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.”

Funky Fish
"Prehistoric weirdo" is a scientific description of the ocean sunfish.

National Names
The French call the ocean sunfish "poisson lune," which means "moon fish." In Germany, an ocean sunfish is referred to as "schwimmender kopek" or "swimming head." Meanwhile, the people of the Philippines who speak in the Bisaya dialect say that ocean sunfish are "putol" or "cut short."

banner fish
Noun

tropical marine fish.

biomechanics
Noun

study of movement in animals.

bony fish
Noun

fish with a skeleton made of bone.

butterfly fish
Noun

brightly colored tropical marine fish.

bycatch
Noun

fish or any other organisms accidentally caught in fishing gear.

cartilage
Noun

strong, flexible connective tissue found in many animals.

cartilaginous fish
Noun

fish with a flexible skeleton made of cartilage, not bone.

characteristic
Adjective

particular feature of an organism.

Noun

edge of land along the sea or other large body of water.

coral reef
Noun

rocky ocean features made up of millions of coral skeletons.

debunk
Verb

to prove false or wrong.

drift net
Noun

extremely large fishing net that can drift with currents or tides.

eclipse
Verb

to block out or overshadow.

efficient
Adjective

performing a task with skill and minimal waste.

evolve
Verb

to develop new characteristics based on adaptation and natural selection.

fin
Noun

device fixed to the bottom of a surfboard to aid in control. Also called a strut or skeg.

flexible
Adjective

able to bend easily.

Frisbee
Noun

brand of round, circular discs used as throwing toys and athletic equipment.

functional
Adjective

usable.

hatch
Verb

to emerge from an egg.

horse mackerel
Noun

small ocean fish.

industriously
Adverb

with hard work and dedication.

initially
Adverb

at first.

jellyfish
Noun

type of marine animal, not a fish, with a soft body and stinging tentacles.

killer whale
Noun

carnivorous whale, actually the world's largest species of dolphin. Also called an orca.

magnificent
Adjective

very impressive.

misconception
Noun

misunderstanding or false statement.

mola
Noun

gigantic ocean fish with a circular body. Also called the ocean sunfish.

ocean sunfish
Noun

gigantic ocean fish with a circular body. Also called the mola.

organism
Noun

living or once-living thing.

parasite
Noun

organism that lives and feeds on another organism.

predator
Noun

animal that hunts other animals for food.

prehistoric
Adjective

period of time that occurred before the invention of written records.

prey
Noun

animal that is hunted and eaten by other animals.

satellite tag
Noun

device attached to animals that can track their movement using satellites and GPS technology.

sea lion
Noun

marine mammal.

shark
Noun

predatory fish.

silver dollar
Noun

coin made of light-colored metal, including silver, tin, and zinc.

skeleton
Noun

bones of a body.

snout
Noun

protruding nose and jaw of an animal such as a pig.

spa
Noun

facility, usually with mineral hot springs, offering health benefits.

swordfish
Noun

ocean fish with a large, pointed upper jaw

tremendous
Adjective

very large or important.

unintentional
Adjective

accidental, or not on purpose.

unique
Adjective

one of a kind.

vertebrae
Plural Noun

(singular: vertebra) bones that make up the spinal column, or backbone, of an animal.