Walk this Way

Many people imagine that sharks are the perfect predators and they are no longer evolving, but this is far from true. New species of sharks are discovered every single year and sharks are still developing new adaptations all the time. For example, some sharks have even evolved the ability to 'walk' short distances on land! So how do they do this? Why do they do it? And what does it mean for the continuing evolution of sharks?

Epaulette sharks have evolved to be able to 'walk' short distances across exposed corals and sand (Image Credit: Anne Hoggett / Lizard Island Research Station / WikimediaCommons)

These Fins are Made for Walking

The epaulette shark (Hemiscyllium ocellatum) lives on the Great Barrier Reef in Australia. These sharks have an especially unique method of getting around. Using two pairs of highly muscular fins, they essentially 'walk' along the substrate, which is a perfect adaptation to their benthic lifestyle (Wise et al, 1998; Allen at al, 2013).

Epaulette sharks have strong fins, which allow them to 'walk' along the ocean floor (Image Credit: Strobilomyces /WikimediaCommons)

But these strong fins mean they are also even able to 'walk' out of the water. 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 fins to propel themselves along exposed corals and rocks between pools of water (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).

Scientists think they evolved this ability so that they would not get stranded at low tide and could venture into different pools to track down food (Wise et al, 1998; Allen at al, 2013).

We Are Family

Epaulette sharks are part of a group of species known as the bamboo sharks (Hemiscyllium). This family is growing because multiple new species continue to be discovered; 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; Dudgeon et al, 2020).

Each if the nine species of bamboo sharks has a limited range and their distributions don't overlap (Dudgeon et al, 2020)

All these species live in different regions with very little overlap in their ranges. This suggests that, despite their specialised 'walking' adaptation, there are limits to how far these little sharks can travel, either out in the air or under the water. That is to say, they have a limited ability to "disperse" (to strike out and colonise new habitats) (Dudgeon et al, 2020).

Allopatric speciation occurs when sub-populations of a species become isolated from each other and evolve separately until they become a whole new species (Image Credit: Andrew Z. Colvin / WikimediaCommons)

I Need Some Space

The geography of a habitat has a massive impact on the evolution of animals that live there. When we think about life underwater, many people imagine that there are no barriers in the ocean and marine species can go anywhere they like. However, geographical changes in the oceans can have a huge impact; even driving new species to evolve (Dudgeon et al, 2020).

Hostile environments barring the way and/or a limited ability for an animal to disperse can mean that sub-populations of a marine species can be isolated from each other in different areas. 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" (Dudgeon et al, 2020).

"allopatric"- From (English): allo = other, (Greek) patra = fatherland

The experts suspect that bamboo sharks evolved by allopatric speciation, when they became isolated from each other by moving land masses (Dudgeon et al, 2020).

As the continents are still drifting and shifting, and conditions are continually altering in the ocean thanks so climate change, it is likely that this group of sharks could become isolated into sub-populations again; driving the evolution of yet more new species over the coming centuries. Who knows what wonderful species are yet to appear and what amazing adaptations they might have in the future!

Epaulette sharks only live in a very small range on the Great Barrier Reef (Image Credit: Jim Capaldi / WikimediaCommons)

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