Dive Inside the Weird World of the Ocean Sunfish

sunfish mola mola — This sunfish (*Mola mola*) in Magadalena Bay, Baja California, Mexico show how captivating and unusual these huge fish are. Rodrigo Friscione/Getty Images

What has a disc-like body, thick lips, boatloads of parasites and probably weighs more than your car? We'll give you a hint: It's a type of fish.

But this isn't some oddball shark. Hammerheads, great whites and their kin are considered cartilaginous fish. That means their skeletons are primarily made of cartilage as opposed to bone. The animal in question belongs to a more modern group: the so-called "bony fishes." (Although, interestingly enough, its skeleton does mostly consist of cartilage. Scientists classify the thing as a 'bony fish' because it evolved from bonier ancestors, but we're getting off-track.)

Ladies and gentlemen, say hello to the ocean sunfish, a creature that's better known by its full scientific name: Mola mola. Compressed laterally (i.e. from side to side) and lacking a traditional tail, adult Mola mola seem kind of awkward at first glance. Something about them just feels off.

Don't be deceived by appearances, however. "[They] may look like the ultimate underdogs (underfish?), but they are truly a well-adapted, immensely successful group of fish with a cosmopolitan distribution and the ability to capitalize on resources that not many other groups consume," Tierney Thys explains in an email.

A marine biologist and science popularizer, Thys specializes in sunfish research. When she isn't conducting research or giving TED talks about these animals, she maintains a website dedicated to them — and she's currently co-editing "the first big academic book on the ocean sunfishes for CRC Press."

The book will have plenty of material to work with. "Just about everything about the sunfishes is unique," Thys says. With their unorthodox swimming style, enormous egg clutches and a thousand other quirks, molas are unlike anything else in the ocean.

Millstones and Swimming Heads

In Latin, the word "mola" means "millstone." Being flattened, roundish and often gray in color, these animals do bear a passing resemblance to their namesake rocks.

All known sunfish have an unusual pseudo-tail that contributes to their vaguely oval-shaped appearance. Most fish possess caudal (tail) fins at the end of their spinal columns. Bass, sharks, sturgeons, and the vast majority of other fish use these things to propel themselves forward.

Yet, bizarrely, sunfish don't have caudal fins. Instead, the rear end of a sunfish is covered by portions of two other fins: the dorsal fin on top of the creature and the anal fin on its underbelly. Together, they form a "pseudo-tail" known as the clavus.

Short, stubby pseudo-tails make sunfish heads seem disproportionately large. In Germany, the sea beasts are popularly called "schwimmender kopf," or "swimming heads." We'll talk about those strange noggins in good time, but first, there's another fin-related issue that warrants discussion.

Remember the dorsal and anal fins that we mentioned earlier? Well, the frontal regions of both appendages are tall, triangular and wing-like. By flapping those structures from side to side, sunfish propel themselves forward. Meanwhile, the clavus functions as a rudder, steering them as they go.

"They are fine swimmers and divers capable of swimming against currents," Thys says. She adds that, in the span of 24 hours, a sunfish may cover 12.4 to 18.6 miles (20 to 30 kilometers) as it swims. Moreover, the fish can dive "up to 1000 meters" (or 3,280 feet) below the surface "multiple times a day into chilly waters" with temperatures as low as 35.6 degrees Fahrenheit (2 degrees Celsius).

Big Fish, Small Fish

Mind you, not every sunfish is a Mola mola. Again, that's just one species. There are other fish within the Mola genus, namely the hoodwinker ocean sunfish (Mola tecta) and the bump-headed sunfish (Mola alexandrini).

The latter species has a pretty impressive distinction. Scientists recently determined that the heaviest bony fish ever caught was a large Mola alexandrine that was dredged up near Japan in 1996. A colossal prize, the big catch weighed 5,070 pounds (2,300 kilograms)!

When it comes to sheer bulk, however, Mola mola is no slouch. Full-grown adults have an average weight of 2,200 pounds (998 kilograms), but they can grow significantly heavier. The distance between the tips of a Mola mola's anal and dorsal fins can be up to nearly 8 feet (2.4 meters). Measured lengthwise, a Mola mola is liable to stretch 6 feet (1.8 meters) long.

Yet they start out small. Mola mola eggs are only about 0.1 centimeter (1 millimeter) in diameter, and a newly hatched larva is the approximate size of a pinhead. By the way, they produce the largest egg clutches of any vertebrate animal; scientists found one gravid Mola mola female who was carrying about 300 million individual eggs!

Sun Kings

Young Mola mola resemble tiny puffer fish (which, not coincidentally, are close relatives of the sunfish). As the creatures get larger, their diets tend to shift. Specimens who measure 1.9 feet (less than 0.6 meters) long will generally eat animals who live in or around the ocean floor, such as crabs and some smaller fish. Bigger Mola mola prefer open water denizens like squid, Portuguese man o' war, planktonic tunicates and — most famously — jellyfish.

Thick-walled intestines help the sunfish guard themselves against stings. Thys says that their eyeballs can be "[pulled] back into their... sockets, presumably to protect them from jelly stings."

Alas, those adaptations don't protect Mola mola from the orcas, great whites and fishermen who hunt them. Another predator is the California sea lion, which — no joke — likes to rip their fins off and then play with the mutilated fish.

And to top things off, it turns out parasites love Mola fish. Nearly 40 different species of these freeloaders have been found taking refuge in and on the slow-moving animals. To keep themselves healthy, Mola mola will hover near — or at — the surface, allowing sea birds and cleaner fish to gobble up some parasitic pests.

This could explain one of their peculiar habits: Juvenile Mola mola have been known to breach out of the water like whales do. "They likely do this when startled, annoyed by overly zealous cleaner fishes ... or maybe to dislodge ectoparasites," Thys says.

While at the surface, Mola are prone to float on one side and "bask" for a while. Indeed, this is why they're commonly called "sunfish." The behavior may be linked to the search for wandering parasite-cleaners.

Its strange antics and stranger anatomy have earned the Mola mola thousands of human admirers around the world. "They are just utterly captivating animals that draw people in and inspire curiosity about the ocean," Thys says. "Curiosity is the first step toward caring and any critter that [encourages] that is worthy of our respect."

Now That's Interesting

Fun fact: Mola mola migrate! In Thys's words, "they typically move with the seasons." The species is naturally found in tropical and temperate ocean waters all over the globe. "In Japan," she says, "they typically move to northern latitudes in the summer months to take advantage of greater food supplies and preferred temps. In South Africa, they don't migrate latitudinally though, presumably since there is [a] plentiful food supply year-round. They often move south in the winter in the northern hemisphere."