No one can accuse the opah, Lampris guttatus, of being a cold fish. Nor could one call it a cold-hearted fish. Even if it were the most emotionally distant and bitter of all fish, the opah is in fact a warm fish. That is to say, the opah is a warm-blooded fish.

Warm-bloodedness, or endothermy, provides many distinct advantages to an organism, including quicker reaction times and increased muscle power. As endotherms are able to move between very cold and very hot environments, they also see increased habitat expansion. Endotherms can conserve heat generated by metabolic processes and maintain their body at temperatures above or below that of their surrounding environment. Endotherms that live underwater, such as whales and dolphins, have adapted to the challenges of retaining body heat in water, which has a particularly high heat capacity, by employing thick layers of blubber.

Initially, endothermy was considered to be a trait found only in mammals and birds. However, as early as 1835, physician John Davy observed that tuna were actually quite warm compared to their surrounding water, sparking debate over their proper classification. It was not until 1996, that scientists agreed that tuna, like lamnid shark and billfishes, are actually just regional endotherms. That is to say, these fish are only able to warm certain tissues, typically muscles, eyes, and brains. Other regions are kept at ambient temperature.

This brings us to the opah: the first fully warm-blooded fish ever documented. Wegner et al. (2015), who recently published their discovery in the journal Science, found that, unlike other fish, opahs have evolved a number of key adaptations that allow them to heat their bodies to temperatures above that of the surrounding water. They do this by first insulating their gills, the primary site of heat loss, using a dense network of vasculature, or retia. Second, rather than undulate their bodies like most fish, opahs swim primarily by oscillating their pectoral muscles. In fact, opahs have one of the highest musculature to body mass ratios in the fish world, including that of regional endotherms. Importantly, musculature is typically an organism’s main heat source.  Third, opahs have a thick layer of fatty connective tissue lining their bodies, which may act as insulation to keep heat in like the blubber seen in whales and dolphins. These, and other modifications, allow the opah to maintain its high level of physiological performance at all times, temperatures, and depths (between 50 and 400m) – a great feat for a little fish.

From Wikimedia Commons, the free media repository
Opah, or Moonfish, Lampris luna (Gmelin) Specimen in Honolulu market weighing 317.12 lbs. From Wikimedia Commons, the free media repository

Peer edited by Deirdre Sackett and Jonathan Susser

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This article was co-published on the TIBBS Bioscience Blog.

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