (Allen at al, 2013). The scientists found that the different species all live in separate regions, with little overlap in their ranges. Therefore, they concluded that the different species of Hemsicyllium evolved via allopatric speciation, when they became isolated from each other by moving land masses (Dudgeon et al, 2020).
The researchers believe this was able to happen because these sharks have a limited ability to "disperse" (when an individual relocates itself to a new area and starts a new population). It is rare for these species of sharks to migrate long-distances and each species lives in it's own restricted geographical range.
However, just because these sharks do not travel far in the ocean, doesn't mean that they don't move at all... In fact they have a unique and remarkable form of locomotion... they 'walk'! Using two pairs of highly muscular fins, these sharks essentially 'walk' along the substrate, which is a perfect adaptation to their benthic lifestyle (Allen et al, 2013).
BUT not only do these sharks use their muscular fins to 'walk' in the ocean, but they can also walk short distances out of the water!
The epaulette shark (Hemiscyllium ocellatum), which lives on the Great Barrier Reef in Australia, is able to 'walk' out of the water during low tide. Every night, when the tide goes out, areas of sand and coral heads become exposed and areas of tide-pools are left isolated from the open ocean. This does not stop the epaulette shark! They use their strong paired fins to propel themselves along exposed corals and rocks (Wise et al, 1998).
As well as their remarkably adapted fins, these sharks have also evolved incredible physiological mechanisms which allow them to survive under extreme oxygen pressure. They are able to survive in the open air, without oxygen for 3 hours at a time! This is possible because they are able to reduce their blood pressure by widening their blood vessels, in order to ensure oxygenated blood reaches the most important organs, like the brain and heart. This is known as "hypoxia tolerance". These adaptations allow these sharks to traverse obstacles in order to move between patches of water to find prey (Wise et al, 1998). They may not move far, but they certainly travel with style!
References
Allen GR, Erdmann MV & Dudgeon CL (2013). Hemiscyllium halmahera, a new species of Bamboo Shark (Hemiscylliidae) from Indonesia. International Journal of Ichthyology.19:3, 123–136. Access online.
Dudgeon CL, Corrigan S, Yang L, Allen GR & Erdmann MV (2020). Walking, swimming or hitching a ride? Phylogenetics and biogeography of the walking shark genus Hemiscyllium. Marine and Freshwater Research, DOI: 10.1071/mf19163. Access online.
Wise G, Mulvey JM & Rehshaw GMC (1998). Hypoxia tolerance in the epaulette shark (Hemiscyllium ocellatum). The Journal of Experimental Zoology, 281:1–5. Access online.
By Sophie A. Maycock for SharkSpeak.