Many of us enjoy swimming, whether it’s for fun, exercise, relaxation, or rehabilitation. When humans swim underwater, we must hold our breath. If we don’t, water would infiltrate our lungs and we would drown. On the other hand, fish use their gills to absorb dissolved oxygen from the water and circulate it around their bloodstream, so the idea of fish closing their gills – essentially "holding their breath" – seems pretty strange. And it had never been documented in any fish species until now.
Researchers have found that scalloped hammerhead sharks are able to close their gills in an effort to keep warm. This is the first time any fish has been recorded doing this, though the behavior may not be unique to scalloped hammerheads.
Mark Royer, a researcher with the Hawai‘i Institute of Marine Biology's Shark Research Lab, conducted an experiment on scalloped hammerhead sharks by attaching sensors to their fins to record the sharks' body temperatures, the water temperature, and the depth the sharks were diving. After dozens of deep dives, the results revealed that the sharks closed their gill slits in an effort to retain body heat in the cold ocean depths.
Because scalloped hammerhead sharks are cold-blooded, they derive body heat from their environment. They typically spend time in warm, tropical waters, so diving deep to catch prey such as squid can be a shock to the system, especially when water temperatures can drop to just 41 degrees F (5 degrees C).
Gills are a major point of heat loss. If a hammerhead loses too much body heat, its eyes, muscles, and brain will begin shutting down. If this happens, it won't be able to swim, which means that it also won't be able to breathe, as hammerhead sharks depend on water flowing across their gills as they swim.
Unlike marine mammals with lungs, such as whales and dolphins, returning to the surface is not a necessity for hammerhead sharks, but it does provide them with a more comfortable environment in which to open their gills again and allow warm water to flow through.
Take a deep breath:
- Scalloped hammerhead sharks (Sphyrna lewini) can grow to around a dozen feet (3.7 m) in length.
- Scalloped hammerhead sharks can dive to more than 2600 feet (800 m) below the surface, though they typically only spend a few minutes at such depths.
- Other fish have evolved unique strategies to stay warm in cold water. Some fish, including tuna and mako sharks, have heat exchange systems at their gills that can significantly raise their body temperatures. Large species, such as whale sharks, are able to stay warm because their huge bodies are good at conserving heat.