An international team of marine biologists has found that members of the genus Hemiscyllium are the ‘youngest’ — as in, the most recently evolved — sharks to ever walk (or swim) our planet.
The bamboo shark Hemiscyllium halmahera. Image credit: © Mark Erdmann.
The ‘walking’ sharks (genus Hemiscyllium), also known as bamboo sharks or longtail carpet sharks, are a genus of nine species of nocturnally active, bottom-living, distinctly patterned sharks.
These creatures are relatively small (usually less than 85 cm, or 33.5 inches, in length) sharks, with the largest species measuring 1.22 m (48 inches).
Rather than swim, they ‘walk’ by wriggling their bodies and pushing with their pectoral and pelvic fins.
The ‘walking’ sharks are found in the Australia-New Guinea region, including islands, reefs, and shoals separated from mainland areas by shallow seas. The Indonesian island of Halmahera is the only location lying outside the core region that is inhabited by these sharks.
“These ornately patterned sharks are the top predator on reefs during low tides when they use their fins to walk in very shallow water,” said team leader Dr. Christine Dudgeon, a researcher in the School of Biomedical Sciences at the University of Queensland.
“They 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 mollusks.”
Dr. Dudgeon and colleagues set out to determine when these creatures evolved from their shark ancestors by using a technique called dated molecular clock methodology.
This uses genetic samples from shark fin clippings to compare the mutations in each shark species to estimate the date when each branched off into a new species.
“We estimated the connection between the species based on comparisons between their mitochondrial DNA which is passed down through the maternal lineage,” Dr. Dudgeon said.
“This DNA codes for the mitochondria which are the parts of cells that transform oxygen and nutrients from food into energy for cells.”
Sharks as a group have actually been around for about 400 million years.
Amazingly, the team discovered that ‘walking’ sharks evolved just 9 million years ago, making them the youngest sharks on the planet.
“Back in 2016, we reported that the nine known species of walking shark are found exclusively in a ring around Northern Australia, New Guinea and the satellite islands of Raja Ampat, Aru and Halmahera in Indonesia,” said team member Dr. Mark Erdmann, of Conservation International.
“Using this clock, we set out to learn how the walking sharks separated into so many distinct species — a process known as speciation.”
“Speciation typically happens when individuals of a given species get separated from their main population — sometimes by walking or swimming or being carried away on a current to an isolated place. If they are lucky enough to survive and breed, eventually evolution will take this new population in a different direction and often leads to a new species.”
“For a shark that isn’t able to swim far and doesn’t move much, we wondered how that separation could possibly be the case.”
The study authors have three main hypotheses to answer this puzzling mystery.
“For most of the walking shark species, our findings support the idea that speciation occurred because the populations slowly expanded their range by walking or swimming, then 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,” Dr. Erdmann said.
“For the four walking shark species found at the Bird’s Head Seascape, we suspect that they actually hitched a ride — on a drifting island.”
“Data suggest the new species evolved after the sharks moved away from their original population, became genetically isolated in new areas and developed into new species,” Dr. Dudgeon said.
“They may have moved by swimming or walking on their fins, but it’s also possible they ‘hitched’ a ride on reefs moving westward across the top of New Guinea, about two million years ago.”
“We believe there are more walking shark species still waiting to be discovered,” she added.
“Future research would help researchers to better understand why the region was home to some of the greatest marine biodiversity on the planet.”
The team’s paper was published in the journal Marine and Freshwater Research.
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Christine L. Dudgeon et al. Walking, swimming or hitching a ride? Phylogenetics and biogeography of the walking shark genus Hemiscyllium. Marine and Freshwater Research, published online January 21, 2020; doi: 10.1071/MF19163
This article is based on texts provided by the University of Queensland and Conservation International.
