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What to do when you’re an embryonic small-spotted catshark still growing in your translucent egg case and a ravenous predator is mere moments away from gobbling you whole? Wrap yourself up in your tail, hold your breath, and then freeze—hoping that your mortal enemy fails to spot you drifting in the tangled seaweed. Catsharks, like several other species of sharks and rays, have evolved the intriguing ability to freeze their respiration for minutes at a time and hide mechanosensory and olfactory cues from passing snails, lobsters, and whelks. But according to a recent study, the ability is being put at risk by a greater enemy still: climate change.
Dan Ripley, a graduate student at the University of Manchester in England and the study’s lead author, says that while scientists know about the sharks’ defensive freezing ability, what wasn’t known was how it might be affected by rising water temperatures. Most sharks are cold blooded, meaning their metabolic rate is modulated by their environment. Ripley and his colleagues hypothesized that, in warmer water, catshark embryos would burn through their oxygen more quickly—and be able to freeze for far less time.
To test their idea, the group placed several embryonic catsharks inside a custom respirometry chamber to measure their rate of oxygen uptake as a gauge of their metabolic rate. After triggering the catsharks’ freezing response by gently tapping each egg case, they compared how long each could hold its breath.
The most striking result, says Ripley, is that catshark embryos that were in water at a more familiar 15 °C could maintain the freezing behavior seven times longer on average than those in 20 °C water. This means that rising water temperatures could make embryonic small-spotted catsharks more vulnerable to predation. But the result, says Ripley, may also extend to the myriad other sharks and rays that lay eggs—many of which are nearing the point of extinction.
For Gareth Fraser, an expert in shark development at the University of Florida, the study provides a valuable insight into how mechanical processes that are typically hidden can affect the vulnerability of embryonic sharks. He hopes that future research will examine other variables, such as ocean acidification. “As you shift the environment in which these organisms live … other predators may evolve or adapt to feed on them,” says Fraser.
Fraser says water temperatures around 20 °C are at the higher end of what catshark embryos are likely to experience in the wild. “It is a fairly extreme temperature change,” concedes Ripley. Nevertheless, Ripley says that as marine heatwaves increase in frequency, it is a temperature “that a shark in the wild could conceivably come across within the not-so-distant future.”