Prior to the discovery of the warm-bloodedness of the opah, all fish were believed to be cold-blooded, as they were unable to sustain warm core temperatures in their bodies. This inability was explained by the respiration of fish by extracting oxygen directly from the water through their gills. This is advantageous, since it allows them to stay underwater at considerable depths, indefinitely. However, the downside of this is that, the fish cannot maintain a warm temperature, since the warm blood (due to muscle activity) that rushes out of the heart immediately passes the gills, and is instantly cooled down to the temperature of the surrounding water. Hence, any heat generated by the fish's muscles is all dissipated, and the coldness affects all functions of the fish. Only a few larger fish, like tuna and some sharks, have been observed to be able to warm certain parts, but not the whole body, hence, they cannot be considered to be warm-blooded. However, one fish has managed to evolve to overcome this limitation: the opah/moonfish. This discovery was made by a research team led by Nicholas Wegner from the NOAA South-West Fisheries Science Center in California. The discovery is quite surprising, since this fish has been used as a food source for a very long time, and yet, this aspect of its biology had never been noticed.

Kingdom:	Animalia
Phylum:	Chordata
Class:	Actinopterygii
Order:	Lampriformes
Family:	Lampridae
Genus:	Lampris
Species:	L. guttatus

❥ It is distributed worldwide, from the West Atlantic to the East Atlantic waters, as well as in the Pacific and Indian oceans. It lives deep in the ocean, at the mesopelagic depths of 50 - 500 meters. The temperature of its habitat is usually in the range of 8 to 22°C. ❥ It is a large, disc-shaped and keeled fish, with a distinct pattern of coloration. It can grow up to a length of 2 m, and a weight of 90 kg (nearly 200 lbs). The body shows a gradual gradation of blue on the top, to rosy pink on the belly/bottom. It has distinct white spots all over its body, with deep vermilion fins placed horizontally instead of vertically, as is the case in most fish. Its jaws are also vermilion, and the eyes are ringed with a golden-yellow color. The body is covered with circular scales, whose iridescent guanine coating is easily abraded. It has a pointed snout, with prominent lips. ❥ Opah are solitary fish, that have sometimes been observed to school with tuna. They swim by moving their fins in a flapping motion, as the fins are horizontally placed, i.e., they have a lift-based mode of swimming. This, coupled with their streamlined and forked caudal fins, allows them to swim at fast speeds of 25 cm/s to almost 4 m/s. ❥ Their diet consists of smaller fish, krill, squid, etc. Their life cycle has not been fully elucidated yet, but they are known to spawn in the spring. They lack dorsal and pelvic fins when they are born, but undergo a rapid transformation soon after to form the characteristic discoid, deep-bodied form. This find offers an intriguing speculation whether warm-bloodedness is a new evolutionary trend for fish inhabiting a cold environment, or whether it is a remnant trait of a common ancestor of modern-day mammals and birds.