The opah is the only known fish to possess a fully warm-blooded physiology.

Nature · Published 3 October 2026 Swipe more like this →

In short: While most fish are cold-blooded, the opah defies the rule. By using a specialized heat-exchange system in its gills, this deep-ocean dweller maintains a body temperature higher than the surrounding water. This unique biological advantage allows it to hunt with superior speed and efficiency in the cold, dark depths.

The opah, or moonfish, is a striking creature characterized by its round, silvery-gray body, rose-red belly, and golden-ringed eyes. Traditionally, scientists recognized two species—the common opah (Lampris guttatus) and the southern opah (Lampris immaculatus)—though recent taxonomic reviews suggest there may be as many as six distinct species. These fish inhabit deep, open ocean waters across tropical and temperate regions, often traveling long distances in response to shifting oceanic temperatures.

What truly sets the opah apart is its status as the only known fully warm-blooded fish. Unlike other fish that rely on the ambient temperature of their environment, the opah generates heat through the movement of its pectoral fin muscles. This heat is preserved through a remarkable counter-current heat exchange system located in the gills. A dense network of blood vessels, known as the rete mirabile, allows warm blood leaving the body to transfer its heat to cold, oxygenated blood returning from the gills. This process ensures that the fish's internal organs, heart, and brain remain warmer than the surrounding water.

This endothermic capability provides a significant competitive edge, enabling the opah to maintain an active lifestyle and faster swimming speeds compared to its prey. While it is an endotherm, it is not a homeotherm; its body temperature fluctuates based on the external environment rather than remaining constant. Despite their fascinating biology, opahs remain somewhat mysterious to researchers. Because they are not a major commercial species and reside in the deep ocean, data on their exact life span and population status are limited. Current research, including electronic tagging programs initiated by NOAA, aims to better understand their movements and life history.

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