On Exhibit: Open Sea
Mainly jellies and other zooplankters, squid, fish, crustaceans, brittle stars
to 14 feet and nearly 5,000 pounds; molas in Monterey Bay up to 1,000 pounds or more
Seasonally distributed in all tropical and temperate oceans
At the Aquarium
Life in the slow lane
Visit our Open Sea exhibit during one of the regularly scheduled feedings and you'll see a blur of motion as tunas and bonitos flash through the water, grabbing their dinner at top speeds. So what's a slow-moving sunfish to do? Our aquarists have learned to feed these gentle giants by placing a colored target at the surface of the water, which signals it's mealtime and to head on over to it!
Fish or craft project?
Ocean sunfish, or molas, look like the invention of a mad scientist. Huge and flat, these silvery-gray fish have tiny mouths and big eyes that vanish into an even bigger body with a truncated tail. Topping out around 5,000 pounds, molas are the world's heaviest bony fish. (This category doesn't count sharks and rays. The whale shark is 10 times bigger.)
With their tank-like bodies, molas were clearly not built for life in the fast lane, but they hold their own against faster and flashier fishes and are able to live in almost all of the world's oceans. They are known to spend time near the ocean surface but tagging shows that molas are also prolific divers and migrate long distances at depth.
Molas hatch from tiny eggs but grow to weigh more than a pickup truck, increasing in size 60 million times along the way. That's the equivalent of a 1-gram tadpole turning into a 60-ton frog! They grow to a maximum of about 10 feet long and are often taller than they are long, up to 14 feet from dorsal fin tip to anal fin tip. They have a truncated tail fin referred to as a clavus—a scalloped fringe of muscle along their blunt rear end, which they use as a rudder.
Breakfast of champions
Inside a mola's tiny mouth are two pairs of hard teeth plates shaped with a slightly curved ridge that look kind of like a bird's beak. Molas eat mainly jellies, from big moon jellies to tiny comb jellies.
To break their dinner into manageable pieces they don't chew; they suck the jellies in and out of their mouths until they're reduced to gelatinous chunks. We think that molas can enjoy this potentially painful diet because of a mucus-like lining in the digestive tract that keeps them from getting stung. Molas also sometimes eat squid, fish, crustaceans, sponges, brittle stars and odd seafloor creatures called crinoids.
Molas are slow and deliberate swimmers. Adult molas lack a gas-filled swim bladder, the organ that gives most bony fish exquisite control over buoyancy. Scientists impressed by their slow-motion swimming at first guessed that molas must drift wherever ocean currents take them. But molas in Southern California have been tracked swimming 26 km in a day, at a top speed of 3.2 km per hour—which, to give them credit, is not far off the speed of a yellowfin tuna when it's just out cruising.
Maybe because molas swim so slowly, they tend to be covered in parasites. Nearly 40 different kinds have been recorded, including a few gooseneck barnacles that were discovered living in a mola's throat. (Some of the parasites that live on molas even carry their own parasites.)
Mola for dinner?
Molas are related to pufferfish, and just-hatched molas are puffy, round and covered with spines like their relatives. Puffers are extremely poisonous in specific parts of their bodies, but scientific studies have so far found no trace of the toxin in molas. In fact, strange as it may sound for a parasite-ridden fish with skin that's thick and rubbery like a car tire, the mola makes a popular meal in parts of Asia and is also used in medicine. Apart from humans, other predators include sea lions, sharks and killer whales.
Because molas spend so much time drifting near the ocean surface, they are vulnerable to fishing boats that use drift gillnets. In California, nearly 30 percent of the catch in a swordfish boat can be molas caught by mistake—rivaling or exceeding the number of swordfish caught.
In the Mediterranean Sea, the Spanish gillnet fishery catches up to 93 percent molas. Gillnets usually don't kill molas immediately, but they cut into their skin, scrape off their protective mucus and flood their gills with air.
Another hazard to molas are discarded plastic bags. When these wind up in the ocean, they float at the surface and look a lot like a jelly—a mola's favorite meal. If the mola doesn't choke as it sucks the bag in, the plastic can clog the fish's stomach, slowly starving the animal. Helping molas is one more reason to carry your own shopping bags with you to the store—and to make sure any plastic bags you use go into the trash can.