These fins are made for walking: How the Epaulette Shark has adapted to life at low tide
Coral reefs provide a unique habitat for many weird and wonderful creatures, so it’s no surprise the first walking shark originates in this highly diverse environment. The Epaulette Shark, Hemiscyllium ocellatum, inhabits the shallow, reef waters surrounding Australia and New Guinea. Regions like these, including the tropical seas of the Great Barrier Reef, can prove to be a tough environment for many of its inhabitants, with new challenges being brought in with each rise and fall of the tide. So how does a small, bottom feeding shark survive in this unforgiving environment? Similar to its reef dwelling neighbours, in order to survive one must adapt. The Epaulette has done this and then some, developing a series of adaptations making them truly the master of the intertidal environment.
The Waiting Game
Twice a day conditions on the reef change drastically as a result of the rise and fall of the tides. At low tide the reef is left as a series of interconnected rock pools. High tide brings with it no solace for the Epaulette either, as with the rising water comes the threat of larger predatory sharks, such as the White Tip Reef Shark seen pictured here. So what strategy does the Epaulette use to survive and thrive in this ever-changing reef environment?
Due to the risk of being eaten at high tide the Epaulette waits for the tide to go out. At this point the larger sharks are forced into deeper waters leaving the Epaulette free to exploit the rich, reef habitat undisturbed. Sounds relatively simple so far, but the reef environment, being the demanding environment it is, brings with low tide a whole host of new problems. Exposed rock pools are no place for a shark especially in the searing heat of the tropics, and if found high and dry one would expect a shark not to survive. However the Epaulette, unlike any other shark, can use its dorsal fins to walk itself out of trouble, navigating its slender body over the exposed reef and into the food abundant rock pools. This is demonstrated and further explained in the video below.
But a vital point has been missed here, no shark can breathe out of water and the Epaulette is no exception to this rule. However the Epaulette has been handed an evolutionary singularity when it comes to the shark species, not only can the Epaulette walk but by shutting down its vital organs one by one, slowing its heart rate and gradually limiting blood flow to certain parts of its brain the Epaulette can survive without oxygen for sixty times longer than a human.
The Epaulette shark walking its way out of trouble – enabling the shark to traverse its way over the complex 3D coral substrate into the rich tide pools. Source.
A Fish out of Water
So how are the Epaulettes fins adapted to allow it to ‘walk’? The Epaulette’s paired fins are heavily reduced in cartilage in comparison to many of its larger shark adversaries. This slight adaptation gives the Epaulette a much larger range of motion in both its pectoral and pelvic fins, allowing them to rotate more freely and push off the substrate.
In truth, the walk of the Epaulette is somewhat clumsy and, as you would expect, it is much more at home in the water. When fully submerged the Epaulette is a perfectly capable swimmer. However even with its highly manoeuvrable and slender body form, perfect for exploiting the different levels of the complex reef habitat, the Epaulette prefers to walk. Undulationg its body over the diverse coral bottom or sandy sea floor as seen in the video below . This is due to the Epaulette shark being primarily an opportunistic bottom feeder, with a specific diet of polychaete worms and crabs.
The Epaulette traversing its’ underwater environment using its pectoral fins to walk. Source.
But walking isn’t enough. To survive outside of the water the Epaulette must also be able to cope for extended periods of time with little to no oxygen. The isolated pools that remain at low tide trap a variety of organisms including smaller fish, crustaceans and segmented worms which are easily preyed upon by the Epaulette, but at a price. At low tide up to 80% of the dissolved oxygen in these pools can be used up through the combined respiration of these few remaining organisms. So how does the Epaulette Shark manage in these Hypoxic conditions?
Not Just a Case of Holding Your Breath
Now here’s where it gets a bit tricky. A shark cannot survive out of water. So for the Epaulette’s low tide strategy to work, it must have many physiological responses to cope in conditions of low oxygen. When the rush of low tide leaves the Epaulette atop the diverse coral substrate, the first survival response is to significantly slow its heart rate down. This slowing in heart rate causes a chain of additional responses; Blood vessels in the Epaulette become dilated and blood pressure falls by almost half. Both of these responses are effective attempts in transfering more blood to the heart and brain.
The brain of a shark is already very advanced, requiring about one third of the ATP needed, in teleost fish, to maintain normal cognitive function. However the Epaulette has taken this to a whole new level. The ever surprising Epaulette can further decrease this energy demand. This is achieved by reducing metabolism to certain areas of the brain and deactivating any non-essential brain functions. By doing this, Epaulettes not only lower their oxygen requirements, but also keep production of free radicals, which can be tissue-damaging, to a minimum.
For the majority of low tide the Epaulette remains in this death-like state. It would be very easy to assume the Epaulette is relatively vulnerable in this condition, however the Epaulette still maintains the ability to walk, hunt and feed despite the drastic modifications it has taken to survive. But low tide can’t last forever, and as high water returns the Epaulette simply switches back into normal behaviour, unscathed by the whole experience.
Evolution in Action?
So could this shark one day take over the terrestrial environment? The answer in short is probably not. The reason for this being the type of fish the Epaulette is: a shark, which is cartilaginous.
Land dwelling vertebrates supposedly evolved from tetrapods around 395 million years ago. One of the main characteristics vital for a transition onto land is the presence of a bony skeleton, something the early tetrapods had. However, the Epaulette sharks body is comprised of a lightweight cartilaginous skeleton, which simply wouldn’t be able to support the sharks body on land.
So why have the fins evolved to walk if not to conquer the terrestrial environment? The ability of the Epaulette to easily traverse its complex reef environment, using its specialised fins to essentially walk, gives the shark an evolutionary advantage when catching bottom dwelling prey. By adapting to life at low tide they have found a way to survive and thrive in a niche completely unique to the Epaulette. Fundamentally they are highly specialised to suit the competitive environment they occupy.