A team of researchers led by Dr Nicholas Wegner of NOAA Fisheries’ Southwest Fisheries Science Center has discovered a whole-body form of endothermy in a deep-water fish, the opah (Lampris guttatus), that produces heat through the flapping of wing-like fins and minimizes heat loss through a series of counter-current heat exchangers within its gills. Unlike other fish, opah distribute warmed blood throughout the body, including to the heart, enhancing physiological performance while foraging in the cold waters.

The opah, also known as the spotted moonfish, cravo, kingfish, and Jerusalem haddock, is a deep-bodied, compressed fish found in eastern North Atlantic, North Sea and Mediterranean.

Opah have beautiful coloration: the body is a steely blue grading to rosy on the belly, with white spots covering the flanks. The body is covered in very small, smooth scales.

Both the median and paired fins are a bright vermillion, contrasting strongly with the body. The large eyes stand out as well, ringed with golden yellow. The mouth is small and toothless.

Opah grow to over 5 feet (1.5 meters) in length and can weigh over 150 pounds (70 kg).

They are apparently solitary but are known to school with tuna and billfish. They propel themselves by flapping their pectoral fins. This, together with their forked caudal fins and depressible median fins, indicates that opah, like tuna, maintain themselves at constantly high speeds.

They regularly dive to depths below 650 feet (200 meters) where they feed on mesopelagic fishes, squids and crustaceans – and where water temperatures are below 4 degrees Celsius. Primary predators of opah are Mako and Great White sharks.

Fish that typically inhabit cold depths tend to be slow and sluggish, conserving energy by ambushing prey instead of chasing it. But the opah’s constant flapping of its fins heats its body, speeding its metabolism, movement and reaction times, according to Dr Wegner and his colleagues at NOAA Fisheries’ Southwest Fisheries Science Center.

“That warm-blooded advantage turns the opah into a high-performance predator that swims faster, reacts more quickly and sees more sharply” explained Dr Wegner, who is the lead author on the study published in the journal Science.

“Before this discovery I was under the impression this was a slow-moving fish, like most other fish in cold environments. But because it can warm its body, it turns out to be a very active predator that chases down agile prey like squid and can migrate long distances.”

The scientists collected temperature data from opah caught during surveys off the West Coast, finding that their body temperatures were regularly warmer than the surrounding water.

They also attached temperature monitors to opah as they tracked the fish on dives to several hundred feet and found that their body temperatures remained steady even as the water temperature dropped sharply.

The fish had an average muscle temperature about 5 degrees Celsius above the surrounding water while swimming about 150 – 1,000 feet (45 – 305 meters) below the surface.

While mammals and birds typically maintain much warmer body temperatures, the opah is the first fish found to keep its whole body warmer than the environment.

“Discoveries like this help us understand the role species play in the marine ecosystem, and why we find them where we do. It really demonstrates how much we learn from basic research out on the water, thanks to curious scientists asking good questions about why this fish appeared to be different,” said Dr Francisco Werner of the Southwest Fisheries Science Center, who was not involved in the study.

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Nicholas C. Wegner et al. 2015. Whole-body endothermy in a mesopelagic fish, the opah, Lampris guttatus. Science, vol. 348, no. 6236, pp. 786-789; doi: 10.1126/science.aaa8902