Something seems fishy here…warm blooded fish?

Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus

Wegner, N.C., Snodgrass, O.E., Dewar, H., Hyde, J.R

Lampris guttatus, a fish who is able to produce its own body heat! (Source: fishbase.org). This fish is found worldwide, though it’s especially common in Hawaii and west Africa.

What data were used?
Captured and freely-swimming opah fish

Methods
Researchers measured the body temperatures of captured and freely swimming fish at their natural depth. Temperatures were taken in multiple places along the fish, including the temperatures of a number of the muscles. These measurements were taken by heat monitoring sensors placed in the muscles of the fish.

Results
Researchers found that the core of the fish (pectoral muscles, heart, etc.) were much warmer than the surrounding environment. The cold, oxygenated blood of the fish is warmed by the conducting of heat from the warmer, deoxygenated blood leaving the respiratory system before the oxygenated blood reaches the respiratory system. This indicates that these fish, just like humans and all other mammals, are able to produce their own body heat (“warm blooded”) as opposed to creatures like reptiles, who rely on external sources, like the sun, to maintain their temperature (“cold blooded”).

Why is this study important?

The temperature of an opah fish as taken by the scientists of this study. Measurements were taken ~4-5 cm below the skin of the fish for 98 cm, the length of the fish’s body.

We’ve all learned from school that critters like reptiles and fish are cold blooded, whereas mammals (like us) are warm blooded. Simple, right? It turns out, it’s not nearly as simple as that! More and more, scientists have begun to discover that there are many animals that don’t fit into these neat categories, the opah fish being the most recent of these. This is important because in the fossil record, we don’t have the luxury of examining animals while they’re still alive, so we need to look for other clues! Dinosaurs and pterosaurs are excellent examples of this-we’ve always thought reptiles were cold blooded. But dinosaurs, like Velociraptor, had feathers! They had larger brains! Pterodactyls could fly by flapping their wings! All of these are examples of warm-blooded behavior. Fish like the opah show us how what we thought we knew might not always be the case!

The big picture
The picture that I want to stress here is that even the big things we thought we understood in science-like who’s warm and cold blooded-are subject to change with new data! Only within the last few decades have scientists begun to ditch the idea that animals fall neatly into categories of “warm” and “cold” blooded. It’s also important to note that discoveries such as these open our interpretations of extinct organisms-like dinosaurs, pterosaurs, and yes, even fish!- and how they were able to generate energy. Since we can’t bring a live pterodactyl (at least, not yet! Maybe we’ll learn more after watching Jurassic World: Forgotten Kingdom) in for testing, data such as these remind us that life isn’t as simple as just ‘warm’ and ‘cold’ blooded.

Citation
Wegner, N.C., Snodgrass, O.E., Dewar, H., Hyde, J.R., 2015, Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus: Science, v. 348, p. 786-789, DOI: 10.1126/science.aaa8902

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