Opah! Warm blooded fish derails conventional science

The study of biology can turn even the most open-minded person into a bit of a snob. As you learn about the infinite complexity of the living world and begin to understand how body systems have evolved to cope with a never-ending struggle for survival it is easy to fall into the trap of thinking one way of doing things is better than another. This can lead to thinking of some species as “primitive” compared to humans and over the course of history this idea has been used to justify some pretty heartless treatment of animals from testing cosmetics to making bears ride unicycles.

A great example of how biology tends to split up animals is the study of thermoregulation, or how animals control their body heat. Although there are animals that exists across a spectrum of temperature controlling abilities we tend to lump all the life on our planet into one of two buckets: warm blooded or cold blooded. Cold blooded animals (also called ectotherms) use forces outside their body (mainly the sun) to regulate their metabolisms. If they need to speed things up, they go lay in the sun for a while. When they begin to overheat, they go lay in the shade. Warm blooded animals, by contrast, can use internal forces like muscle contraction and widening or narrowing blood vessels to keep things in check, regardless of whether or not the sun is shining.

Generally speaking, the warm blooded camp includes mammals and birds  and team cold-blood includes everything else. The upshot is that since humans are warm blooded mammals and we are the ones who publish the bulk of the scientific papers on this planet, cold bloodedness tends to get spun as being “less advanced.” Fortunately, science builds its understanding of things over time and recent research is beginning to demonstrate that things aren’t quite as neat and tidy as some of us wish they were.

Fish, for example, are the quintessential primitive, cold blooded animals. When they leave the warm and sunny waters near the surface for a deep dive, they become sluggish as their bodies cool down… But not all fish. On May 15, 2015 researchers reported in the journal Science that a fish called the opah seems to defy this description.

The opah is a cool animal to begin with. It is roughly circular in shape, with powerful looking fins. It is red with white polka dots, and can grow to 90 kg (200 lbs) in weight and be about the size of a car tire. But it turns out that the coolest thing about the opah is that it isn’t really cool at all. The opah is the first fully warm blooded fish ever discovered. Through flapping its fins and changing the way warm and cool blood exchange heat in its gills, the opah can keep its body temperature 4 to 5 degrees C (7 to 9 F) above the temperature of the surrounding water. This gives it an advantage in hunting cold blooded fish at depth and means it never has to move to the surface to warm up.

Other fish like tuna and some sharks have been found to be able to heat up specific parts of their bodies when gearing up to hunt, but to avoid damaging their organs they eventually need to move to warm water.

The process used by the opah is actually pretty similar to how humans stay warm when its cold outside. When our body gets cold it sends a message to the hypothalamus in our brains, which acts as our internal thermostat. The brain constricts blood vessels near the skin and in the extremities to reduce heat loss (this is why your ears, fingers and toes get so cold so fast). The hypothalamus can also use muscle contractions and hormonal reactions to produce heat in the body.

Interestingly, the opah isn’t the only animal to throw a wrench in the gears of cold blooded classification. Dinosaurs, in case you didn’t already think they were awesome, defy conventional categorization and have forced scientists to create a third label “mesotherms” to explain how they regulated heat. The trouble with dinosaurs is that there aren’t any around to study, but by looking at growth rates compared to how much energy is burned in living animals, scientists have been able to piece together how  dinosaurs bodies might have worked. They lie in between cold bloodedness and warm bloodedness… More akin to tuna than lizards.

So once again the human urge to lump things into groups is thwarted by the complexity of nature. Just another reason why the guys who wrote the biology texts in the 19^th century should have been more afraid of sharks and dinosaurs.