New Study Reveals Evolutionary History of 'Walking' Sharks
A newly published paper has shed light on the evolution of 'walking' sharks found in the waters between Australia and New Guinea. The 12-year study also described four new species during the process, bringing the total number of known species to nine.
The study, conducted by researchers from the Commonwealth Scientific and Industrial Research Organisation (CSIRO), University of Queensland, the Indonesian Institute of Sciences and the University of Florida, published earlier this week in CSIRO's Marine and Freshwater Journal, found that walking sharks – also known as epaulette sharks – evolved relatively recently, within the last nine million years.
Walking sharks are members of the family Hemiscylliidae, and have been known to science since the late 18th century, although only five species were described prior to 2008. They are confined to the waters of Indonesia, Australia and Papua New Guinea, although an as yet undescribed species has been photographed in Seychelles. Growing up to one metre long, the sharks use their pectoral and pelvic fins to 'walk' along the seafloor, a skill that becomes especially useful in low tides, where the sharks can crawl across the reef tops in search of prey.
'Their ability to withstand low oxygen environments and walk on their fins gives them a remarkable edge over their prey of small crustaceans and molluscs,' said Dr Christine Dudgeon of the University of Queensland. 'These unique features are not shared with their closest relatives the bamboo sharks or more distant relatives in the carpet shark order including wobbegongs and whale sharks.'
The discovery is important within the evolutionary history of sharks, which first evolved more than 400 million years ago. The oldest extant species of shark in the oceans – including frilled sharks and sixgill sharks – have remained unchanged for some 150 million years, hence the recent, and quite rapid evolution of walking sharks makes them the youngest known species of shark on the planet.
'We set out to determine when these curious little critters evolved from their shark ancestors by using a technique called “dated molecular clock methodology,"' said Mark Erdmann of Conservation International, co-author of the study. 'This uses genetic samples from shark fin clippings – just like nail clippings for humans – to compare the mutations in each shark species to estimate the date when each branched off into a new species'
The new species appear to have evolved after becoming genetically isolated from their original populations. Factors which may have influenced the separation include the shifting of reefs through tectonic activity in the region. 'They may have moved by swimming or walking on their fins,' said Dr Dudgeon, 'but it’s also possible they "hitched" a ride on reefs moving westward across the top of New Guinea, about two million years ago.'
'By analysing sea-level rise and tectonic shifts over the past 50 million years, we know that island fragments around Australia and Southeast Asia have been constantly spinning, splitting apart and smashing against each other,' said Mr Erdmann in a Q&A session with Conservation International. 'We also know that beginning around 10 million years ago, one particular set of island fragments moved northwest from southeastern New Guinea along the coast of the island, until it eventually joined together to form the modern-day Halmahera Island. These islands potentially transported walking sharks from southeastern New Guinea all the way to the Bird’s Head Seascape in Indonesia, where they likely radiated into the four species we can now find in West Papua.'
The scientists believe that the ongoing evolutionary process in the area helps to explain why the region is home to some of the greatest marine biodiversity in the world's oceans. Further research will help to determine the sharks' survival strategies in a changing climate, and may lead to the discovery of new species of walking sharks in the future.