Looking like a giant silver dollar with fins, the ocean sunfish’s appearance is striking. But its unique shape is only one of many characteristics that cause the creature to stand out, according to Tierney Thys, a National Geographic Emerging Explorer who has traveled the world studying ocean sunfish, also called mola.

“This is the world’s heaviest bony fish,” she says. Some sharks are heavier, but sharks have a light, flexible skeleton of cartilage, not bone. Mola can weigh up to 5,000 pounds (2,268 kilograms). 

Thys lists other unique characteristics of mola. “It produces more eggs in one individual than any other vertebrate on the planet. It’s the growth champion of the world, so that from hatching size to adult size it puts on 60 million times its original weight, which just eclipses pretty much everything out there in the vertebrate world. So, it’s this magnificent creature with all these world records to its name, and we don’t really know that much about it at all.”

Like many others, Thys was initially drawn to the ocean sunfish because of its appearance. Having studied fish biomechanics at Duke University, the scientist was puzzled by the shape of the ocean sunfish. It didn’t appear to be very functional.

“You look at the sunfish, and at first glance, it really doesn’t look stunningly efficient,” she says. “It doesn’t really look streamlined. It doesn’t really look very intuitive. So, it grabbed my attention. Why make a fish without a tail? What’s the use of that?”

According to Thys, ocean sunfish don’t really need speed because they pursue a diet of slow, floating jellyfish. A streamlined body or powerful tail are not required. “It’s not a big speedster,” she says. “It doesn’t need to be.”

She and other scientists have debunked a misconception about the fish. Previously, ocean sunfish were perceived as lazy. They seemed to simply float along on the water’s surface, which is one reason they received the common name “sunfish.” Thys and other biologists, however, found that ocean sunfish are actually quite active. “We put these tags on them and found that they are diving industriously up and down, up and down, up and down, up and down to depths as deep as 800 meters, 2,100 feet down,” she says. “They are capable of a tremendous amount of activity.”

Studying Sunfish

The ocean sunfish, found in every ocean in the world, is not endangered. Thys has traveled to Indonesia, Japan, Taiwan, and South Africa to study them.

One unusual characteristic that Thys has observed in sunfish around the world is the high number of parasites on their bodies. Parasites are organisms that feed on other organisms. Most of the parasites that live on mola do not harm the fish. Thys says ocean sunfish are home to parasites because the fish are slow moving, which allows floating parasites to easily secure a place on the sunfish’s thick skin.

On her research expeditions, Thys has observed the ocean sunfish relax in what scientists call “cleaning stations.” In these areas, smaller fish remove the parasites from the larger fish’s body.

“In Bali, coral reef fish, banner fish, and butterfly fish will go in little groups off the reef and find these mola to clean them and give them a spa treatment,” she says.

Even though an ocean sunfish can reach 3.1 meters (ten feet) from snout tip to tail fin, they are prey to larger predators including sharks and killer whales. Off the coast of California, many ocean sunfish are killed by sea lions.

“Certainly in Monterey [California], sea lions rip the fins off and turn them into Frisbees and eat their guts,” says Thys.

Future of the Sunfish

Thys says the greatest danger facing ocean sunfish are not natural predators but fishermen, who often unintentionally catch ocean sunfish while harvesting other species. In the Mediterranean, ocean sunfish are the unintentional victims of drift net swordfish fishing. Off of South Africa, they are caught as fishermen try to bring in horse mackerel.

Thys contends ocean sunfish are one of the most evolved fish in the sea. “[I]t looks like this prehistoric weirdo, but it’s actually a modern marvel in fish terms.”

Recently, Thys received funding from the National Geographic Society to study a population of ocean sunfish located around the Galapagos Islands. In 2011, Thys will head to the Galapagos with a team of four scientists for several weeks. There, the research crew will attach satellite tags to the area’s ocean sunfish. The satellite tags record the fishes’ locations, diving depths, and diving temperatures.

Thys says her primary goal is to characterize the ocean sunfish’s use of the Galapagos ecosystem. Monitoring the fish will allow Thys to determine how they use the area’s animals and plants for food, shelter, or activities like cleaning station “spa treatments.”