Opah

Overview

The Opah (Lampris guttatus), also celebrated worldwide as the moonfish, sunfish, or Jerusalem haddock, is a magnificent, deep-bodied pelagic marine teleost that achieved biological fame in 2015 when marine biologists made a historic discovery: it is the first and only fully warm-blooded (whole-body endothermic) fish known to science.

With its enormous, disc-shaped silvery-blue body speckled with white polka dots, bright crimson fins, and large golden eyes, the opah cuts an unforgettable visual figure in the open sea. Unlike other deep-water fish that are sluggish ambushers, the opah’s warm internal body temperature allows it to be an active, high-speed predatory pursuit hunter in the chilly, dim twilight depths of the global ocean.

Biology and Physical Characteristics

Lampris guttatus possesses an extraordinarily deep, laterally compressed oval body resembling a shimmering silver plate. Its dorsal coloration is a rich steely-blue that grades into delicate rose-pink along the belly, adorned with hundreds of irregular, gleaming white spots. Its large, sickle-shaped pectoral, dorsal, and anal fins are a brilliant, glowing vermilion to bright scarlet, while its huge gold-ringed eyes are adapted to capture faint downwelling light.

The opah’s revolutionary circulatory system is a masterpiece of thermal engineering: its massive pectoral swimming muscles generate continuous internal heat, which is conserved by thick layers of insulating blubber. Within its gills sits an intricate counter-current heat exchanger (rete mirabile) that warms cold, oxygenated blood returning from the gills before it circulates into the body, maintaining core muscle and cranial temperatures up to 5°C (9°F) warmer than ambient water.

Behavior and Ecology

Because its warm-blooded physiology grants enhanced muscle performance, faster neural processing, and sharper eyesight in freezing waters, the opah operates as an aggressive, high-speed chase predator. While most mesopelagic fishes hover passively to conserve energy, the opah flaps its powerful, crimson pectoral fins like wings, cruising effortlessly between 100 and 400 meters depth to pursue agile deep-sea squids and schooling lanternfish.

Opah are solitary wanderers that rarely school, roaming vast oceanic gyres and submarine ridges alone. They undergo daily vertical migrations, staying in deep, cold mesopelagic zones during daylight hours and following squid upward into shallower layers at night. Spawning takes place offshore in open ocean waters, where females broadcast millions of floating pelagic eggs.

Conservation and Human Impact

The Opah is classified as Least Concern on the IUCN Red List due to its enormous circumglobal distribution in open international waters. While not typically targeted by dedicated commercial fisheries, it is frequently taken as highly prized incidental bycatch in commercial pelagic longline fisheries for tuna and swordfish in Hawaii, California, and the South Pacific.

Its rich, fatty, multi-textured flesh has made it increasingly popular in upscale gourmet restaurants, where it is marketed as a premium seafood delicacy. Regional pelagic fishery observer programs closely monitor opah catch rates to ensure that longline bycatch mortality remains within sustainable ecological boundaries.

Interesting Factoid

The opah is the only known fully warm-blooded fish on Earth; specialized heat exchangers in its gills allow it to keep its heart, brain, and muscles up to 9 degrees Fahrenheit warmer than the freezing deep ocean waters it hunts in.