Ocean Sunfish (Mola mola) — identification, habitat & look-alikes

The ocean sunfish is a flat, disk-shaped fish that looks like it is all head, and it is one of the largest bony fish in the world. It is seen off both the Pacific and Atlantic coasts of the US, often floating on its side at the surface, likely to warm up after deep dives. It is also called the common mola.

Appearance

Appearance: Adults average about 6 ft (1.8 m) long and often weigh 550–2,200 lb (250–1,000 kg). Instead of a tail it has a short, rounded rudder called the , and tall dorsal and anal fins that it sculls together to swim.

Look-alikes

Hoodwinker sunfishMola tecta

The hoodwinker has a slimmer, smoother body with no bump on the head, no protruding snout, and no lumps along the edge of the clavus. Experts often need clear photos to confirm it.

Why they get mixed upIt was only described in 2017, and several have washed ashore in California (near Santa Barbara in 2019, Arcata in 2021) where people first assumed they were ocean sunfish.

Sharptail molaMasturus lanceolatus

Look at the back end: the sharptail has a pointed, triangular lobe sticking out from the middle of its rudder (clavus), while the ocean sunfish's rudder edge is short and rounded.

Why they get mixed upIt is a big, disk-shaped sunfish of warm seas and has been recorded off the US Atlantic and Gulf coasts.

Where it lives

Typical habitats: Open ocean
Coastal waters
Range: Lives in tropical and temperate waters of every ocean. In the US it is recorded in national marine sanctuaries such as Olympic Coast (Washington), Cordell Bank (California), and Stellwagen Bank (Massachusetts).
Preferred conditions: Spends much of its time hunting deeper than 650 ft (200 m) and prefers water warmer than 50°F (10°C). Long stays in water at 54°F (12°C) or colder can leave it disoriented and kill it.

Behavior

Diet: Once thought to live on jellyfish, it actually eats a wide mix: small fish, squid, crustaceans, fish larvae, and jellyfish and other soft-bodied animals.
Activity: Often floats on its side at the surface, likely to warm up after cold, deep dives. Its tall dorsal fin breaking the surface is sometimes mistaken for a shark fin. It can quickly darken its skin when threatened.
Social life: Usually solitary. Seabirds and small fish pick parasites off its skin. It is gentle, harmless to people, and often approaches divers.

Life cycle

Reproduction: Females release huge numbers of eggs into the water, where they are fertilized. The often-quoted figure of 300 million eggs comes from a count of unripe eggs in one ovary, so the real number per spawning is uncertain.
Larva: Hatchlings are about 0.1 in (2.5 mm) long and look like tiny spiny pufferfish. Survivors may grow up to 60 million times their starting weight.
Lifespan: Not well known in the wild. In aquariums they have been reported to live up to about 10 years.

Conservation

IUCN Red List: VUVulnerableHigh risk of extinction in the wild
Status: is the main threat: sunfish made up nearly 30% of the catch in California's drift gillnet swordfish fishery. They also swallow plastic bags they mistake for jellyfish.

Culture and stories

Culture and stories: The name mola is Latin for millstone, which the round, gray, rough-skinned fish resembles.
Monterey Bay Aquarium received the first ocean sunfish exhibited in the US in 1986, and by 1998 its staff could keep one for more than a year. They are hard to keep because they bump into tank walls and need very large tanks.
Selling sunfish as food is banned in the European Union, but it is eaten in Japan, Korea, and Taiwan.