Walking Sharks Defy Biology: Reproduction Without Energy Cost

New research from James Cook University has made the extraordinary discovery that epaulette sharks can reproduce and lay eggs without any measurable rise in energy use. JCU's shark physiology research team, led by Professor Jodie Rummer, has published new research in Biology Open measuring the energy use of epaulette sharks, otherwise known as "walking sharks," during their short reproduction 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." Reproduction is generally considered a heavy investment for most species. But until this research, nobody had ever measured the direct 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." Epaulette sharks typically produce two eggs per three-week reproduction cycle, with peak egg-laying occurring from September to December. Prof. Rummer's team tracked five female epaulette sharks before, during and after egg case encapsulation. Sharks were housed in large temperature-controlled tanks in the Marine and Aquaculture Research Facility Unit at James Cook University, Townsville. "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. Lead author of the research and recent JCU Ph.D. graduate, Dr. Carolyn Wheeler, described how they also tracked blood and hormone changes during the egg laying process. "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."