Warm Fishes?
Maybe you’re thinking, “I kind of remember high school biology, and I’m pretty sure this isn’t a thing. Fishes are cold blooded!” And you are correct! Mostly. Turns out some fishes have found creative ways to overcome this environmental obstacle and we’re here to share the awe.
Just in case you don’t remember high school, or didn’t do the high school thing, let’s have a quick review. Cold-blooded animals—or ectotherms—cannot produce body heat and must rely on the environment to regulate their body temperature. Cold-blooded animals include reptiles, insects, amphibians, annelids (worms and stuff), and fishes. Warm-blooded animals—or endotherms—can generate their own internal heat and can maintain body temperatures that differ from the environment’s temperature. Mammals and birds are endothermic. Okay, now that we’re all on the same page, let’s take this info below the surface and into the blue. Fish are ectothermic, so they all must be the same temperature as the ocean water around them, right? No! Because science is so cool, we are discovering species that break the rules and find ways to heat themselves up!
What kinds of fishes would do this? And why? Do other animals do this kind of crazy thing? On land, we observe reptiles and insects making the best use of their thermal environment all the time. Now that summer is here, we can see so much more activity in these animals than we did all winter long. Lizards and snakes come out to sun themselves on the rocks on sunny days and retreat to their dens in the cool evening hours. Ants busy themselves out on sidewalks and trails.
It turns out hammerhead sharks are similarly making use of the warmer ocean temperatures and holding their breath when they dive into the cold. A new study published in Science in May of 2023 indicates that hammerhead sharks may be suspending gill function when they dive to deep, cold water to hunt.
Researchers fitted biologgers (tags that collect and store data) to adult scalloped hammerheads and looked at the muscle temperatures of the sharks as they dove. They found that, while the sharks traveled from warm (~26C/78F) surface waters to depths exceeding 800m/2,624ft with temperatures as low as 5C/41F, they maintained warm muscle temps.
Cooling of the muscles only started on the ascent portions of the shark’s dive. They noticed that once the muscles started cooling, they cooled very quickly. These observations lead the researchers to believe that the sharks are blocking off their gills, essentially holding their breath in the colder water and breathing again once they start to head back up toward warmer water. The researchers noted the similarities between this behavior and that of marine mammals’ breath-hold diving. Why would a shark want to hold its breath? As you can see from the research, it’s all about having warm muscles. How fast and agile are you when you’re freezing cold? What about when you’re warmed up? Which condition do you think is better for hunting? Of course, sharks rely on speed and agility to hunt, and so warmer muscles would benefit them while they’re looking for food in the frigid depths!
Sharks holding their breath—so cool! Are there other warm fishes? Why yes! In fact, there are! Bluefin tuna and mackerel sharks (think great white, goblin, and megamouth sharks) are warm blooded.
I’ll let that sink in for a second. Yes. Warm-blooded fish. How? Their circulatory systems are specially designed to work as heat exchangers. Talk about green energy! As the warmer blood exits the muscles on its way to get re-oxygenated, it passes by the colder arterial blood that’s been freshly oxygenated. They use the old, warmer blood to warm up the colder, new blood.
This trick preserves heat in the body of the fish instead of losing it to the ocean through the gills. Bluefin tuna in colder water have been measured to have body temperatures as much as 20C/68F above the water temp. The method is different from the hammerheads, but the reason is the same—warm muscles are fast muscles and fast muscles catch prey.
