QuestionOcean sunfish
Why Do Ocean Sunfish Lie on Their Side? Basking at the Surface
Ocean sunfish lie on their side at the surface mostly to warm up. They hunt in cold water hundreds of meters down, then rise and turn their broad, flat body toward the sun-warmed surface water to raise their body temperature. Basking on the surface also lets seabirds and small fish pick parasites off their skin.
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Warming up after deep dives
Like most fish, the ocean sunfish cannot produce enough body heat to stay warm. Its body temperature follows the temperature of the water around it. That is a problem for a fish that feeds deep below the surface, where the water can be far colder than in the sunlit layer above.
Tagging studies show that sunfish make repeated dives during the day, often well below the thermocline, the boundary where warm surface water gives way to cold deep water. A study off Japan, published in 2015 by Itsumi Nakamura and colleagues, fitted sunfish with sensors and cameras. The fish dove again and again to feed on siphonophores, long, fragile relatives of jellyfish, and their body temperature dropped during each deep excursion. Between dives they returned to the surface and stayed there until they had warmed up again. Fish that had cooled down more tended to spend longer near the surface before diving again.
Lying on its side helps. A sunfish is tall and flat, so turning sideways exposes as much of its body as possible to the warmest water and the sun. The study also suggested that sunfish may be able to warm up faster than they cool down. A large body helps too, because it loses heat slowly, letting the fish stay longer in the cold depths where its prey lives.
Cleaning stations at the surface
Sunfish carry a heavy load of parasites. More than 40 species have been recorded on and inside them, including flatworms and small crustaceans attached to the skin. Basking at the surface gives the fish a chance to get rid of some of them.
Seabirds such as gulls have been seen landing on or beside basking sunfish and pecking parasites from their skin. Sunfish sometimes seem to invite this by lifting a fin or their mouth out of the water. Below the surface, small fish do the same job. Off California, sunfish visit floating kelp where small cleaner fish nibble at their skin, and off Bali, sunfish rise from deep water to reef cleaning stations where bannerfish and other small fish pick them over.
Sunfish are also occasionally seen leaping clear of the water and crashing back down, which may help dislodge parasites.
Other possible reasons
Warming up and cleaning are the best-supported explanations, but researchers have suggested others. Some deep water is low in oxygen, and time near the surface may help a sunfish recover after hunting there. Basking may also simply be a way to rest between dives.
Not every sunfish at the surface is basking, however. Many swim upright just below the surface with the dorsal fin sticking out, and some lie still because they are sick or injured. Sunfish that stay too long in water colder than about 12 °C (54 °F) can become sluggish and disoriented, and some strand on beaches in cold spells.
How do sunfish swim?
Without a proper tail, a sunfish swims with its two tall fins. The dorsal fin on top and the anal fin underneath beat from side to side at the same time, working like a pair of wings to push the fish forward. The rounded flap at the back, called the clavus, acts as a rudder, and sunfish can also steer by squirting jets of water from the mouth or gill openings.
This system is slower than a fish tail but efficient. Tagged sunfish usually cruise at about 3 km/h (2 mph) and can travel around 26 km (16 mi) in a day. Far from being lazy drifters, they spend much of their time actively diving and hunting.
Sunfish fin or shark fin?
When a sunfish swims at the surface, its dorsal fin often breaks the water and is sometimes mistaken for a shark’s. In British and Irish waters, the most likely shark to cause confusion is the basking shark, which also feeds at the surface in summer.
| Clue | Ocean sunfish | Shark |
|---|---|---|
| Fin movement | Flops from side to side, sometimes tipping over | Stays upright and moves steadily forward |
| Tail | No tail fin showing | Tip of the tail fin often visible behind the dorsal fin |
| Behavior | Often lies flat with one fin out of the water | Swims along the surface in a straight line |
Where and when to see basking sunfish
Ocean sunfish are found in temperate and tropical seas worldwide, but they are most visible when they come to the surface in warm months. In Europe, they are seen off Cornwall, the Isles of Scilly, Ireland and Wales mostly in summer and early autumn, and also in the Bay of Biscay and the Mediterranean, where ferry passengers sometimes spot them. In North America, they are regularly seen off California, including Monterey Bay, and in New England waters from summer into fall.
Sunfish often appear near drifting seaweed and in areas rich in jellyfish, such as moon jellyfish, one of the many gelatinous animals they eat. If you see one from a boat, slow down, keep your distance and never try to touch or ride it. Basking sunfish are easily hit by boats, and their skin and mucus layer are easily damaged. To learn how large these fish can grow, see How big do ocean sunfish get?
Frequently asked questions
Is a sunfish lying on its side sick or dead?
Usually not. Healthy sunfish often lie flat at the surface to bask and will right themselves and dive if disturbed. A sunfish that is stranded on a beach or trapped in very shallow water may be in trouble, and the best thing to do is report it to local marine wildlife responders rather than handle it.
Why are they called sunfish?
The English name comes from their habit of lying at the sea surface as if sunbathing. In many European languages they are called moonfish instead, because of their round shape, and the scientific name Mola is Latin for millstone.
How long do sunfish stay at the surface?
It varies. Tagged sunfish alternate between dives into cold water and periods near the surface throughout the day, and the colder the water they have been hunting in, the longer they tend to spend warming up afterward.
Is that a shark fin or a sunfish fin?
A sunfish's dorsal fin flops slowly from side to side and sometimes tips over, while a shark's dorsal fin stays upright and moves steadily forward, often with the tip of the tail fin showing behind it.
When is the best time to see ocean sunfish?
In the northeast Atlantic, including Cornwall, Ireland and the Bay of Biscay, most sightings are in summer and early autumn. Off California and in New England they are seen most often from summer into fall, when the sea surface is warmest.
Sources and further reading
- Nakamura, I., Goto, Y. and Sato, K., "Ocean sunfish rewarm at the surface after deep excursions to forage for siphonophores", Journal of Animal Ecology 84 (2015)
- Pope, E. C. et al., "The biology and ecology of the ocean sunfish Mola mola: a review of current knowledge and future research perspectives", Reviews in Fish Biology and Fisheries 20 (2010)
- IUCN Red List of Threatened Species, "Mola mola"
Cite this page
“Why Do Ocean Sunfish Lie on Their Side? Basking at the Surface.” Animal Atlas, https://animals.bigworldlist.com/questions/why-do-ocean-sunfish-lie-on-their-side/.
