Cause of the Month: Ocean Sunfish
Don’t worry, that fish isn’t injured…it’s just your average Mola mola—the world’s largest bony fish!
Ocean sunfish aren’t the little sunnies you caught at the lake when you were a kid (seriously, no relation). Growing up to 10 feet long, 14 feet tall (including the fins), and weighing nearly 5,000 pounds, the ocean sunfish (scientific name Mola mola) is an absolute unit of a fish that can be found in tropical and temperate oceans all around the globe.
A cross between a rock and a pancake, the mola has a truncated tail fin, or clavus, which is a scalloped fringe of muscle along its backside that acts like a rudder. They use their top and bottom fins to swim and pretty much flop back and forth to get where they’re going. Not the most elegant or speedy method of transport, but it certainly gets the job done; molas have been observed cruising at a top speed of two miles per hour.
Molas are usually white or silvery-grey and have thick, rough, rubbery skin that protects them from predators like sea lions, large sharks, and orcas. They can be seen floating at the surface for long periods of time and remain so still that people sometimes mistake them for a dead or injured animal.
Motionless molas even had scientists thinking they were a very large kind of plankton, unable to swim and at the mercy of the ocean’s currents. However, more recent studies have discovered that mola can move independently of the current and have been tracked swimming as many as 16 miles in one day.
As their common name suggests, ocean sunfish float at the surface to expose their broadest surface to the sun. It’s the fastest way to warm themselves up after deep water dives!
Because molas are heavily infested with parasites (they can carry up to 40 different kinds!), some researchers speculate that basking also attracts birds and fish who feed on the parasites and help keep the mola clean.
The mola’s wonderfully strange appearance has earned them some equally weird names. Our German friends may call them Schwimmender Kopf, which means “swimming head,” while our friends in Poland might call them samogłów, which means “head alone” or “lonely head.” Even the word “mola” comes from the Latin word for “millstone,” which is not an unfair comparison.
Hatching from a tiny egg, the mola increases its weight 60 MILLION times before it’s fully grown, quickly packing on pounds until it weighs as much as a pickup truck. According to the Monterey Bay Aquarium, that’s the equivalent of a 1-gram tadpole turning into a 60-ton frog!
With a growth rate like that, you’d expect ocean sunfish to be voracious predators. And they are, but you may be surprised to learn that they achieve peak physical condition on a diet mostly made up of jellyfish. Researchers believe that molas have a protective lining in their digestive tract that prevents stings. They may occasionally feed on other gelatinous zooplankton like salps, squid, fish, crustaceans, and algae.
Despite the teeth plates that give molas their bird-like appearance, they can’t actually chew anything. Instead, molas will suck jellies in and out of their mouths until they’re reduced to smaller, more manageable chunks of jelly. (Perhaps you or a child you know has done something similar while eating Jell-o? Or is that just us?)
Because jellies are mostly made up of water, they’re low in calories and nutrients so ocean sunfish must gobble up hundreds and hundreds of them every day to stay fed and healthy. Their ability to consume lots of jellyfish also makes them a very important part of the marine food web.
Without animals like ocean sunfish and leatherback sea turtles to keep them in check, jellyfish populations would explode and they’d start eating up all the plankton, which are a staple of the marine food chain that many other animals rely on to survive. In extreme cases, jellyfish overpopulation can actually cause the wholesale collapse of an entire ecosystem.
Ocean sunfish are thought to be naturally rare and their conservation status is currently rated “vulnerable” by the International Union for the Conservation of Nature and Natural Resources (IUCN).
While ocean sunfish have some predators, their large size and tough skin makes them difficult prey. Unfortunately, their greatest threats are man-made.
Ocean sunfish are prone to entanglement in fishing gear as well as bycatch, a phenomenon in the fishing industry where animals are accidentally caught in fishing gear intended to catch other species.
Drift gillnets are a particular hazard because molas spend so much time at the ocean’s surface. While gillnets don’t kill molas right away, they can cut into their skin, scrape off their protective mucus, and flood their gills with air.
It’s not uncommon for molas to mistake plastic bags for jellyfish and eat them. And the funny way molas eat jellies makes them especially prone to choking on plastic debris. If they do manage to suck it all down without choking, plastic bags and other debris can clog their stomachs and cause them to starve.
Help raise awareness about the impact of plastic pollution and the importance of protecting molas with our limited edition Ocean Sunfish Awareness Bracelet. Made from the plastic you see our crews recovering every day, your purchase funds our global cleanups and helps advance our mission to end the ocean plastic crisis.