Species: Mola mola
A common rule in evolution is: the strongest, most fit species are the ones to survive. Typically, a species has more than one adaptation that allows it to survive long enough to reproduce. Adaptations can be variations of speed, camouflage, strength, armaments, or some combination of those features..
The Mola mola is the exception to that rule.
The ocean sunfish, Mola mola, is the world’s heaviest bony fish, weighing in at 5,000 pounds. This makes the ocean sunfish 10 times smaller than the whale shark, which is the largest cartilaginous fish.
The Mola mola looks as if some god got bored and stopped after making half a fish. Its large bulbous head looks like it belongs to a fish much bigger than it is, especially with the lack of a caudal (tail) fin. Even looking incomplete, the sunfish is 14 feet from the bottom of its anal fin to the top of its dorsal fin, and it is about 10 feet from head to end.
Mola is the Latin term meaning “millstone,” referencing the creature’s disc-like shape. Humans are not always clever with their naming schemes, though you can count on us being obvious.
Without a functioning caudal fin, the Mola mola awkwardly swims by waggling its dorsal and anal fins. It does not swim very fast, making it easy prey for sharks, killer whales, and sea lions. With their thick, stretchy skin and their typical diet of jellyfish, ocean sunfish may not be a worthy meal for their predators.
Ocean sunfish are also preyed upon by parasites that live in and on their bodies. While an open water fish, they have been spotted near reefs while attempting to remove parasites. Reefs often contain organisms that feed on the parasites found on bigger fish and turtles. Mola molas will also swim flat on the surface of the water to allow birds to pick some of the parasites off.
Jellyfish don’t provide a lot of nutrition, but sunfish eat a lot of them, along with algae, squids, and some crustaceans. Ocean sunfish don’t chew. Instead, they have an infused tooth-plate on the top and bottom parts of its mouth. When they suck jellyfish into their mouths, the action against the plates turns the jellyfish into a gelatinous material that’s easier to consume. Yum!
Ocean sunfish are a slow-moving, easy target for predators to catch. So, how is it still around? How does it populate almost every ocean? A single female Mola mola can release 100 million eggs into the open water each year. Therefore, if 100 females reproduce in a given year, that’s still about 10 billion eggs.
If only those eggs were cash, am I right?
Despite how many eggs they produce, their global populations are on a decline. They get caught in gill nets that trail behind fishing boats; the nets cause them injury and can even suffocate them. Sunfish also fill their stomachs with indigestible plastic bags, which look like jellyfish floating in the water, and the plastic eventually causes them to starve to death.
You can help Mola mola by picking up trash off the beach when you can or when you’re out in public anywhere. Even if you don’t live right next to the water, those discarded plastic bags you see can still make it to the ocean by way of wind. As for the gill nets, try supporting fisherman and fishing companies that have been reported to fish sustainably. Typically, there is a label of some kind on fish products that have been harvested or caught sustainably.
Please do what you can; every little bit helps these goofy-looking fish!
Sources and links:
Reef Fish Identification: Florida Caribbean Bahamas 4th edition by Paul Humann and Ned Deloach
Ocean The Definitive Visual Guide made by the American Museum of Natural History
https://oceansunfish.org/species-and-distribution/ ⇐has more info on sunfish and research relating to them.