Ocean Sunfish

The ocean sunfish is the world's heaviest bony fish, with adults commonly weighing over 1,000 kg. Its extraordinary body shape—flattened laterally and truncated at the rear—makes it instantly recognizable among pelagic fish. Rather than a true tail fin, it has a rounded pseudo-tail called a clavus formed by the fusion of dorsal and anal fin rays.

Found in temperate and tropical oceans worldwide, sunfish are famous for "basking," lying on their side at the surface, a behavior thought to help thermoregulate after deep dives into cold water and to allow seabirds to pick parasites from their skin. Despite their bulk they are gentle plankton feeders, specializing on gelatinous prey like jellyfish and salps that offer little nutrition, requiring them to eat enormous volumes.

• Disc-shaped, laterally flattened body with no obvious tail
• Rounded clavus (pseudo-tail) replaces a normal caudal fin
• Tall, pointed dorsal and anal fins set far back on the body
• Thick, rough, silvery-gray to brown skin, often scarred or scraped
• Small beak-like mouth fused into a parrot-like structure
• Can exceed 1,000 kg and 3 m across, dwarfing all other bony fish

Look-alikes: Sharptail mola (Masturus lanceolatus) has a pointed projection on its clavus; southern sunfish (Mola alexandrini) is bulkier with a different clavus shape and bump on the head.

Ocean sunfish roam open water in temperate and tropical seas worldwide, from near the surface down to several hundred meters. They regularly make deep dives below the thermocline, sometimes past 600 m, to feed on deep-water jellyfish and siphonophores, then return to sunlit surface waters to rewarm. They are most often encountered offshore over deep water but occasionally drift close to shore, especially where currents concentrate jellyfish blooms. Juveniles can school in shallower coastal water before adults disperse into the open ocean. Sunfish tolerate a wide range of temperatures but favor productive current boundaries and upwelling zones rich in gelatinous zooplankton.

Sunfish are solitary drifters, spending much of their time cruising slowly or basking motionless at the surface. Basking is thought to help them warm up after cold, deep foraging dives and may also attract cleaner fish and seabirds that remove external parasites, of which sunfish carry an unusually high number. They feed mainly on jellyfish, salps, and other soft-bodied plankton, consuming huge quantities to meet energy needs. Ocean sunfish are among the most fecund vertebrates known, with females capable of releasing hundreds of millions of tiny eggs in a single spawning event, though survival to adulthood is extremely low. Growth from larva to adult involves an enormous increase in body mass.