Mola Mola (Ocean Sunfish): Size, ID and Facts

An ocean sunfish looks like a huge fish head that has been sliced off behind the fins, and nothing else in the sea looks like it. The body is flattened side to side, silvery grey, and covered in rough skin. A tall triangular dorsal fin sticks up and a matching anal fin points down, and the animal often sculls them in an oar-like way that people mistake for a shark.

The giveaway is the back end. A mola has no normal tail. Instead it has a clavus, a scalloped band of muscle along the blunt rear edge that works as a rudder. Look also for:

• A tiny round mouth that does not close, set in a big face
• Large eyes relative to the small mouth
• Two small pectoral fins low on the sides
• Skin that looks scuffed, often carrying pale blotches and barnacle scars

Inside that small mouth are two pairs of hard tooth plates with a slightly curved ridge, closer to a bird's beak than to rows of teeth. A sunfish sucks food in and blows it out again to break it into pieces it can swallow. If you have a photo and are not sure what you are looking at, you can photograph it and the app identifies it.

Molas in Monterey Bay grow to around 1,000 pounds (454 kg), and Oceana gives adults of the species at roughly 3 metres long with individuals up to 2,235 kg (4,927 lb). The larger relative, the giant sunfish Mola alexandrini, reaches about 6,000 pounds.

The heaviest bony fish ever measured was a Mola alexandrini found floating dead off Faial Island in the Azores on 9 December 2021. It weighed 2,744 kg (6,049 lb) and measured 3.25 m (10 ft 8 in) in total length, with 3.6 m between the fin tips. Guinness World Records lists it as the heaviest bony fish, and the measurements were published in the Journal of Fish Biology on 11 October 2022. The previous holder, also a bump-head sunfish, came from Japanese waters in 1996 at 2,300 kg.

Getting there is the strange part. A mola hatches from an egg smaller than a grain of rice and multiplies its weight roughly 60 million times on the way to adulthood. Oceana notes that young molas can add up to 0.82 kg (1.8 lb) in a single day.

Small molas are generalists. They forage on crustaceans, squid, small fish, other invertebrates and algae. Once a mola passes about 70 cm the diet narrows and gelatinous prey takes over: moon jellies, comb jellies and similar soft-bodied drifters.

That is a thin diet for such a large animal, which is part of why molas dive deep and travel far. They are strong divers that move between the warm surface and much colder water below, and they cover long distances at depth rather than staying put.

The diet also creates the biggest everyday hazard they face. A floating plastic bag looks very much like a jelly from below, and Monterey Bay Aquarium lists discarded bags as a known danger to molas.

The basking is where the name sunfish comes from. After diving into cold water a mola comes back up and lies flat on its side at the surface, soaking up warmth.

There is a second reason to come up. Molas carry a remarkable parasite load — Monterey Bay Aquarium counts nearly 40 different kinds of parasite on and in them, and researchers have even found gooseneck barnacles inside a mola's throat. Basking at the surface puts the fish where seabirds can pick parasites off the skin, and near reefs and kelp it lets cleaner fish work the same job.

Adult molas are still eaten. Sea lions, sharks and killer whales all take them.

The main pressure on molas is fishing gear they were never the target of. Drift gillnets catch large numbers of them as bycatch, and Oceana estimates populations are falling by at least 30% every 24 to 30 years. Plastic bags mistaken for jellies are the other named threat.

A mola at the surface is not a danger to people. It has no sting, no biting teeth to speak of, and no interest in swimmers. The risk runs the other way: a 1,000-pound animal drifting just under the surface is a real hazard to small boats, so slow down rather than steer close.

If you find one stranded or clearly injured on a beach, contact the local marine mammal and wildlife stranding network or state fisheries office rather than moving it yourself.

Mola mola are docile, but that is not an invitation to touch them. Wikipedia's summary of the research describes ocean sunfish as docile and posing no threat to divers, and notes that at some dive sites they have reportedly become familiar with divers. The US Fish and Wildlife Service asks divers not to touch molas, not to shine lights at them and not to harass them, because a disturbed mola leaves the cleaning station before small fish have removed its parasites. That can lead to secondary infections and can stop the fish from returning to the site.

The skin is also not what it looks like from a boat. It is rough, coated in mucus and home to more than 40 kinds of parasite (Wikipedia). Touching strips mucus from the fish and gains you nothing.

The injuries on record run the other way: a mola leaping clear of the water. Wikipedia records a 2005 case off Pembrokeshire, Wales, in which a 1 m sunfish jumped onto a family boat and slightly injured a four-year-old boy. Collisions between boats and sunfish are common in some waters and damage hulls and propellers. The practical rule is to watch from a distance and slow the boat down.

This page does not tell you that sunfish is safe to eat, and the legal and toxicological record gives good reasons not to.

• It is eaten in parts of Asia. Wikipedia lists sunfish as a delicacy in Japan, Korea and Taiwan, with Taiwan and Japan as the largest markets. USFWS notes that in Taiwan, Japan and the Philippines the intestines are eaten, sold as "dragon intestine".
• It is banned for sale in the European Union. Under Regulation (EC) No 853/2004, fishery products from the family Molidae may not be placed on the EU market because they contain toxins harmful to human health (as summarised by Wikipedia).
• The toxin question is unsettled. Molas belong to the same order as pufferfish, Tetraodontiformes, and share the four-toothed beak. Some sources say their internal organs contain tetrodotoxin, the puffer poison; others dispute it (Wikipedia). With a disputed toxin, a family-wide EU ban and a Vulnerable listing, there is no responsible case for eating one.

The short answer to "why can't we eat sunfish" is therefore: in the EU it is illegal to sell, its organs may carry the same class of toxin as puffers, and the IUCN lists the species as Vulnerable with a falling population. If anyone develops numbness or tingling of the lips or hands after eating any fish from this group, call 911 and Poison Control at 1-800-222-1222 in the US, or the local emergency number and poison centre elsewhere. See the puffer fish page for how tetrodotoxin poisoning develops.

The species is not rare in the sense of being confined to one place: molas live in tropical and temperate oceans worldwide, and USFWS says sunfish can range farther north than the Arctic Circle and farther south than Cape Town. What makes a sighting feel rare is behaviour and decline.

• They spend most of their time deep. USFWS reports dives to 1,000 m (3,280 ft) and up to 40 deep dives a day; Cape May Whale Watch cites dives to 800 m. You only see a mola when it comes up to bask.
• Sightings are irregular even where they occur. Cape May Whale Watch logged molas on 26 May 2022 as its research vessel left the inlet and again on 4 June 2022 during a 24-hour offshore trip, and gives no seasonal pattern beyond that.
• They are easy to mistake for something else. The dorsal fin breaking the surface is regularly taken for a shark (Wikipedia).
• Numbers are falling. The IUCN lists Mola mola as Vulnerable and decreasing, with drift gillnets the main threat (Wikipedia, Monterey Bay Aquarium).

Seeing one in an aquarium is rarer still. Monterey Bay Aquarium, which in August 1986 became the first US aquarium to hold one, lists the species as not on exhibit. Osaka's Kaiyukan is one of the few aquariums that display them, and the captivity record belongs to a sunfish named Kukey at Kamogawa Sea World, which lived 2,993 days and grew from 72 cm to 187 cm (Wikipedia).

Most popular accounts, including the diet section above, describe the ocean sunfish as a jellyfish specialist. That picture comes from older observation and still fits large adults feeding at the surface. Genetic analysis of gut contents tells a broader story: Wikipedia summarises the work as showing sunfish are generalist predators that eat mostly small fish such as flounder, eel larvae, squid, other molluscs and crustaceans, with jellyfish and salps making up only about 15% of the diet. They sometimes swallow eelgrass too.

The two views are less contradictory than they look. Diet changes with size, and a mola that hunts below 200 m (Wikipedia) is feeding where no observer at the surface can see it. The practical takeaway for anyone reading older sources: "eats only jellyfish" is an oversimplification, and plastic bags remain a real hazard because jellies are still on the menu.

Several features set the mola apart from every other fish in this site's lookup pages:

• Most eggs of any known vertebrate. A female can release up to 300 million eggs at once, and a larva hatches at about 2.5 mm and under one gram (Wikipedia).
• No swim bladder. Buoyancy comes from a stiff gelatinous layer under the skin that is about 90% water, held in a mesh of collagen and elastin (Wikipedia).
• A tail that is not a tail. The clavus formed where the dorsal and anal fins meet, with 11 to 14 fin rays, replaced the original tail fin in the course of evolution (Wikipedia).
• A name from a millstone. The Latin word mola means millstone, for the grey colour, rough skin and round body (Wikipedia).
• Babies that look like puffers. Sunfish fry resemble small spiky pufferfish more than adult molas, a reminder of the shared order (Wikipedia).
• A long-distance traveller. USFWS reports molas covering nearly 20 miles in a day and leaping more than 10 feet out of the water.

If you photograph something huge, flat and grey drifting at the surface, the app can confirm whether it is a mola; the fin shape and missing tail usually settle it.