“Ocean sunfish produce more eggs than any other known vertebrate animal—a single female can release up to 300 million eggs at once.”
The ocean sunfish (mola mola) is the world's heaviest bony fish, a flattened disc that basks at the surface and drifts through open oceans feeding largely on jellyfish. It is listed as Vulnerable.
By An Nguyen · Published · Updated
Mola molaTetraodontiformesMolidae
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A closer look at habitat, diet, behavior, reproduction, and conservation.
The ocean sunfish (Mola mola) is one of the heaviest bony fish in the sea and one of the strangest-looking — a tall, flattened disc that seems to end abruptly, with no ordinary tail. It swims by sweeping its tall dorsal and anal fins, steers with a scalloped rear rudder, and, despite its bulk, lives largely on jellyfish and other soft, watery prey. Tracking shows it is no passive drifter: it travels actively up and down through the water column to feed.
The sunfish's most obvious feature is what it lacks — a proper tail fin. Over evolutionary time its ancestors lost the rear vertebrae and caudal fin, and the back edge of the body became a stiff, scalloped rudder called the clavus, formed from modified dorsal and anal fin rays. Thrust comes almost entirely from beating the tall dorsal and anal fins together, top and bottom.
Other oddities follow from its lifestyle. It has no functional swim bladder, unusual for a bony fish that dives through great changes in depth. It has no scales; instead a thick, leathery, elastic skin — up to three inches deep and coated in mucus — covers the body. And like its pufferfish relatives (both are in the order Tetraodontiformes), its teeth are fused into a rigid, beak-like plate it cannot fully close, suited to shredding soft prey. Large individuals reach roughly 6 to 10 feet across and can weigh well over 2,000 pounds, though the biggest records are exceptional rather than typical.
The ocean sunfish is found worldwide, across all tropical and temperate ocean basins — the Atlantic, Pacific, and Indian — and off coasts from the United States and Portugal to South Africa, Australia, and Japan. Although it is most often seen basking at the surface, tagging reveals it is really a deep-water animal, spending much of its time below the thermocline and sometimes diving past 2,500 feet into cold water to forage before returning to warm up.
The sunfish feeds mainly on gelatinous zooplankton — jellyfish, comb jellies, and salps — supplemented by squid, small fish, crustaceans, and even bottom-dwelling animals when scavenging. Because this prey is mostly water and very low in calories, the sunfish must eat large quantities, and it hunts on deep daytime dives where such prey gathers. Unable to close its beak, it feeds by suction, rapidly drawing water in and jetting it back out to tear the soft tissue of jellyfish and salps into swallowable pieces.
The sunfish's famous habit is "basking" — lying flat on its side at the surface after long, cold, deep dives. This is mainly rapid rewarming: exposing its huge surface to the sun restores body temperature so its heart and metabolism keep working. Basking has a second payoff. Sunfish skin harbours dozens of parasite species, and a fish at the surface invites cleaners — seabirds such as gulls and albatrosses land to pick parasites off the skin, while on coastal reefs wrasses and other cleaner fish do the same job.
Sunfish are broadcast spawners — males and females release eggs and sperm into open water for external fertilisation, with no parental care, offsetting heavy early losses with sheer numbers. One large female was famously estimated to carry on the order of 300 million developing eggs, a figure that describes potential fecundity in one examined fish, not a count released each spawning.
The growth that follows is extreme. A newly hatched larva is about 2.5 millimetres long, so an adult represents an increase in mass of roughly 60 million times. The young change shape dramatically along the way: hatchlings look like tiny spiky pufferfish with a normal tail, then lose the spines, resorb the tail into a clavus, and flatten into the adult disc. Wild sunfish are thought to live around 20 to 25 years.
The ocean sunfish is listed as Vulnerable with a declining global trend. Its biggest threat is bycatch: in offshore gillnet and longline fisheries targeting swordfish and tuna, sunfish are caught in large numbers as unintended catch. Their slow surface habits also make them prone to boat strikes. And because they hunt translucent, drifting prey, they readily mistake plastic bags and balloons for jellyfish, and swallowed plastic can block the gut and cause starvation. Orcas, great white sharks, and sea lions prey on them naturally, but human pressures drive the decline. How the sunfish actually processes jellyfish is examined in how ocean sunfish eat jellyfish.
Kingdom
Animalia
Phylum
Chordata
Class
Actinopterygii
Order
Tetraodontiformes
Family
Molidae
Genus
Mola
Species
M. mola
Ocean sunfish produce more eggs than any other known vertebrate animal—a single female can release up to 300 million eggs at once.
Despite their gargantuan adult size of over 4,000 pounds, newly hatched sunfish larvae are tiny, measuring just 2.5 millimeters long!
Sunfish lack a standard tail fin; instead, their dorsal and anal fins fold around the back to create a rudder-like structure called a clavus.
They frequently float completely flat on their sides at the ocean surface, a behavior called 'basking' that warms their bodies up after diving into cold, deep water.
Their thick, leathery skin harbors more than 50 different species of marine parasites, prompting sunfish to seek out kelp gulls and cleaner fish for pest removal.
Because their teeth are fused into a rigid beak, sunfish cannot fully close their mouths.
Select a question to reveal the answer.
The ocean sunfish, or Mola mola, is a massive, unusual marine fish with a truncated, circular body and tall vertical fins that make it look like a giant swimming head.
Ocean sunfish live in the pelagic open water zone of tropical and temperate oceans worldwide, migrating across vast oceanic expanses.
They feed primarily on gelatinous marine animals like jellyfish and salps, but also consume squid, small schooling fish, zooplankton, and brittle stars.
Sunfish bask on their sides at the surface to absorb solar heat after plunging into cold deep ocean waters to hunt, and to allow seabirds to pick parasites off their skin.
No, ocean sunfish are exceptionally docile and harmless to swimmers and divers, though their massive bulk can cause damage if struck by boats.
The Mola mola is listed as Vulnerable due to massive fisheries bycatch in offshore gillnets, boat collisions, and accidental ingestion of floating plastic bags mistaken for jellyfish.
No, ocean sunfish eat more than jellyfish. Their diet also includes salps, squid, small fish, zooplankton, brittle stars, and some algae, with prey changing across depth and location.
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A sunfish draws gelatinous prey into its small mouth, breaks it into manageable pieces, and traps it with teeth deeper in the throat.
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An ocean sunfish looks unfinished only if you expect every fish to have a long tail.
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