Wildlife Guide · Temperate and tropical oceans worldwide & Open ocean, often near the surface

Ocean Sunfish

Mola mola (Linnaeus, 1758) — family Molidae

A fish that looks like a head with fins, grows 60 million times its birth weight, and basks on its side.

Vulnerable — the heaviest bony fish alive

Ocean sunfish drifting upright in open blue ocean

Image: AI-generated educational wildlife portrait — World Animal Rescue Network · AI visualisation, not a photograph

In brief

The ocean sunfish (Mola mola) is the heaviest bony fish alive at over two tonnes, has no true tail fin, feeds largely on gelatinous plankton, and grows to around 60 million times its larval weight.

By the WARN Research & Conservation TeamChecked against IUCN Red List & CITES sourcesLast updated July 2026

The ocean sunfish is the heaviest bony fish alive, reaching over two tonnes, and one of the strangest-looking animals in the sea. It appears to be all head — the body ends abruptly in a rudder-like structure called a clavus, formed where a normal tail fin would be, giving it a truncated, disc-shaped profile. It begins life as a larva a few millimetres long covered in spines, and can grow to roughly 60 million times that weight, one of the most extreme size changes of any vertebrate.

2,300 kg

Heaviest recorded — the heaviest bony fish

~60 million×

Weight gain from larva to adult

No tail fin

Steers with a clavus formed from fin rays

~600 m

Depths reached while foraging

Figures sourced from IUCN Red List — Mola mola, Smithsonian Ocean — Fish .

Ocean Sunfish: Quick Facts

Vital statistics and key facts about the Ocean Sunfish
Maximum weight	~2,300 kg
Relatives	Pufferfish and triggerfish (Tetraodontiformes)
Tail	None — steers with a clavus
Buoyancy	No swim bladder; cartilage and gelatinous tissue instead
Dive depth	Several hundred metres
Main threat	Bycatch in drift and gill nets

Scientific Classification

Scientific classification of the Ocean Sunfish
Kingdom	Animalia
Phylum	Chordata
Class	Actinopterygii
Order	Tetraodontiformes
Family	Molidae
Genus	Mola
Species	Mola mola

Conservation Status

IUCN Status: Vulnerable
Population: Unknown
Trend: Decreasing
Assessed: 2015
CITES: Not listed

Sunfish are rarely targeted but form a very large share of the catch in some drift-net fisheries. Plastic ingestion is a documented cause of death.

Did you know?

An ocean sunfish larva is a few millimetres long and covered in spines; the adult can weigh 60 million times as much.

Key Facts: Ocean Sunfish

Ocean sunfish are the heaviest bony fish alive, exceeding two tonnes.
They have no true tail fin — a rudder-like clavus takes its place.
Larvae are a few millimetres long and covered in spines.
They dive to several hundred metres and bask at the surface to rewarm.
Bycatch in drift and gill nets is the principal threat.

A body built by subtraction

Sunfish lose the tail fin during development. What remains is the clavus, a stiff rudder formed from modified fin rays, which the fish uses for steering while the tall dorsal and anal fins provide propulsion, sculled from side to side. The skeleton is largely cartilage rather than heavy bone, and the body carries a thick layer of gelatinous tissue under the skin. That combination keeps a two-tonne animal buoyant without the swim bladder that most bony fish rely on.

Basking, and why

Sunfish are often seen lying on their side at the surface, which produced the common name. This is thermoregulation: after diving into cold water at several hundred metres to feed, the fish returns to the surface to rewarm in sunlight. The posture may serve a second purpose. Surface-basking sunfish are visited by seabirds that pick parasites off the skin, and the fish also visit cleaner-fish stations on reefs — a large animal with a heavy parasite load using two different cleaning services.

Diet and the jellyfish problem

Sunfish were long described as feeding almost entirely on jellyfish. Better dietary studies show a broader diet including small fish, crustaceans, squid and salps, though gelatinous prey remains important. That gelatinous preference creates a specific hazard: floating plastic bags closely resemble jellyfish, and sunfish ingest them. Plastic ingestion is a documented cause of death in the species.

Why they are Vulnerable

Sunfish are rarely targeted commercially, but they are caught in enormous numbers as bycatch. In some drift-net and gill-net fisheries sunfish have made up a very large share of the total catch, and they are also taken in longlines. They are listed as Vulnerable. Plastic ingestion, vessel strikes at the surface and the sheer scale of incidental capture are the pressures; targeted fishing is not.

What WARN Does

WARN publishes free, sourced species education for animals that decline through incidental capture and marine litter rather than deliberate hunting. Sunfish illustrate both at once — a species almost nobody fishes for, dying in nets set for something else and eating plastic mistaken for jellyfish.

If this guide helps you understand wildlife and the pressures it faces, a gift to WARN supports habitat protection and free public education in our partner countries.

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How can I help save the ocean sunfish?

There are three practical things that make a real difference for the ocean sunfish — and the first is the one that funds everything else.

Fund frontline rescue

Back the partner-led teams, veterinary care and protection that reach ocean sunfishs where the need is greatest — WARN channels donations straight to that work.

Ease the pressures

Avoid the wildlife trade and souvenirs, never buy wild animals as pets, and choose sustainable, habitat-friendly products — the threats behind the ocean sunfish's decline fall when demand does.

Spread the word

Share accurate, sourced information about the ocean sunfish. Wider awareness is what turns concern into lasting protection.

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Ocean Sunfish: Frequently Asked Questions

How can I help save the ocean sunfish?

The most effective way to help the ocean sunfish is to fund the frontline rescue and habitat protection that keeps the species alive — WARN channels donations to partner-led teams working where the need is greatest. You can also ease the pressures behind its decline by avoiding the wildlife trade and souvenirs, never buying wild animals as pets, and choosing sustainable products, and by sharing accurate information to build wider support. See the ways to help below, or support WARN's Support Habitat Protection.

What is the heaviest bony fish?

The ocean sunfish, at over two tonnes. Larger fish exist — whale sharks and basking sharks — but those are cartilaginous, so the sunfish holds the record among bony fish.

Why do sunfish lie on their side at the surface?

To rewarm. Sunfish dive to several hundred metres into cold water to feed, then return to the surface and bask in sunlight to raise their body temperature. Seabirds also pick parasites from basking sunfish, so the behaviour serves twice.

Do sunfish only eat jellyfish?

No — that was the traditional description, but better dietary studies show a broader diet including small fish, crustaceans, squid and salps. Gelatinous prey is still important, which is why floating plastic bags are dangerous to them.

Why don't sunfish have a tail?

They lose the tail fin during development. A stiff rudder called the clavus, formed from modified fin rays, takes its place, while the tall dorsal and anal fins provide propulsion by sculling side to side.

What do ocean sunfish / molas eat?

Ocean Sunfish / Molas eat 2,300 kg — Heaviest recorded — the heaviest bony fish. The ocean sunfish (Mola mola) is the heaviest bony fish alive at over two tonnes, has no true tail fin, feeds largely on gelatinous plankton, and grows to around 60 million times its larval weight.

Where do ocean sunfish / molas live?

Ocean Sunfish / Molas live in Temperate and tropical oceans worldwide, Open ocean, often near the surface. The ocean sunfish (Mola mola) is the heaviest bony fish alive at over two tonnes, has no true tail fin, feeds largely on gelatinous plankton, and grows to around 60 million times its larval weight.

Are ocean sunfish / molas dangerous?

Ocean Sunfish / Molas are not generally dangerous to people. The ocean sunfish (Mola mola) is the heaviest bony fish alive at over two tonnes, has no true tail fin, feeds largely on gelatinous plankton, and grows to around 60 million times its larval weight. Give wild individuals space and do not corner or handle them.

How long do ocean sunfish / molas live?

A precise wild lifespan for ocean sunfish / molas is not firmly established in the sources cited on this guide. The ocean sunfish (Mola mola) is the heaviest bony fish alive at over two tonnes, has no true tail fin, feeds largely on gelatinous plankton, and grows to around 60 million times its larval weight. Where longevity figures appear in the literature they vary by region, sex and whether animals are wild or captive.

How big is an ocean sunfish / mola?

An Ocean Sunfish / Mola is about ~2,300 kg. The ocean sunfish (Mola mola) is the heaviest bony fish alive at over two tonnes, has no true tail fin, feeds largely on gelatinous plankton, and grows to around 60 million times its larval weight. Conservation status: Bycatch in drift and gill nets.

Sources & References

Population figures, conservation status and biological facts on this page are drawn from authoritative scientific and intergovernmental sources.

Terms used on this page

Plain-English definitions, from our wildlife and conservation glossary.

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