Have you ever seen a scary movie where the murderer is stalking a potential victim, and then it cuts to a close-up of their prey frozen still, holding their breath for dear life? Believe it or not, shark embryos have a remarkably similar freeze response when predators are nearby. Small-spotted catshark (Scyliorhinus canicula) embryos develop for 21 weeks in egg cases called mermaid's purses. After the 10th week, these shark embryos produce secretions that open slits into the egg case, and for the remainder of their development, the tiny sharks have to move to exchange seawater to ensure enough oxygen is available to continue their development. However, these movements attract shark predators. Fortunately, the youngsters are able to detect the presence of the threat and stop moving and breathing – a behavior known as the freeze response – although they are only able to do so for a limited amount of time as they have to resume respiration and moving once again. As shark embryos are particularly vulnerable to predation, any factor that affects the freeze response is a risk for shark populations in the wild. As temperatures are expected to increase and affect the metabolic rate of marine animals, Daniel Ripley and researchers from the University of Manchester, UK, investigated the effect of increasing temperatures on the freeze response of small-spotted catshark youngsters.
First, the team transported ∼3-week-old small-spotted catshark embryos from Germany to the UK and placed the developing youngsters in either 15 or 20°C water until they were 15 weeks old. Then, the researchers took the shark embryos and gently flicked them underwater, to imitate a predator inspecting an egg case, while measuring how long the youngsters froze and the amount of oxygen that they consumed at their respective temperatures. Finally, the team measured how fast the embryos fanned their gills to inhale oxygen at the different temperatures.
Despite the difference in the embryos’ temperatures, the team did not find a difference in youngsters’ resting metabolic rates. However, they did discover that the embryos fan their gills faster at 20°C than at 15°C. Additionally, as the resting metabolic rate or incubation temperature of embryos increased, the duration of their freezing response decreased. This means that the length of time that the embryos hold themselves still does not depend solely on their metabolic rate. Most alarmingly, the researchers also found that the warmest embryos only froze for a seventh of the time that the cooler embryos remained stock still. This decrease in the length of the freeze response has significant implications as temperatures increase as a result of climate change. The freeze response is a behavior that helps embryos avoid predators and improves their chances of survival, but small-spotted catshark embryos will likely be at increasing risk from passing predators as temperatures rise and this response declines.
For these shark embryos, the duration of this freeze response can be the difference between life and death. The research team also advises that predator avoidance behavior is another factor that needs to be taken into account when thinking about the conservation of sharks and the impact of rising temperatures owing to climate change.