Sunfish get their common English name from their habit of floating on their sides at the ocean’s surface, as if basking in the sun. Sailors and fishermen who spotted these enormous, round fish lying flat near the waterline thought they looked like they were sunbathing, and the name stuck. But that’s only one layer of the story. The sunfish has picked up vivid names in nearly every language, each one inspired by a different feature of this bizarre animal.
The Basking Habit That Inspired the Name
Ocean sunfish spend a surprising amount of time at the surface, drifting on their sides with one flat flank exposed to the sky. Early observers assumed the fish were simply lounging in the warmth, and “sunfish” became the natural description. Scientists initially thought the same thing, proposing that sunfish bask to rewarm their bodies after deep dives into frigid water. That idea, sometimes called the “thermal recharging” hypothesis, turned out to be weak. Studies found no real connection between how long a sunfish spent in cold, deep water and how much time it later spent at the surface.
The more likely explanation is parasites. Sunfish are famously loaded with them, including burrowing crustaceans that cause real tissue damage. When a sunfish floats at the surface, seabirds like albatrosses land nearby and pick parasites directly off its skin. Researchers now believe sunfish seek out this cleaning service on purpose, making the basking behavior less about sunbathing and more about pest control. Still, the visual impression of a giant fish soaking up rays is what gave the animal its English name centuries ago.
What the Latin Name Means
The scientific name, Mola mola, comes from the Latin word for millstone. A millstone is a large, round, flat, rough-surfaced disc used for grinding grain, and if you’ve ever seen an ocean sunfish head-on, the resemblance is hard to miss. The fish is almost circular in profile, grey, and covered in a thick, sandpapery skin. Carl Linnaeus made the name official when he formally described the species in 1758, and the millstone comparison has held up ever since. Adults can reach over 3 meters (about 11 feet) in length and weigh up to 1,300 kilograms (roughly 2,900 pounds), so the comparison to a massive stone disc is more literal than poetic.
Moonfish, Millstone, and Other Names
Not every culture looked at the same animal and thought of the sun. In French, the ocean sunfish is called “poisson lune,” meaning moonfish. German speakers call it “Mondfisch,” also moonfish. These names focus on the fish’s round, pale, disc-like shape rather than its basking behavior. If you picture a full moon reflected on the water, you can see the logic. In Japanese, the fish is called “manbÅ,” a name with its own distinct cultural history. The variety of names highlights how different observers latched onto different traits of the same strange creature.
Freshwater Sunfish Are a Different Story
If you searched this question thinking about the smaller, colorful sunfish found in North American lakes and ponds (bluegill, pumpkinseed, and their relatives), those fish earned the name for a different reason. Many freshwater sunfish have bright, warm-colored markings in oranges, yellows, and reds that reminded early American settlers of the sun. Some species also fan out circular nests in shallow, sun-drenched water, reinforcing the association. These fish belong to the family Centrarchidae and are not related to the ocean sunfish at all. The shared name is pure coincidence, two completely different groups of fish independently earning the same label from people who thought “sun” when they looked at them.
A Species Hidden in Plain Sight
For most of modern history, scientists recognized only two species in the Mola genus. Then in 2008, genetic analysis of tissue samples from a large number of sunfish revealed a third distinct species hiding in the data. It took nearly another decade to formally describe it. In 2017, researchers officially named it Mola tecta, from the Latin word “tectus,” meaning hidden. They also gave it the common name “hoodwinker sunfish” because it had been swimming in plain sight, misidentified in museum collections for years. Close examination of preserved specimens at New Zealand’s national museum revealed that roughly half of their stored sunfish were actually the new species. It was the first new sunfish species described in over a century, a reminder that even a fish the size of a small car can slip past scientists when nobody looks closely enough.