Sunfish or Mola mola, is one of the two genera of round-tailed ocean sunfishes (Molidae) seeing in the Indian seas, the other genus being Ranzania. These sunfishes are oval-bodied, with thick leathery skin and a small mouth with teeth fused together to form a single sharp-edged bony plate or beak in each jaw. Pelvic fins are absent and the dorsal and anal fins are extended vertically. The caudal fin is short and spread out along the entire rounded hind margin and hence the epithet 'round-tailed'. Their coloration is uniformly grey to olive brown with silvery reflections on the sides. It is reputed to grow a little over three metres and weigh more than a tonne. Because of absence of caudal portion of the body and backward disposition of the dorsal fin, the fish appears to have an enormous head only and is sometimes referred to as a 'head fish'.

The Sunfish is an inhabitant of the open oceans, often seen basking in the sun, laying on its side at the surface during calm weather, drifting more or less passively with ocean currents and feeding upon small fish, jellyfishes and crustaceans. With a semi-rounded body and no caudal portion, propulsion through water with speed must be a real problem and its dependence on ocean currents seems natural. Some people in the past attributed intelligence to this fish on account of its seemingly large head region, but it will allow a boat to come right up to it without making any effort to escape. It is found to have a very small brain despite its huge body.

The fish has a very thick skin having a layer of five to seven centimetres thick gristly material, making harpooning difficult. The skin is said to be impervious even to a bullet from a gun. When accidentally captured in a net it is said to make grunting sounds. Its outstanding feature is the enormous number of eggs (300 million) found in its ovary. The newly hatched larva is very tiny and elongated like a normal fish but later on it loses its tail portion and thereafter a short tail is developed on the rounded edge of the truncated body.