Staying at the top of the food chain takes a lot of effort. Some fish, such as sharks and tunas, have even gone to the extent of warming their brains. Kathryn Dickson from California State University Fullerton, USA, explains that having a warm brain probably allows fast-moving foragers to maintain nervous function and see well as they dive deep in cold waters. But it wasn't clear whether deep-diving squid-chasing opah had gone to the same lengths as other pelagic predators to ensure their place at the top of the pyramid(p. 461).

Catching opah in the Pacific Ocean, the team measured the temperature behind the fish's eyes, in the brain and in the myotomal muscle, and found that the fish's eyes were 2.1°C warmer than the rest of their bodies and their brains were also significantly warmer. They were warming their eyes and brains, much like sharks and tunas.

Curious to find out where the heat was coming from, the team dissected some fish heads and found that one of the muscles attached to the eyeball, the well insulated lateral rectus muscle, was probably the heat source. Closer inspection also showed that the blood flow through the muscle is arranged to minimise heat loss.

So opah seem to warm their brains and eyes, which probably gives them an advantage over their prey when plunging into cold water.

Runcie, R. M., Dewar, H., Hawn, D. R., Frank, L. R. and Dickson,K. A. (
). Evidence for cranial endothermy in the opah(Lampris guttatus).
J. Exp. Biol.