Ocean Sunfish

The ocean sunfish (Mola mola), also called the headfish, is so named because of its unique shape: it looks as if it is all head and no body.

The ocean sunfish is a very large species that lives in tropical and temperate waters. The ocean sunfish has a flattened, oval body that may measure 11 ft (3.5 m) in length and weigh as much as 2,000 lb (1,000 kg). In contrast to its huge size, it has a vertebrae column of only 0.5 in (12 mm) in length. The body is oval and has a thick leathery skin. Most ocean sunfish are gray, olive brown, sometimes nearly black, with light undersides.

The ocean sunfish has a snout which protrudes out beyond a small mouth, which consists of both an upper and lower jaw. The jaws are toothed and are joined to form a single, sharp-edged beak.

The fins of the ocean sunfish are distinctive: there is a single long dorsal fin extending from the top and an equally long anal fin. The body ends abruptly with a low tail fin, and a rounded and wavy tail. The pointed-tailed sunfish (Mola lanceolata), has a tail drawn out into a point in the middle. The oblong-shaped sunfish (Ranzania truncata) has a tail with a more rounded margin. These two species are smaller than the Mola mola, with the Ranzania truncata seldom exceeding 2 ft (60 cm).

The young of ocean sunfish are a relatively normal fish shape compared to the shape of the adult. A captured female sunfish had approximately 300 million eggs in its ovaries. The larvae of the sunfish are about 0.10 in (2.5 mm) long and similar in shape to conventional fish. The shape soon changes with the growth of both the anal and dorsal fins, and the body becomes covered with spines. This coat of spines is then lost until there are only five spines left. These five long spines shorten until they are lost completely. After this stage the bulky, disc-like, body begins to form. The young ocean sunfish is then about 0.5 in (12 mm) long.

In order to steer its way through the ocean, the ocean sunfish waves both the anal and dorsal fins in unison from side to side. These fins add a twisting motion as they wave. The small, continuously flapping, pectoral fins are thought to only act as stabilizers, having no effect on the propulsion or steering of the animal. The tail is used as a rudder. Steering is accomplished by the use of the gills. The sunfish steers itself by squirting a strong jet of water out of one gill opening or the other, or out of its mouth. The food of the ocean sunfish consists of plankton, jellyfish, shellfish, crustaceans, squid, and small fish, so speed is not essential. The life of ocean sunfish is very simple and does not require much intelligence, and its brain is smaller than its spinal cord.

The ocean sunfish has frustrated harpooners for many years. When pierced by a harpoon, a sunfish makes no attempt to take evasive action, but rather makes sounds described as sighing, groaning, or grunting. These sounds are made by grinding their throat teeth together and may or may not indicate distress. No evasive action is necessary because the ocean sunfish has about 2-3 in (5-7.5 cm) of gristle under its tough skin. Harpooners have been known to try dozens of times before piercing this skin. Indeed, it has even been said to be bullet-proof.

Often ocean sunfish are seen sunning themselves on the ocean surface, most often during calm weather. At times, the ocean sunfish may go down to a depth of about 650 ft (200 m). The sunfish is most often seen singly or in pairs, but at certain times of the year they may come together in schools of a dozen or more. Basking in the sun by sunfish has often been disputed. Some believe that sunfish seen at or near the surface must either be dead or dying. A biologist has investigated this phenomenon and concluded that ocean sunfish in this position are, for the most part, sick or dying. Other underwater investigations have shown that the ocean sunfish, when at rest, goes to a darker color and when it begins to swim the color changes to a very light shade.