These “walking sharks” can lay eggs without spending extra energy, rewriting what scientists thought they knew about reproduction.
Scientists at James Cook University have reported an unexpected discovery. Epaulette sharks can reproduce and lay eggs without showing any measurable increase in energy use, challenging long-standing ideas about the cost of reproduction.
The findings come from a study by JCU’s shark physiology research team led by Professor Jodie Rummer. Published in Biology Open, the research measured how much energy epaulette sharks, often called “walking sharks,” used throughout their short reproductive cycle.
“Reproduction is the ultimate investment … you are literally building new life from scratch,” Prof Rummer said.
“We expected that when sharks make this complex egg, their energy use would shoot up. But there was no uptick in energy use, it was completely flat.”
Rethinking the Cost of Shark Reproduction
For most animals, reproduction is assumed to require a significant increase in energy. Until now, however, no study had directly measured the metabolic cost of egg laying in sharks.
“These sharks appear to have adapted their physiology to be able to optimize their energy use,” Prof Rummer said.
“This work challenges the narrative that when things go wrong – such as warming oceans – that reproduction will be the first thing to go.
“Epaulette sharks appear very resilient, but it’s important to determine just how resilient to warming oceans these species are.”
How the Study Was Conducted
Epaulette sharks usually produce two eggs during each three-week reproductive cycle, with egg laying peaking between September and December. To understand what happens during this process, the research team monitored five female sharks before, during, and after egg case encapsulation.
The sharks were kept in large, temperature-controlled tanks at the Marine and Aquaculture Research Facility Unit at James Cook University in Townsville. This setup allowed the researchers to carefully track changes in energy use under stable conditions.
“We measured their oxygen uptake rates, which are a proxy for their metabolic rate … the more oxygen you burn, the more energy you use,” Prof Rummer said.
Stable Metabolism Under Stress
Lead author Dr Carolyn Wheeler, a recent JCU PhD graduate, said the team also examined blood chemistry and hormone levels as the sharks laid their eggs.
“Everything was remarkably stable, so this research challenges our fundamental assumptions about chondrichthyan fishes (sharks, rays, skates, and chimaeras),” she said.
“Under environmental stress many species will choose between survival and reproduction, but the epaulette shark might be able to continue to produce eggs, even under such stressors.
“That’s encouraging, because healthy sharks equal healthy reefs.”
Reference: “Assessing the metabolic and physiological costs of oviparity in the epaulette shark (Hemiscyllium ocellatum)” by Carolyn R. Wheeler, Cynthia A. Awruch, John W. Mandelman, Jodie L. Rummer, 5 November 2025, Biology Open.
DOI: 10.1242/bio.062076
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