Its name sounds Hawaiian, but is actually Latin, meaning “millstone” which is an appropriate name for one of the world’s heaviest fish. The Mola mola, or ocean sunfish, can weigh over 4,000 pounds and measure over 10 feet in length. It is a strange disc-shaped fish that has rough grayish skin and an array of unusual fins. Sunfish have a pair of small, fan-shaped pectoral fins along with a unique tail fin known as a clavus, which it has instead of separate dorsal and anal fins. The clavus has a dozen little finlets and is helpful to the fish in steering it much like a rudder. This fin is the one that boaters often see poking above the surface and mistake as a shark’s dorsal fin.
The Mola mola can weigh over 4,000 pounds and measure over 10 feet in length. Shutterstock
In fact, sunfish are somewhat similar to sharks in that they have a fair amount of cartilaginous tissue, much as sharks do. Even though they are quite heavy, the cartilaginous material is lighter than bone and is part of what allows them to grow so large. But that cartilage, in addition to the fact that sunfish are typically solitary, is where its similarities to sharks end. Whereas sharks are an apex predator in the marine food chain, sunfish prefer to eat jellyfish, small fishes and other crustaceans. In turn, sharks are among the sunfish’s predators. Sunfish are also prolific reproducers, unlike sharks that produce only a handful of pups each year through an odd combination of internal gestation followed by the release of an egg in a signature shark egg case. Sunfish can produce up to 300 million eggs at a time — more than any other vertebrate! These larvae look like tiny pufferfish, one of their closest relatives.
Its swimming pattern is something that also distinguishes a sunfish. Their common name, sunfish, refers to their habit of “sunning” themselves near the surface in order to store up warmth before they dive into deeper water to feed — sometimes up to 600 feet below the surface! They do this by swimming on their side to absorb as much light as possible, their clavus moving back and forth as they float. Even if you can’t see its orb-like body under the surface, the swishing motion of the clavus can help to easily differentiate a sunfish from a shark whose fin moves in one direction through the water. Sunfish are decent swimmers but not particularly speedy — only 2 miles per hour or so. They typically spend more time drifting with the currents than actively swimming, but they do occasionally leap up out of the water, making a substantial splash with their large bodies. Regardless, sunfish are harmless to swimmers and are fun to watch as they float around. The only danger they might pose is to boaters who don’t see them and can run into them accidentally. Their size and weight can cause some significant damage to hulls and propellers.
While there isn’t a market for them in the United States, sunfish are valued in the seafood market in places like Japan, Korea and Taiwan, where they are considered a delicacy. They are typically caught in gillnets both as part of target fisheries and also incidentally along with swordfish out in California and also in the Mediterranean, a couple of the many places around the globe where sunfish roam.
Next time you’re out on the water and see an odd fin sticking up above the surface, take a closer look. You might get a glimpse of one of the biggest bony fishes out there waving its clavus above the surface as it soaks up some sun.
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