The Mola mola is an astounding fish that has not really been researched enough due to it’s eccentric looks and habits. You see, ocean scientists refer to this fish as the “swimming head,” which is an apt description of this fish that can weigh as much as an adult rhinoceros and which generally is seen lounging at the surface of the ocean, but that description overlooks the rather special evolutionary niche which this fish occupies.
According to an excellent article on the Scientific American website, these swimming heads – also known as sunfish, are, from an evolutionary standpoint, somewhere in the middle between a fish and a shark:
Biologists have affectionately described Mola, or ocean sunfish, as a “a swimming head.” And while they seem to just float aimlessly at the surface, scientists are finding that these fish — which occupy a crucial evolutionary link in the fish family— are actually warming up after epic daily treks into deep water.
These fish live off the California coast and around the world in temperate and tropical areas. But many people have never heard of them, let alone seen one.
Mola mola are not endangered and not eaten in the United States. In fact, females can produce up to 300 million eggs, more than any other bony fish. But the hapless fish ends up tangled in fishing nets, as bycatch for more valuable target species. They make up the largest bycatch component (29 percent) in the California drift-gillnet swordfish fishery.
So why does it matter if Mola mola are caught in mesh nets?
Mola are pelagic, which means they live in the open ocean. Like humans, and many other fish, they have a bony internal skeleton. Sharks and rays, however, have a cartilaginous skeleton. According to some scientists, mola could provide a missing link to understand their open ocean neighbors animals, like sharks.
“Sunfish are one of the most advanced bony fish, but they have a lot in common with cartilaginous fish. What they have in common may be adaptive to pelagic life and to study it may lead to solve evolution of pelagic species,” says Itsumi Nakamura, a biologist at the University of Tokyo.
Mola have lost the calcium carbonate that makes their skeleton hard, so it’s more like a shark skeleton, says Christopher Lowe, a professor of marine biology at Cal State Long Beach State. Also like sharks, they lack a swim bladder that helps most bony fish stay afloat. Being lighter means using less energy, which is important when you are searching for hard to find and low calorie dinner items, common for deep-sea eaters, he says.
Nakamura has studied the fish since 2009 and recently revealed mysteries of their strange behavior. Mola are often seen just lounging at the surface, he says. But new research published in the Journal of Animal Ecology from Nakamura and his team have found that mola actually make daily treks to the deep sea more than 2,600 feet beneath the surface — a place reserved for creatures like giant squid and diving sperm whales.
Why do they dive so deep? The article continues:
They are eating jellyfish-like creatures, called siphonophores, says Nakamura. And it turns out, after observing the camera from one mola, they might be dining on the most nutritious part of the animals- their sex organs.
“Of course I was surprised, because it is very novel that they eat only calorie rich parts of the jellyfish,” he says.
Relaxing at the surface has another benefit for the sunfish; it’s a trip to the spa. Mola line up at cleaning stations, while smaller fish peck parasites from their body. For a more thorough cleaning, mola swim to the surface and seagulls jab through their flesh, feasting on parasitic worms.
There’s definitely more to study about these strange fish. For now, additional details can be found in the excellent article on the Scientific American website.
Source: ScientificAmerican.com – “Face to Face With the Ugly, Marvelous Mola Mola“
Featured Image Credit: Cal State Long Beach Shark Lab