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Despite looking like it is missing half of its body, the ocean sunfish is the largest bony fish, with a front-to-back length of 10 feet, top-to-bottom height of 14 feet, and weight of 4,900 pounds being fairly common.
The ocean sunfish's leathery skin is almost three inches thick on the ventral surface, and is covered by a layer of denticles and mucus instead of scales. Overall coloration ranges from brown to silvery-grey or white, with a variety of mottled skin patterns. This fish exhibits the ability to vary skin coloration from light to dark, especially when under attack.
The teeth in both the upper and lower jaws are joined to form a single sharp-edged beak, making it impossible for the ocean sunfish to completely close its relatively small mouth. As it grows, the caudal fin of the ocean sunfish is replaced by a structure known as a clavus, which is used more like a rudder than for propulsion. The dorsal and anal fins are quite prominent. Adult ocean fish do not possess a lateral line, and only one gill opening is visible on each side, which is located near the base of the small pectoral fins.
Distribution and Habitat
Ocean sunfish are found in the temperate and tropical regions of the Atlantic, Indian, and Pacific Oceans, and in the Mediterranean Sea. They are thought to migrate to higher latitudes during the spring and summer months to pursue their migrating zooplankton prey.
They prefer the open ocean but will occasionally venture into kelp beds and deep coral reefs in order to take advantage of the "parasite removal services" offered by reef-dwelling fish. Although they are often seen at or near the surface, ocean sunfish are known to live and feed at depths of about 600 feet, and may go even deeper.
Jellyfish make up the bulk of the ocean sunfish's diet, but it also feeds on zooplankton, small crustaceans and fish, and even eel grass.
Little is known about the breeding behaviors of ocean sunfish. It is known, however, that it begins life as a very small (about 1/10 inch) "normal looking" fish, complete with full tail fin. As it grows the fish becomes round and the body becomes covered with spines (right). Those spines are gradually lost until only 5 long ones are left, which then shorten until being lost completely and the adult body shape begins to appear. Based on observations in marine aquariums, ocean sunfish appear to go from larval to adult shape in about 15 months.
Other Habits and Behaviors
Ocean sunfish are usually seen singly or in pairs, but may come together in schools of a dozen or so at times. Those schools most likely form when there is an abundance of parasite-eating fish, and possibly when zooplankton concentrations are high.
Not surprisingly, ocean sunfish are fairly clumsy swimmers. They use their dorsal and anal fins as their primary means of locomotion. They flap these fins in a synchronous motion, which also allows them to swim on their side, and by waggling their dorsal and anal fins to move and steering with their clavus.
Ocean sunfish have been seen diving below the thermocline during the day, possibly to forage for zooplankton that migrate vertically. They have also been observed basking at the surface of the water on their side, drifting with the ocean current, which may be an attempt to re-warm core body temperature after diving into colder water. Ocean sunfish will also "fly out of the water" to a height of 10 feet or so in the air and then land with a great splash, an action which is most likely undertaken as a way to get rid of parasites.
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This page was last updated on August 27, 2018.