Warm-Blooded Fish?
Not
exactly. But some fish do keep parts of their bodies warm.
Most fish don’t even try to stay warm. Their bodies are only slightly warmer than the water they swim in.
Since they breathe water, they’d have a big problem trying to warm up. Fish get their oxygen from the water around them by pumping blood through tiny vessels in their gills. As the blood goes through the gills, it picks up a full load of oxygen. The blood also comes to the same temperature as the water.
You can see why a fish living in cold water is about as cold-blooded as an animal can get.
Cold-blooded animals have an advantage in that they don’t have to use energy to keep warm. So they don’t need much food. But when they do get cold, they are likely to be sluggish. Their body machinery slows down, and they can’t swim or think as fast.
A few fish have ways to get around the temperature problem. The swordfish, for example, has a special built-in heater that warms the blood going to its eyes and brain. The heater is made of tissue that looks like muscle but can’t contract. The tissue spends all of its energy just to make heat. The heater allows the swordfish to go down into deep, cold water in search of prey.
The
Tuna’s
Tricks
Tuna
have a more complicated series of inventions. A large
tuna is a marathon swim-mer and can keep going at speeds
of almost nine miles an hour for long periods. It swims
with a tail motion that is worked by a big slab of muscle.
That hard-working muscle generates a lot of heat. In most
other kinds of fish, this heat is lost when blood from
the muscle gets back to the gills for another load of
oxygen. But the tuna has a special trick that saves some
heat.
Close to the muscle is a place where warm blood from the muscle goes past cold blood coming in from the gills. The two bloodstreams race past each other in tiny thin-walled tubes. An engineer would call that a countercurrent heat exchanger.
The heat exchanger uses blood that is leaving the muscle to warm the incoming blood. In bluefin tuna, muscle tempera-tures may be more than twenty-five degrees Fahrenheit above the outside water temperature.
The bluefin’s brain is kept warm by another small heat exchanger. Because of these two systems, one scientist has called the fish an endotherm: a warm-blooded animal.
In-Between
Cases
No fish can keep its body warm the way you do. But that
may be the most interesting part of all. We have been
used to thinking that animals had to be either warm-blooded
endotherms or cold-blooded ectotherms. But some fish are
in-between cases—just a part of the body is kept warm.
So it seems that the inventions for keeping warm can occur
in stages.
If you are a dinosaur fan, then you might know of the arguments about whether the dinosaurs were warm-blooded or cold-blooded. Now you can see that there are in-between possibilities. Maybe the dinosaurs, like some fish, kept only parts of their bodies warm.
