Ocean sunfish eat a surprisingly wide range of prey, from jellyfish and salps to small fish, squid, and crustaceans. Long assumed to survive almost entirely on jellyfish, recent DNA analysis of their stomach contents tells a more complex story. A 2016 study published in Scientific Reports found that crustaceans and small fish actually made up the majority of consumed prey, with jellyfish accounting for only about 16% of the diet.
A More Varied Diet Than Expected
For decades, scientists and nature documentaries described ocean sunfish as dedicated jellyfish eaters. That reputation came largely from indirect evidence: fatty acid profiles of a handful of fish caught off Nova Scotia, and the observation that sunfish tend to show up in the same waters as large jellyfish species like barrel jellyfish, compass jellyfish, and lion’s mane jellyfish. But when researchers used DNA barcoding to identify the actual contents of sunfish stomachs, the picture shifted dramatically. The diet turned out to be cosmopolitan, spanning crustaceans, small bony fish, hydrozoans, siphonophores, and ctenophores alongside the expected jellyfish.
Age and size play a role in what sunfish prefer. Younger, smaller sunfish (under about a meter in length) eat a mixed diet of bottom-dwelling and open-ocean prey, including small fish, squid, crustaceans, and jellies. Juveniles sometimes school together near shore to feed on these varied food sources. As sunfish grow larger, they shift toward gelatinous prey. Camera footage from tags attached to adult sunfish confirmed this preference, capturing feeding events on siphonophores, true jellyfish, and comb jellies in open water.
How Sunfish Catch and Process Prey
Ocean sunfish have a distinctive beak-like mouth formed by fused teeth, similar to their relatives the pufferfish. But the real feeding innovation is deeper in the throat. Sunfish have a set of recurved, spike-shaped pharyngeal teeth arranged in three loosely connected rows on their upper throat. These teeth sit nearly hidden under soft tissue at rest, but a strap-like muscle running across the jaw can evert them outward, functioning somewhat like a cat extending its claws.
Researchers at the University of California tested these throat teeth in flume experiments that simulated the jetting motion sunfish use to expel water after gulping prey. The teeth caught simulated gelatinous prey with 70% to 100% success and stayed locked in their sockets even at 50 times the normal jetting force. In effect, the throat acts as a retention cage, trapping slippery, soft-bodied prey that would otherwise slide right back out. This is a rare evolutionary solution, convergent with the throat spikes found in leatherback sea turtles and other unrelated species that face the same challenge of eating gelatinous animals.
Deep Dives and Foraging Strategy
Sunfish don’t just drift at the surface waiting for jellyfish to float by. Tracking data from the southern California Current system shows they actively migrate up and down through the water column on a daily cycle, following the vertical migration of their prey. During the day, sunfish dive deep. Occasionally they descend beyond 500 meters (about 1,600 feet). At night, they move into shallower water as zooplankton rise toward the surface.
They also target specific oceanographic features. Coastal upwelling fronts, where cold, nutrient-rich water meets warmer surface water, create concentrated patches of prey. Zooplankton surveys off central Baja California, Mexico, found dense populations of salps on the warm side of these fronts, exactly where tagged sunfish were spending their time. This kind of targeted foraging suggests sunfish are far more strategic feeders than their slow, drifting reputation implies.
The Low-Calorie Problem
Jellyfish and salps are mostly water. Bomb calorimetry measurements of three common jellyfish species found energy densities ranging from just 0.10 to 0.18 kilojoules per gram of wet mass. For comparison, a small fish packs roughly 20 to 40 times more energy per gram. Salps are similarly dilute, with an estimated 12% of their dry weight consisting of water trapped in their tissues rather than usable nutrition.
This creates a volume problem. Adult ocean sunfish typically weigh between 247 and 1,000 kilograms, and they need to eat 1% to 3% of their body weight each day. For a 500-kilogram sunfish, that means consuming 5 to 15 kilograms of food daily. If most of that food is gelatinous prey with minimal caloric value, the sunfish has to eat enormous quantities just to break even. This likely explains why sunfish also eat higher-energy foods like fish and crustaceans when available, and why they spend so much time actively searching productive ocean fronts rather than passively drifting.
Their digestive system reflects this diet. The stomach is small and underdeveloped, but the intestine is exceptionally long and deeply coiled, occupying most of the abdominal cavity. Sunfish also lack a pyloric caecum, the finger-like pouches that many fish use for additional digestion. Instead, the sheer length of the intestine provides extended surface area to extract whatever nutrition exists in their watery prey.
Plastic: A Growing Risk
Because sunfish feed on translucent, drifting organisms, they face the same plastic ingestion risk as sea turtles. Floating plastic bags and film can closely resemble jellyfish in the water column. Documentation of this problem in sunfish is still limited compared to turtle research, but a study from the western Mediterranean Sea confirmed plastic fragments in the digestive system of a sunfish for the first time. The fragment was classified as a mesoplastic, a piece between 5 and 25 millimeters. Given how much gelatinous prey sunfish consume and how visually similar some plastics are to that prey, the actual frequency of ingestion is likely higher than current records suggest.