Round yet flat, the “swimming heads” have silver-gray skin, thick and rubbery like a car tire, an apt comparison for animals that may grow to weigh more than a pickup truck. The missing back end is actually a broad, stiff, scalloped lobe formed from a degenerated spine, an evolutionary remodeling of the standard-issue fish tail fin used for propulsion. Having a truncated back end also makes for a more rigid body, further compromising side-to-side swimming motion like that enjoyed by other fish. But molas aren’t hampered by inflexible bodies and nonexistent tail fins because they are instead empowered with statuesque dorsal and anal fins, which do the propelling; the modified tail lobe used instead for steering. To set their ship in motion, Molas simultaneously scull the dorsal and anal fins to generate lift-based thrust. Surprisingly, this setup makes for highly efficient swimming.
Females can produce a staggering 300,000,000 eggs at a time, but for a12-foot-long, 4,000-pound adult, space likely isn’t a problem. What seems impossible is how a fish can get so big eating mostly jellyfish, which are 99 percent water. The bumper crop of jelly prey invading La Jolla may explain the visiting molas that subsequently arrived. Molas don’t chew but suck jellies in and out of their mouths until they are of manageable size for swallowing. To escape what could be a painfully stinging meal, molas may produce mucus-like lining. Molas do augment their diet with mollusks, crustaceans, small fish and even eel grass, indicating they feed from the surface to the deep. The long-held perception that sunfish mostly loll on the surface because they are sluggish, weak swimmers has been disproved by tagging studies. The fish were active predators and powerful swimmers, migrating long distances and diving to depths of up to about 2,000 feet.
Between surface lolling and having a body not built for speed, it would seem that mola mortality would mostly be at the behest of large predators like sharks, but it’s the small but mighty that are more likely to bring down the giants. A whopping 54 parasite species are known to inhabit molas, both internally and externally, but sunfish don’t voluntarily offer themselves as habitat. They have been seen breaching more than 10 feet out of the water to dislodge and reduce parasite numbers. Molas also flag down seabirds by waving their pectoral fins, an invitation to dine on their skin parasites. Once a bird polishes off one side, the host graciously rolls over to offer seconds. Molas must suffer from their burden because all the adults I’ve seen are covered with open sores from the afflictions. It’s not known for certain where molas pick up parasites but gelatin-based prey are known to commonly host various parasite larvae.
Recreational fishers surley find the parasite load and body sores unappetizing enough to leave molas alone, as is true for most commercial fisheries worldwide except Japan and Taiwan. Mola threats from humans are incidental when they are caught in fishing drift nets set for other species. The California swordfish fishery estimates that molas make up almost 30 percent of all bycatch, far outnumbering the target species. The Mediterranean swordfish industry estimates that molas comprise up to 90 percent of their total catch. In some areas, fishers who regard molas as worthless bait thieves hack off the majestic fins, then toss the fish back alive. This cruel practice results in a slow death as molas, now unable to propel themselves, sink to the seafloor.
So much loss, and we know so little about these fish and their importance to the broader ecosystem, from their deep-diving behaviors; long-distance migrations; global and localized population status; hunting strategies; and effects of incidental bycatch, particularly since mola bycatch is unregulated worldwide. Molas also face threats from jelly doppelgangers, those floating plastic bags that look like prey but instead can choke and suffocate molas or fill their stomach so they starve. Modern technology and techniques now make it possible for scientists to pursue international collaborations to understand the ecological importance of fish that are much more than just swimming heads.
— Judith Lea Garfield, naturalist and underwater photographer, has authored two natural history books about the underwater park off La Jolla Cove and La Jolla Shores. Send comments to firstname.lastname@example.org.