Walking Sharks Are Breaking the Rules of Evolution and Reproduction
There is a shark that can walk on land. This fact alone seems like a biology demonstration. But more impressively, the epaulette shark – a small spotted reef dweller also known as the “walking shark” – has added another surprise to its roster: it can reproduce without paying the usual metabolic price.
New research suggests that these sharks are quietly rewriting some of the rules that scientists thought governed energy, evolution and reproduction in the ocean. Published in Open BiologyThe study reveals that female epaulette sharks can produce and lay eggs without a measurable increase in energy consumption. For animals living in some of the harshest environments on Earth, this efficiency may be exactly what makes them such successful survivors.
What are walking sharks?
Epaulette sharks are famous for their unusual way of moving. As explained by Oceanic Societyinstead of relying solely on swimming, they “walk” along the seafloor using a crawling, undulating motion powered by highly flexible pectoral and pelvic fins. This adaptation allows them to navigate shallow, obstacle-filled coral reef systems, slipping through cracks and crevices where other predators cannot follow.
Walking shark (Hemiscyllium ocellatum) swimming.
(Image courtesy of Johnny Gaskell)
When the tides go out, these sharks can even move between coral heads and briefly exposed reefs or land. This ability is enhanced by another remarkable adaptation: extreme tolerance to low oxygen. According to a study in Marine and freshwater researchEpaulette sharks can slow their heart rate and breathing and selectively reduce blood flow to certain parts of the brain, allowing them to survive conditions that would quickly overwhelm most fish.
There are nine known species of walking sharks, all found exclusively in Australia, Indonesia and Papua New Guinea. Genetic analyzes show that these are also newcomers during evolution. By comparing small fin clippings – similar to human fingernail clippings – researchers found that walking sharks evolved only 9 million years ago, making them the youngest sharks on the planet.
Their diversification likely occurred as populations slowly expanded and then became isolated by rising sea levels and changing landscapes.
Learn more: Some orcas turn over juvenile white sharks and devour their livers
How walking sharks evolved to defy expectations
Living in warm, shallow reef environments is energy demanding. Food availability can fluctuate, oxygen levels drop, and predators are always nearby. Over time, epaulette sharks appear to have evolved a physiology that prioritizes efficiency above all else.
This effectiveness can extend well beyond movement and breathing. According to the new study, reproduction – typically one of the most energy-intensive processes in an animal’s life – barely registers metabolically in these sharks.
“Reproduction is the ultimate investment. […] You’re literally building a new life from scratch,” Jodie Rummer said in a James Cook University press release. “We expected that when sharks made this complex egg, their energy consumption would increase significantly. But there was no increase in energy consumption, it remained completely stable.
A shark reproduction strategy like no other
The research team followed five female epaulette sharks before, during and after egg capping. Housed in large temperature-controlled tanks at James Cook University, the sharks were monitored for their oxygen consumption, blood chemistry and hormonal changes throughout their three-week breeding cycle.
Walking shark embryo (Hemiscyllium ocellatum).
(Image courtesy of Martijn Johnson)
Epaulette sharks typically produce two eggs per cycle, with peak egg-laying occurring between September and December. Yet despite this investment, researchers found that metabolic rate, blood markers and hormones remained unchanged.
“Everything was remarkably stable, so this research challenges our fundamental assumptions about chondrichthyan fishes (sharks, rays, rays, and chimaeras). Under environmental stress, many species will choose between survival and reproduction, but the epaulette shark may be able to continue producing eggs even under such stressors. This is encouraging, because healthy sharks mean healthy reefs,” explained lead author Carolyn Wheeler.
In a warming and increasingly unpredictable ocean, a shark that can walk, tolerate low oxygen, and reproduce on an energy budget could be just the kind of evolutionary success story the ocean needs.
Learn more: 115 million-year-old giant shark fossil discovered in Australia rewrites evolutionary timeline
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