I’m going to share a secret with you. I’ve never actually been diving with those big, charismatic animals. I see my friends’ pictures of whale sharks, mantas, dugongs, and all of the amazing giant animals of the sea with increasing jealousy and malcontent. All of my dives have been with very cool, but very small ocean animals. So until the day that I can finally come face to face with some megafauna, I have to seek solace in learning what makes them charismatic. So if you, like me, are still on the sideline of large animal dives, why not take some time to learn about the illustrious Mola mola or ocean sunfish in preparation for the trip where you finally see one.
Molas belong to the Tetraodontiformes order along with boxfish, pufferfish, triggerfish, and filefish. Basically it’s the order of the fish weirdos. They belong to the family Molidae which contains four species, all called sunfish of some sort as their common names. The ocean sunfish, the Mola mola, is the type species for the family and also the largest and heaviest bony fish in the ocean. The word mola is Latin for millstone, which, once I looked up what a millstome was, is a surprisingly accurate physical description. With the largest recorded Mola weighing in at 2,300 kg (5,071 lbs), they’re about as heavy as one as well.
Molas have arguably the most extreme size growth of any vertebrate with the potential to grow more than 60 million times their birth size. They start out as tiny fish larvae more resembling pufferfish than their parental forms. As they grow, they lose their baby spines and form the large, round, laterally flattened, tailless giants we more commonly know them as. They can spawn up to 300 million eggs at a time, more than any other vertebrate, thanks to their large body accommodating all those tiny eggs.
Surprisingly, the largest bony fish in the ocean not only has fewer vertebrae than any other fish, its skeleton is also mostly composed of cartilaginous tissues and not bony tissue. The cartilage is lighter than bone, so it allows the fish to reach sizes impractical to others of their class. With only 16 vertebrae, the body is shorter in relation to their size of any fish, giving them a short, truncated look. In many languages the common name alludes to a swimming head or head-only fish. Which is also surprisingly descriptive as it really does look like a head attached to a long, flat, boneless pseudotail – formed by the convergence of the elongated dorsal fin and the equally elongated anal fin. Because of these fins, molas can be equally as long as they are wide.
To swim, they scull their caudal and anal fins to propel forward, steering with their pseudotails. They can further refine their movements by squirting a vigorous jet of water out of their mouth or gills. Since sunfish derive their names from their propensity to sunbathe at the surface causing their dorsal fin to break the surface and occasionally be mistaken for that of a shark. However, this behaviour could be more than just a way to warm their large bodies.
More than 50 species of parasites have been identified living on Mola molas. Over time, this has brought about some interesting interactions with other animals. Molids will swim up to halfmoon cleaner fish on flotsam in the water and swim vertically with its head near the water’s surface. This signals to the cleaner fish to start eating the molid’s parasitic worms. If that still doesn’t get the job done, the ocean sunfish will swim further to the surface and signal to a gull or another seabird with its dorsal fin. The bird will then dig out the more entrenched parasites out of the fish’s skin.
Sunfish are not usually an intentionally sought after food fish. They eat mostly jellyfish which there is an abundance of these days. However, M. mola is still listed as Vulnerable on the ICUN Red List due to being frequently caught in drift gillnets. In California, ocean sunfish comprise almost 30% of bycatch in the swordfish fishery. That percentage jumps to almost 90% in the Mediterranean swordfish industry. And we simply don’t know too much about these open ocean giants to paint a clearer picture on their numbers or habits. Adults are usually solitary except during breeding seasons where they are found in groups in certain congregation areas, so finding one is literally like finding a drop in the ocean.
Sunfish haven’t taken all this human fishing pressure laying down, though. On the contrary, they are sometimes reported to jump into boats and trap people under their massive weight, or at the very least, knock them out for a spell. Which is of course, not as funny in practice as it is in the abstract. But don’t fear, the chances of you dying from a sunfish are still much lower than from a refrigerator. Especially if your interaction with these strange looking, enigmatic giants is below the surface. So get out there and go see these sea beasts in their native habitats…just don’t brag to me about it, please.
‘Mola mola’ was written by marine biologist Roya
Photo Credits & Links: