Imagine a shark… What do you see? Chances are you are thinking of something like a blacktip reef shark, set against a beautiful, vibrant coral reef. However, there are many other species of underappreciated small sharks on coral reefs. These smaller sharks may not “command” the reef like their larger kin, but in many ways they are more impressive. Of these smaller sharks, one group in particular, epaulette sharks, has developed a unique trait, achieving something that most of us do daily-- they can walk.
While most sharks have rigid fins, epaulette shark fins are flexible, which lets them walk across the rough and jagged reef and actually move from tide pool to tide pool. A fish out of water is generally seen as a bad thing and being on land is not where fish (including sharks) are generally found. However, not only do epaulette sharks walk along the reef, they can also walk out of the water! How does a shark that needs to live in the water to breath, walk on land? Epaulette sharks are capable of turning off half their brain when in air and they can do this for up to 2 hours! Incredibly, these sharks have adapted to live in extreme conditions such as areas with little or no oxygen. But what actually makes the epaulette shark walk?
Coral reef flats, home to the epaulette shark, are not only known for their extreme oxygen environment, but also for having pretty extreme temperatures. Reef flats can be 3-4°C (about 6-8°F) warmer than on the reef slope where we typically think of sharks living and this is even worse during summer. This may not sound like much, but imagine walking outside during summer and the temperature going from a warm 28°C (83°F) to spiking up to 33°C (92°F) in just a few hours! Summer temperatures can be even more extreme during low tides reaching upwards of 32°C (89°F). The oceans where these amazing epaulette sharks live are only predicted to get warmer, with coral reef flats potentially getting up to 35°C (95°F)! How exactly do they handle the heat? By walking of course!
Most sharks are like lizards in that their body temperature is based on the temperature of the environment around them. Lizards use behavioural thermoregulation. This simply means using a behaviour to control body temperature. One famous behaviour is basking, where lizards lie on a warm rock or out in the sun. By doing this, they can increase some processes like digestion. Many fish are thought to use movement in the same way, moving from place to place to occupy a preferred temperature, warming (or cooling) their body. A fish’s preferred temperature can be tested in a laboratory, but that doesn’t necessarily tell us what is going on in the real world. How exactly are sharks (and other fish) using temperature? Is there a preference they are always following? Or are they “easy-going”, tolerating whatever the ocean throws at them? This is where my research comes in.
This winter, I plan to travel to Heron Island at the most southern end of the Great Barrier Reef. Heron Island is a densely forested sand cay about 0.8 km (0.5 mile) long. This beautiful island is home to many sharks including blacktip reef sharks and lemon sharks, and is thought to have the largest population of the amazing epaulette sharks. While on Heron, I will use data loggers to monitor water temperatures on the reef flat and collect epaulette sharks. Following fieldwork, the sharks will be shipped back to my home university, James Cook University, where I will look at their preferred temperatures. Once I understand what the sharks prefer, I can compare this to the actual body temperatures of the sharks living on the reef flat. All going well, I plan to return to Heron Island during the summer months to collect information on the seasonal changes that occur. During winter, sharks may not have the same behaviours or use the same temperatures as they do during the summer when it is a lot warmer.
Understanding how these sharks use temperature is critical to understanding and ultimately conserving fish species as conditions continue to warm. If we learn what fish do to cope with changing conditions right now, we may be able to predict how fish will respond in the near future.
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