Found: First Warm-blooded Fish Identified

Catching an opah was believed to bring good luck, and therefore, fishermen who caught it often gave it away, rather than sell it.

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.

Scientific Classification
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, 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 fish is able to sustain its core body temperature at least 5°C above the ambient temperature. This is possible due to the presence of a special network of insulated blood vessels between the heart and the gills.

The insulation acts to keep the blood pumped out of the heart to remain warm, so that it would reheat the oxygenated blood leaving the gills before it went to the rest of the body, thereby allowing the body core temperature to remain warm.

This is possible since the blood vessels carrying the warm blood from the heart to the gills are arranged around the vessels passing through the gills going to the heart.

Due to this arrangement, deoxygenated blood reaches the gills, where it is cooled and oxygenated. But once it leaves the gills, the proximity of the insulated vessels reheat the oxygenated blood, causing the entire body to receive warm oxygenated blood.

The entire process by which heat is transferred between the two blood vessels flowing in opposite direction is known as counter-current heat exchange. This is aided by the generation of heat due to the muscle activity of the fins.

The heat is retained throughout the body with the help of an insulating layer of fat along the skin. Due to all these factors, it is the first endothermic fish to be discovered.

Being warm-blooded in a cold environment is of great advantage, that allows this fish to be an active predator. Due to its endothermic nature, it can move faster and actively capture prey as compared to its cold-blooded counterparts who are slow and feed by waiting for the prey to drift nearby.

It also enable it to dive to deeper depths in search of food or to evade its own predators, and have a better sense of vision and speed. Also, cold-blooded fish cannot stay in deep waters for a long amount of time due to the danger of their organs shutting down because of the temperature.

Fortunately, the warm-bloodedness of the opah overcomes this limitation, allowing it to stay deep for long periods of time.

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.