The ocean sunfish is the heaviest bony fish on Earth, reaching weights of over 2,000 kilograms and lengths of 3 meters. Few fish look quite like it: the sunfish appears to be all head, a massive oval body with two tall fins, one on top and one on the bottom, that it uses to propel itself through the water with a slow, deliberate sculling motion. It cruises open ocean with a calm that suggests it has nothing to prove, and given its size, it probably doesn’t.
The sunfish’s primary food is jellyfish, which are almost entirely water and have almost no nutritional value. To sustain a 2,000-kilogram body on jellyfish, a sunfish must eat an enormous quantity of them continuously, which it does, cruising through open ocean and consuming whatever it encounters. It also eats salps, squid, and small fish when available, but jellyfish are the staple.
The sunbathing behavior that gives the fish its name is real and has a practical function. Sunfish dive to considerable depths to feed, where the water is cold, and they return to the surface to warm up, lying on their sides in the sun to raise their body temperature before diving again. They are also visited at the surface by seabirds that pick parasites from their skin, a cleaning service the sunfish appears to actively seek out. A sunfish lying at the surface is not sick or dying. It is thermoregulating and getting a cleaning.
The skin is up to 7 centimeters thick, tough and rubbery, and covered in a layer of mucus. It has no scales. The skeleton is largely cartilaginous rather than bony, which reduces weight for an animal that needs to move large volumes of water to stay upright. The tail fin, called a clavus, is not a true tail but a fusion of the dorsal and anal fins — meaning the sunfish is essentially swimming without a tail in the conventional sense.
The Parasite Load
Sunfish carry more parasite species than almost any other fish. Over 40 species of parasites have been recorded on a single individual, living in the skin, gills, and internal organs. The surface-cleaning behavior described above — where birds and smaller fish remove external parasites — is one response to this. Sunfish have also been observed leaping completely out of the water, a behavior thought to dislodge parasites through the impact of re-entry. It is a dramatic solution to an itching problem from an animal that does not look built for jumping.
Sunfish in the Open Ocean and Conservation Status
The leaping sunfish is also a hazard to small boats. A 2,000-kilogram fish landing on a vessel after a breach has caused damage and injury in documented cases- a risk most ocean-going animals simply do not present.
Sunfish are found in tropical and temperate oceans worldwide, spending most of their lives in open water far from shore. Despite their size, they are vulnerable to human activity. They are caught as bycatch in longline and drift net fisheries, and they are affected by plastic pollution- mistaking plastic bags for jellyfish and ingesting them. The species is listed as Vulnerable on the IUCN Red List, with detailed life-history data for Mola mola catalogued by NOAA, showing populations declining due to bycatch mortality across their range.
A fish that weighs two tons and survives on jellyfish, that sunbathes to warm up after deep dives and leaps clear of the ocean to shake off parasites- the sunfish is one of those animals that makes you reconsider your assumptions about what a fish can be. Discover more wildlife stories on the Wild Wonders Daily blog.