Taxonomy

Rarity

Abundance rates how many are in suitable habitat. Find difficulty rates how hard it is to locate one on a daylight snorkel or freedive through 50 ft. A higher find score means a harder search. The rarity category also reflects the kind of encounter; it is not an average of the 2 scores.

Seasonality

Location

Ocean sunfish live in open water and spend most of their time in the upper few hundred feet, though they have been recorded to nearly 5,000 feet. Tracking work has put fish in the top 30 feet for more than 30 percent of their time and in the top 650 feet for more than 80 percent of it. Adults also come in over slopes next to deep water, where 2 things happen: they shelter, and they seek out cleaner fish. FishBase describes them as usually shy but notes that in some places they become familiar with divers.

The famous behavior is the drifting. A sunfish will lie on its side at the surface, apparently doing nothing, which is where the name comes from and why the fish was assumed for centuries to be a passive drifter. It also swims upright near the surface with its tall dorsal fin sticking out of the water, which is how it gets mistaken for a shark.

Diet is a longer list than the jellyfish reputation suggests. Ocean sunfish eat fishes, mollusks, drifting plankton, jellyfish, crustaceans, brittle stars, salps, comb jellies and siphonophores, the last of which are colonial relatives of jellyfish. Juveniles are eaten by California sea lions in Monterey Bay. That is the only named predator in any source consulted for this entry, though a fish this size clearly has few once grown.

Deep Dive

An ocean sunfish is built wrong on purpose, and every strange thing about it is a consequence of one decision: the animal gave up its tail.

Where a normal fish has a tail fin, an ocean sunfish has a stiff rudder-like structure called a clavus, from the Latin word for rudder, edged in a wavy frill with 11 to 14 fin rays. The propulsion comes instead from the dorsal and anal fins, which are tall, short-based, and flapped from side to side in unison like a pair of wings. The result moves faster than it looks. Everything else follows the same pattern of reduction. The skeleton is largely cartilage rather than bone. There are no scales, and the skin instead is extremely thick and elastic. The gas bladder that most fish use to control depth is absent in adults. The gill openings have shrunk to a small hole at the base of each pectoral fin. The mouth is tiny, and the teeth in each jaw are fused into a single plate, giving the animal a beak like a parrot's.

From that unpromising design comes the largest bony fish in the world. FishBase records a maximum length of 10.9 feet and a maximum published weight of 1.3 tons; the Australian Museum puts the weight at over 2,500 kilograms, which is closer to 2.8 tons. The species holds the Guinness record for the heaviest bony fish and for the fish with the most eggs.

The eggs are the other superlative. A 5 foot female was found carrying 300 million of them, and the ovaries hold oocytes at different stages of development, which suggests the fish spawns repeatedly rather than releasing everything at once. That 300 million figure is the largest clutch estimate recorded for the species.

For a long time the surface drifting was read as laziness, and a 2015 study took an instrumented look at it. Researchers put an accelerometer and a camera with a light on wild sunfish and found something different from the picture. The fish were active during the day and made repeated deep excursions, feeding mainly between 330 and 660 feet, where the water sat below 54 degrees Fahrenheit. What they were catching down there was mostly siphonophores. The fish's own body temperature fell during each dive and recovered at the surface, and the heat-budget models found that a sunfish takes on heat 3 to 7 times faster than it loses it. The authors' conclusion was that the main purpose of surfacing is to rewarm, and that the warming lets the animal spend more time hunting in cold deep water than it otherwise could. Their measured thermal preference for the fish was around 61 to 63 degrees.

That is one study on one set of instrumented animals, and it is worth saying that the question is not closed. The Australian Museum notes other research suggesting surface basking is not thermoregulation at all. Both readings are in print.

One last oddity, recorded because it is the sort of thing only a fish this strange produces. A sunfish that stranded in the Azores in 2004 was found with a living colony of goose barnacles attached inside its esophagus. The barnacles had a good arrangement, with a steady food supply and shelter from currents and predators. Whether the fish was harmed, helped, or merely inconvenienced was not established.

Where to Find Them

Researched habitat and range candidates

These locations have suitable habitat within the species range. An assignment is a possible encounter, not a confirmed sighting at every beach. Season, depth, and local conditions still matter. Historical habitat without present-day evidence is excluded.