Movies like Jaws and Deep Blue Sea have fostered an innate fear of sharks in many people, and shark researchers have now discovered new species of sharks that can walk! But don’t worry yet, these sharks are still very far from chasing humans through the waves and across the beach (or survive in a Sharknado).
Epaulette sharks (Hemiscyllium ocellatum) are small, slender sharks that live among corals where they feed on bottom-dwelling invertebrates. They tend to be creamy or brown in color, with small dark spots, and grow to be about 3 feet in length. Due to them feeding around shallow corals and rock pools, they risk being stranded and low tide, however they are able to survive up to an hour without any oxygen. These sharks are found in the western Pacific Ocean, around New Guinea and northern Australia. However, in Australian waters they can be found as far south as Sydney and New South Wales.
Since 2008, four new species of these sharks have been discovered, and this discovery sheds light on a very niche adaptation and peculiar evolutionary history: epaulette sharks can walk!
Epaulette sharks are extremely slender, with a short and stumpy snout. Their uniquely adapted pectoral and pelvic fins are broadly rounded and paddle-like. Muscular and skeletal modifications in these fins allow the sharks to lift their bodies and move the fins at a 90-degree angle, to use the fins as “feet” in a walking motion.
Walking On Fins
Although the walking ability of epaulette sharks has been known for several years, a new study published in Marine and Freshwater Research has revealed some new information about this unique group of sharks. This study collected data on these sharks over 12 years. The study identified four new species of walking epaulette sharks, resulting in a total of nine species all belonging to the genus Hemiscyllium. Although all nine species reside in the waters around Papua New Guinea and Australia and they all have a similar anatomy, they vary greatly in skin colors and spot patterns.
What makes these epaulette sharks so incredibly unique is that they are able to walk across the seafloor. They do this by walking on specially adapted pectoral and pelvic fins, giving them a movement similar to that of a lizard scuttling across rocks.
The question is, why would epaulette sharks have evolved the ability to walk on their fins, when swimming is a perfectly good means of locomotion? In deep water, it is beneficial to use their fins for steering while swimming. However, when the tide rolls out, it becomes evident why they have adapted to walking on the seabed.
The coral reefs and intertidal zone where epaulette sharks live are influenced by tides and at low tide, a portion of the sharks’ habitat becomes exposed, forming isolated rock pools (or tidal pools). The unique ability of epaulette sharks to survive without oxygen for up to an hour, and their adapted pectoral and pelvic fins, allows the sharks to move between rock pools across the areas exposed to air at low tide. This adaptation allows the sharks to search for prey trapped in rock pools where they are unable to escape the sharks.
The Youngest Sharks
In addition to discovering four new species of epaulette sharks, the published study also studied genetic samples taken from the sharks. The genetic material sourced from these sharks, as well as museum specimens, have shown that epaulette sharks are approximately 9 million years old. With most shark species evolving around 100 million years ago, and other species such as the sixgill shark estimated to have already existed 200 million years ago, it is possible that epaulette sharks are the most recent shark species that have evolved in the ocean. Most shark species have undergone very little evolutionary changes over the millions of years they have existed. Therefore, epaulette sharks may hold the key to finding out why some shark species change over time while others don’t. It is predicted that the limited range of epaulette sharks as well as their preference for tidal reefs and the tectonic activity within their habitat were drivers in their recent and rapid evolution.
A distinct species, such as the walking epaulette shark species, usually forms when some members of a species are physically separated from others. The scientists therefore also attempted to determine how that speciation (i.e. when populations evolve to become distinct species) occurred in the case of these walking sharks. The authors of the paper states that their findings support the idea that speciation of walking sharks occurred because the shark populations slowly expanded their range (by walking or swimming), and some individuals eventually became isolated by environmental factors, such as sea level rise or the formation of large river systems that broke up their habitats. The adaptations of epaulette sharks to be deprived of oxygen for long periods of time and the use of fins for walking are not shared with their closest relatives, bamboo sharks (Hemiscylliidae).
But Wait, There Is More!
Ongoing discoveries of new species and their specialized adaptations indicate that there is still so much we don’t know about life in the ocean, and that there is still a lot more to discover. Of the nine identified species of epaulette sharks, there is only enough information and data on four of the species for them to be listed on the IUCN red list for conservation. This means we cannot be sure of the conservation threat status (whether they are endangered or not) of these sharks. It is also possible that there are even more species of epaulette sharks, waiting in the tidal pools to be discovered.
Luckily for humans, it is “very unlikely” that larger shark species, like Great White Sharks (Carcharodon carcharias), evolve to walk on land. Where epaulette sharks are small and able to withstand the force of gravity when walking between tidal pools, larger sharks are incredibly heavy, and it does not seem feasible that their fins become muscular enough to lift their bodies out of the water.