Walking sharks are only a small piece of our planet’s shark universe, but they’re remarkable for their ability to propel themselves along the seafloor using bodies and fins. And, actually, on land – literally fishes out of water. New walking shark research by an international team of scientists has found that, in terms of evolution, they are the most recent group of sharks to arrive on the scene.
DESPITE A COMMON MISCONCEPTION, only a small number of the planet’s 400-plus species of sharks need to swim continuously to breathe. In fact, some species make their way by “walking” on the sea bottom.
Actually, walking sharks sort of amble along more than “walk,” propelling themselves by wriggling their bodies and pushing along with their pectoral and pelvic fins.
And, their walking isn’t restricted to the seafloor. When necessary, these remarkable fishes can cope with low-oxygen environments for extended periods. And, they can survive for an hour or so totally in the open air, wriggling and pushing their way across dry land between bodies of water.
This is important due to their penchant for pursuing diets of small fishes and invertebrates in reef and shoreline tidepools.
During low tides, it’s not uncommon for foraging sharks to become stranded in isolated pools, a situation in which the amount of dissolved oxygen may drop to hypoxic levels. Thanks partially to the chemical adenosine, which plays a role in the flow of blood, the epaulette shark (Hemiscyllium ocellatum), for example, is said to be able to survive for more than three hours in a low-oxygen situation.
By slowing down his heartbeat and powering down his brain, an epaulette can survive completely out of the water for more than an hour while he crawls his way back to the ocean.
FEW SPECIES, NEW ARRIVALS
Walking sharks constitute a small group – only nine species. All are restricted to tropical waters around Australia’s northern coast, Papua New Guinea and Indonesia. In fact, most of them spend their lives in severely restricted ranges, rarely venturing more than a mile from the reefs where they hatched.
Newly published research on walking sharks’ origins has determined that, while sharks on the whole have been swimming the earth’s oceans for some 400 million years, walking sharks arrived on the scene only some nine million years ago. Evolutionarily, they’re the youngest group of sharks in the shark dynasty.
VIDEOS – OUT & ABOUT ON LAND AND SEAFLOOR
WALKING SHARK BASICS
Members of the Hemiscylliidae family of sharks, walking sharks are usually described as small, although not necessarily as small as the recently defined 16-inch pocket sharks. Hemiscylliidae species range in size from 18 inches (Indonesian carpet shark, Hemiscyllium freycineti) to more than three feet (epaulette shark, H. ocellatum). Theoretically, some species can grow to 48 inches, but most are typically in the 20 to 30 inch range.
Their long tails tend to exceed the length of the rest of their bodies, accounting for their other common name, longtail carpet sharks. You might think their lives on the seafloor might account for the carpet part but, ostensibly, it reflects someone’s judgment that their exotic body decor – spots and mottled patterns – resembles fancy carpets.
At the other end, Hemiscylliidae species tend to have short snouts, nostrils near the tip and short barbels that serve as sensory organs in the search for their tiny prey in reef crooks and nooks.
They’re somewhat clumsy and easily caught. Although they may nip if handled, by and large they’re deemed harmless to humans. They’re popular in aquariums, and a burgeoning ornamental fish market is a concern. Fortunately, many areas where these fishes are found are in protected preserves. But some are not.
Whether their walking skills reflect adaptation to their habitats or their habitats reflect best use of their singular walking and breathing skills (or, of course, some of each), walking sharks flourish in shallow waters.
They’re nocturnal. They tend to spend their days hiding under ledges and in crevices. They devote their nights to looking in those kinds of nooks and crannies for their preferred prey – small fishes, snails, worms, crabs, clams and the like. That’s why, if they’re not careful, they can get trapped in scattered pools during low tides.
The newly released study, Walking, swimming or hitching a ride? Phylogenetics and biogeography of the walking shark genus Hemiscyllium, was published in the journal Marine and Freshwater Research in January. It attracted attention with headlines that scientists had “discovered four species of walking sharks, almost doubling known species count.”
In fact, as part of their 12-year-long of walking shark research, the international team of scientists had announced the identification of nine species in 2016. There are likely more, one researcher says. Some species have long been known, like the Indonesian speckled carpetshark (H. freycineti), first identified in the early 19th Century.
The study’s primary focus was on the geographic and physical processes that led to the emergence of nine walking shark species. Analyzing genetic samples from fin clippings, they were able to determine how long ago each species arrived. The answer they found was nine million years ago, making them mere toddlers in the shark family tree.
BUT HOW DID THEY GET THERE?
A mystery is: How these sharks, who really can’t swim long distances, arrive in Australia/Papua New Guinea/Indonesia waters. They may have originally moved by swimming or walking away from their more conventional shark ancestors, become genetically isolated and evolved into new species, the researchers suggest.
But, they add, the little guys could also have “hitched” a ride on reefs that drifted into place during plate tectonic movements.
MORE SURVIVAL SKILLS
In addition to their low-oxygen tolerance, their walking skills and their all-around charm, the Hemiscylliidates are of interest to researchers for another characteristic: They’re remarkably able to tolerate warming water. In an era of climate change, one of the researchers suggests, their genes may have unique, useful information still to come.
The research involved scientists from the Australian Science Agency CSIRO, the nonprofit Conservation International, the Indonesian Institute of Sciences, the University of Florida and Australia’s University of Queensland.
PRINCIPAL SOURCES: Walking, swimming or hitching a ride? Phylogenetics and biogeography of the walking shark genus Hemiscyllium, was published in the journal Marine and Freshwater Research; Walking sharks discovered in the tropics, University of Queensland News Office; Discovery affot: New study cracks mystery of how “walking” sharks split, Conservation International; Epaulette Shark, Florida Museum Department of Ichthyology, University of Florida; Epaulette Shark Walks on Land Video, Nature on PBS; Hemiscylliidae, et.al., Wikipedia.