Unraveling the Mysteries of the Ocean Sunfish- page 3 | Science | Smithsonian
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The ocean sunfish is the heaviest bony fish in the world; it can grow more than 10 feet long and pack on a whopping 5,000 pounds, and yet its flat body has no real tail to speak of. (Mike Johnson)

Unraveling the Mysteries of the Ocean Sunfish

Marine biologist Tierney Thys and researchers at the Monterey Bay Aquarium are learning more about one of the largest jellyfish eaters in the sea


In its lifetime, a mola grows from a larva one-tenth of an inch long to an adult more than 60 million times its starting weight. That is comparable to a human baby ultimately weighing the equivalent of six Titanics. But what is the fish’s average lifespan? By extension, at what rate do they grow in the wild?

Michael Howard, head of the Monterey Bay Aquarium’s mola husbandry team, would certainly like to know the answer to that last one.

At the aquarium, Howard takes me to the top lip of the million-gallon Open Sea tank, where I have a front row seat to a mola feeding. The event is carefully orchestrated, as is just about everything related to an exhibition where hammerhead sharks, sea turtles, tunas, huge schools of sardines and other animals are meant to peacefully coexist. The turtles are stationed in one area while a staff member, crouched on a gangplank over the tank, dunks a pole with a ball on the end of it into the water. The mola is trained to come to the target, expecting a meal. The fish rises, a murky shadow at first. Then, once the mola’s botoxed-looking lips break the surface, the feeder drops some squid, shrimp and a gelatin product into its gaping mouth.

The aquarium has exhibited molas fairly consistently for 16 years, but in many ways, the husbandry staff is still shooting from the hip—especially when it comes to managing the fish’s growth in captivity.

In the late 1990s, a 57-pound mola ballooned to 880 pounds in just 14 months. The fish had to be airlifted out of the aquarium by helicopter and released back into the bay. “It worked great, and it was a rush. It took seven months to plan. We had 24 people on staff and FAA approval to cordon off the building that day we released it,” says O’Sullivan. “It is a great story. But wouldn’t it be better if we just got the animal up to half that weight, had a much more relaxed deaccession, replaced it with another animal a fraction of its size and started the whole process over?”

Howard, who has led the program since 2007, has been working toward this end. He and his team conduct ongoing captive growth studies; they record the mass of each type of food fed to the mola at its twice-daily feedings and follow up with routine health exams every two or three months, making any necessary adjustments in the fish’s diet. Each day, they aim to feed the mola a ration of food equal to 1 to 3 percent of its body weight. A few years ago, aquarists captured some moon jellies from the bay and had them analyzed. With the results, they worked with a company to produce a comparable gelatin product comprised of 90 percent water. “That really helps us get the daily volume up while keeping calories low,” says Howard. Depending on their stage in life, molas require only three to ten calories per kilogram of animal mass. To put that into perspective, adult humans need 25 to 35 calories per kilogram. Tunas at the aquarium get 30 calories per kilogram, and otters get 140 calories per kilogram. On the new diet, the aquarium’s last mola gained an average of .28 kilograms per day, whereas the airlifted mola nearly quadrupled that rate.

“As long as a mola’s behavior is healthy, we can consider working and caring for the fish until it approaches about six feet in length,” says Howard. That usually equates to a two-and-a-half-year stay. When it comes time for the fish to be released, which is always the end goal, says Howard, the team can then feasibly hoist the mola out of the tank on a stretcher, place it in a holding tank, first on a truck and then on a research vessel, and let it go a few miles offshore.

For Howard, the mola has been the trickiest species he has encountered in his 15 years of aquarium experience. “But who doesn’t enjoy a good challenge?” he says. 

The peculiar fish prompts a slew of questions from aquarium visitors—about the species and the ocean in general. “If that happens,” says O’Sullivan, “then we are being successful in our mission.”


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