Epaulette shark (Hemiscyllium ocellatum), a shark that became famous thanks to its ability to “walk”.

Universal Images Group (via Getty Images)

Reproduction is often framed as biology’s ultimate energy expenditure. (As someone who recently became pregnant, I can definitely say it was exhausting!) For many animals, producing offspring means eating more, moving less, and/or diverting energy to growth and survival. This idea is particularly strong for sharks and their relatives. This is because sharks tend to mature slowly, reproduce slowly, and invest heavily in each offspring. So there was little doubt that reproduction must come at a high energetic cost.

Until now.

new research from james cook university‘s Shark Physiology Research Teamled by Professor Jodi Lamar and Published in biology openhave accomplished something no one has been able to do before: directly measure the metabolic cost of spawning in sharks. Their subject was a local Great Barrier animal known as the epaulette shark (Hemiscilium ocellatum), a small carpet shark that became famous many years ago for its ability to “walk” across reef flats using its fins. This species also featured in a David Attenborough documentarythis predator has become an unlikely poster child in Queensland waters. But despite their fame, there’s still a lot we don’t know about them. What we know: Epaulette sharks are oviparous (laying eggs rather than giving birth to live young), with each female typically laying two egg cases every three weeks, with peak spawning in the (Australian) spring and early summer. The researchers focused on five female epaulette sharks kept in large temperature-controlled tanks at the JCU Marine Aquaculture Research Facility in Townsville, tracking them before, during and after eggshell formation, and monitoring oxygen uptake as a proxy for metabolic rate. Simply put, the more oxygen an animal uses, the more energy it expends.

What they discovered was unexpected even to the scientists themselves. “We expected that when sharks make these complex eggs, their energy use would spike,” Lamar explained in the article. university press release. “But there was no increase in energy use, it was completely flat.” This goes against decades of assumptions about chondrich fishes, which include sharks, rays, skates and chimeras. Reproduction in these animals has long been thought to be a slow, energy-intensive process, especially compared to most teleost fishes, but until this study, no one had directly measured how much energy sharks actually use during spawning. In addition to metabolism, the researchers also measured reproductive hormones and blood parameters related to oxygen transport. main author Dr. Carolyn Wheelera recent JCU PhD graduate, said the results were surprisingly consistent. Hormone levels were shown to be largely stable throughout the reproductive cycle, except for a brief initial testosterone peak. Blood indicators such as hematocrit and hemoglobin concentration also showed no meaningful changes. “Everything has been incredibly stable,” Wheeler said. “So this study challenges some of our basic assumptions about chondrich fishes.”

Why does shark metabolism remain flat during a process that intuitively seems expensive? One explanation may lie in how epaulette sharks manage energy over time. In the wild, these sharks breed seasonally, probably using energy stored in their livers to encourage spawning. In captivity, where temperature and food availability are held constant, sharks may have shifted to what biologists call an “income breeding strategy,” using energy from regular feeding to support continued reproduction. This means that your energy usage stays even throughout instead of dramatic peaks and troughs. Another factor to consider is scale. Organs called oviductal glands, which are involved in the formation of egg sacs, may have high metabolic demands but are small compared to other parts of the shark’s body. This means that the extra energy used therein can be effectively lost in noise when measuring whole-animal metabolism. In other words, important work may be happening, just not large enough to move the metabolic needle.

The implications of these discoveries extend beyond a single small shark species. Many of our concerns about climate change and reproduction are rooted in the idea that reproduction is one of the first processes to fail under stress. When food becomes scarce or temperatures rise, animals are expected to choose survival rather than producing offspring. But what if some species evolved ways to buffer reproduction against environmental changes? “This work challenges the narrative that when things go wrong, such as ocean warming, reproduction happens first,” Lamar said. “Epaulette sharks appear to be very resilient, but it is important to determine how resilient these species are to ocean warming.” That attention is important, Lamar noted. These experiments were conducted under controlled conditions, rather than during the seasonal temperature changes that sharks experience on coral reefs. Future research that mimics natural warming and cooling cycles may uncover hidden costs that only emerge under certain conditions.

Healthy sharks are closely related to healthy coral reefs. If some species can continue to reproduce under difficult conditions, their resilience could buy ecosystems valuable time. But resilience is not invincible. It is important to understand where the limits lie, especially as the oceans continue to warm. So how many of our assumptions about shark biology are built on inference rather than direct measurements? More research should tell us! One thing is for sure: sharks still have a lot to teach us, even about processes we thought we understood. What other “obvious truths” about marine life might be revealed under close scrutiny? And how might these revelations reshape the way we think about conservation in a rapidly changing world?