Fish
Ocean Sunfish
Mola mola · also called common mola, mola
The ocean sunfish looks like a mistake — a giant fish head that swims off before its body finished growing. It is in fact the heavyweight champion of the bony fish, a specialised jellyfish-eater that drifts and dives across the open ocean and produces more eggs than any other backboned animal.
Overview
Ocean sunfish live in warm and temperate seas worldwide, ranging from the surface to around 600 metres. They spend their time alternating between deep foraging dives in cold water and long spells recovering at the surface, where they are often spotted lying on one flank like a floating dustbin lid. They are usually solitary. Despite their bulk they are gentle, curious, and harmless to people.
An unusual body
A sunfish is built as a tall, flattened oval. It has:
- No tail. The rear of the body is a stiff, wavy-edged rudder, the clavus, formed as the tail fin’s precursors fold inward during development.
- Towering dorsal and anal fins that beat side to side in unison to drive the fish forward — the main means of propulsion.
- A small, beak-like mouth with fused teeth, permanently slightly open, suited to sucking in soft prey.
- Rough, leathery skin up to several centimetres thick, coated in mucus, in place of normal scales.
Diet
Sunfish were long assumed to be lazy jellyfish specialists, and jellyfish and other gelatinous plankton — salps, comb jellies, siphonophores — do form a large part of the diet. But studies of stomach contents and diving behaviour show they also eat small fish, squid, crustaceans, and seafloor invertebrates, hunting at a range of depths. Because gelatinous prey is so low in energy, a sunfish must eat a great deal of it.
Reproduction
The ocean sunfish is the most fecund vertebrate known: a single large female can carry an estimated 300 million tiny eggs. Fertilisation is external, in open water, and the larvae are minute, spiky, and pufferfish-like — the sunfish belongs to the same order as pufferfish and triggerfish. Almost all of the hundreds of millions of offspring die young; only a handful survive to the enormous adult size.
Threats and conservation
Mola mola is listed as Vulnerable. It has little commercial value as food in most countries, but it is caught as bycatch at very high rates — hooked on longlines, tangled in driftnets set for swordfish and tuna, and scooped up in trawls. Ingested plastic and, in some places, targeted fishing add to the pressure. Its slow reproduction to maturity and the scale of incidental catch make the declines hard to reverse without changes to fishing gear and practice.
Ocean Sunfish: common questions
How big does an ocean sunfish get?
It is the heaviest bony fish alive. Ordinary adults weigh 250 to 1,000 kg, and exceptional individuals exceed 2,000 kg — the record, a bump-head sunfish caught off Japan, weighed 2,744 kg and measured 3.3 m long. Only cartilaginous fish like the whale shark and basking shark are heavier. From an egg 2 mm across, a sunfish can grow to 60 million times its birth weight.
Why do sunfish lie on their side at the surface?
Several reasons. After deep dives into cold water to feed on jellyfish, a sunfish is chilled and returns to the surface to re-warm in the sun — hence "sunfish." Lying flat also lets seabirds such as gulls pick parasites off its skin, and the fish will visit cleaner-fish stations on reefs for the same service. It sometimes breaches clear of the water, possibly to dislodge parasites.
Does the ocean sunfish really have no tail?
Correct — it never develops a true tail fin. During growth the rear of the body folds in on itself, and the tall dorsal and anal fins take over propulsion, sculling from side to side while a stiff, frilly rudder called the clavus steers. This gives the sunfish its strange half-a-fish silhouette and a slow but surprisingly capable swimming style; tagged sunfish cover long distances and dive repeatedly to hundreds of metres.
Are ocean sunfish endangered?
The common ocean sunfish is listed as Vulnerable, with populations declining in parts of its range. The main threat is bycatch — sunfish are caught in enormous numbers in gillnets, longlines, and trawls targeting other species, and in some Mediterranean and South African fisheries they make up a large fraction of the catch. They also frequently eat floating plastic bags, mistaking them for jellyfish.