Four species of walking sharks have recently been identified in the waters of northern Australia and New Guinea. The news may spark images of horror films where giant predators chase bathers up the beach, but the reality is less alarming. Even in Australia, not everything is trying to kill you. Now the first genetic comparison of the genus is teaching us about these unusual reef-dwellers and the region's ecological history.
“At less than a metre long on average, walking sharks present no threat to people but 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 in a statement.
All walking sharks swim, but use their fins to walk across reefs at low tide when the water is shallow enough to make that the most energy-efficient way of getting around.
No sharks outside the genus, even walking sharks' closest relatives the bamboo sharks, have adopted similar forms of locomotion, although several families of bony fish have hit upon the concept.
Until recently, however, marine biologists considered many walking sharks part of a single species Hemiscyllium ocellatum. Dudgeon is part of a project that has studied the sharks in more detail and gradually demonstrated that those inhabiting waters around New Guinea are different enough from the Great Barrier Reef-dwelling H. ocellatum to deserve classification as four new species, bringing the total number to nine.
Dudgeon told IFLScience the body shapes and behaviors of all Hemiscyllium are similar, with the differences only being visible in their color schemes. The variation hinted at in their patterns, however, is confirmed in the sharks' genetics. Moreover, Dudgeon said: “None of the species' geographical ranges appear to overlap,” although she is interested to see if studies in the Torres Strait may change this.
In Marine and Freshwater Research Dudgeon reports using mitochondrial DNA to map how each species relates to the others, and roughly when separation occurred.
With more mobile creatures we might not be able to make much use of this, but Dudgeon told IFLScience walking sharks appear remarkably tied to their location for marine species. “Bony reef fish are site-attached as adults, but distribute themselves during the larval stage,” she said, while most sharks are quite mobile as adults.
Walking sharks, on the other hand, appear to conquer territory slowly. New species formed when populations became cut off from each other, either by changes in sea level or tectonic uplift around highly volcanic New Guinea. These never moved into each other's territory when the barriers were removed, leaving the region's geologic and climatic history written in their genes.
