Jellyfish are carnivores that feed primarily on tiny drifting animals collectively called zooplankton, with copepods (small crustaceans roughly the size of a grain of rice) making up the bulk of most species’ diets. Beyond that staple, jellyfish also consume fish eggs, fish larvae, other jellyfish, and an assortment of whatever small organisms drift into their tentacles. What makes their feeding remarkable is less about pickiness and more about method: jellyfish have no brain, no centralized mouth in the way we think of one, and yet they can strip a local patch of water clean of prey in hours.
The Staple Diet Across Species
If you could peek inside the stomach of most jellyfish species, you would find copepods above almost everything else. A study examining the gut contents of the South Atlantic jellyfish Lychnorhiza lucerna found that calanoid copepods alone made up about 53% of the prey, with other copepod types adding another 26% on top of that. Bivalve larvae (tiny young shellfish) rounded out much of the remainder at around 7%.1Marine Biology. Diet, prey selection, and individual feeding rates of the jellyfish Lychnorhiza lucerna (Scyphozoa, Rhizostomeae) Studies of other species in Alaskan waters found similar patterns: the guts of moon jellyfish, lion’s mane jellyfish, and crystal jellyfish contained mainly copepods, larvaceans, and cladocerans, along with a mix of larval organisms.2Marine Ecology Progress Series. Predation on zooplankton by large jellyfish, Aurelia labiata, Cyanea capillata and Aequorea aequorea, in Prince William Sound, Alaska
The pattern holds remarkably well across oceans and species. Jellyfish are not so much selective hunters as they are living drift nets. Their diet composition tends to mirror whatever is floating nearby in proportional abundance. If the surrounding water is full of copepods, the jellyfish eats copepods. If larvaceans happen to be plentiful, they show up in the gut too. This is not a sign of indifference to what they eat, though. The size of the jellyfish, the length and stickiness of its tentacles, and the speed at which it pulses all shape which prey it actually captures from the available buffet.
Fish Eggs and Larvae on the Menu
One of the more consequential items on the jellyfish menu is ichthyoplankton, the eggs and larvae of fish. Research on the mauve stinger (Pelagia noctiluca) in the northwestern Mediterranean found that larger medusae contained relatively high numbers of fish eggs and larvae in their guts. In areas where jellyfish and anchovy larvae overlapped geographically, medusae consumed between 0.1% and 0.9% of the anchovy larval population per hour, which sounds small until you consider it compounds over a full day and across thousands of individual jellyfish.3Journal of Plankton Research. Natural diet and predation impacts of Pelagia noctiluca on fish eggs and larvae in the NW Mediterranean The younger jellyfish stages (ephyrae) actually consumed anchovy larvae at even higher rates, taking out between 1.5% and 10.4% of anchovy larvae per hour in some areas.
Moon jellyfish (Aurelia aurita) show a similar appetite for fish larvae. In a Danish fjord system, predation by Aurelia was intense enough that the half-life of cirripede larvae and fish larvae in the water column dropped to less than one day at several locations.4Marine Ecology Progress Series. Clearance rates of jellyfish and their potential predation impact on zooplankton and fish larvae in a neritic ecosystem (Limfjorden, Denmark) That means jellyfish were removing half the standing population of those larvae every 24 hours. For fish species that depend on high larval survival to sustain their populations, this predation pressure is a genuine ecological force.
How Jellyfish Actually Capture Food
Jellyfish do not chase prey. They are ambush predators in the truest sense. The basic capture strategy involves trailing tentacles equipped with specialized stinging cells called nematocysts. When a copepod or larva brushes against a tentacle, nematocysts fire in milliseconds, injecting venom or physically entangling the prey with tiny coiled threads. The prey is then transferred toward the oral arms or the central mouth. The whole process is passive enough that a jellyfish can feed while barely moving, which is part of why they are so energetically efficient for their size.
Different species have evolved variations on this theme. Species in the order Rhizostomeae, which includes some of the largest jellyfish in the world, lack the long trailing tentacles you might picture from nature documentaries. Instead, they have elaborate, frilly oral arms covered in tiny mouth openings. These jellyfish feed by filtering water through the branching folds of their oral arms, trapping small particles and plankton in mucus. The prey never encounters a dramatic tentacle sting; it is simply caught in the sticky architecture of the oral arms and funneled inward.
For species with long tentacles, bell size and pulsing rhythm matter. Larger jellyfish with wider bells sweep a greater volume of water with each pulse, increasing encounter rates with prey. Research in Alaskan waters showed that the number of prey in a jellyfish’s gut was significantly correlated with both the medusa’s diameter and the density of prey in the surrounding water.2Marine Ecology Progress Series. Predation on zooplankton by large jellyfish, Aurelia labiata, Cyanea capillata and Aequorea aequorea, in Prince William Sound, Alaska Bigger jellyfish eat more, and jellyfish in richer waters eat more. The math is straightforward: individual medusae were estimated to consume hundreds to thousands of prey items daily.
Jellyfish That Eat Other Jellyfish
Not all jellyfish stick to tiny crustaceans and fish larvae. Some eat other jellyfish. The lion’s mane jellyfish (Cyanea capillata), which can grow tentacles over 30 meters long, is a well-documented predator of the moon jellyfish (Aurelia aurita). Both species occupy similar waters in the North Atlantic and other boreal regions, and both feed on the same copepods and fish larvae. But Cyanea also feeds directly on Aurelia, creating what ecologists call intraguild predation, where a predator eats a competitor.5Marine Biology. Intraguild predatory interactions between the jellyfish Cyanea capillata and Aurelia aurita
This jellyfish-on-jellyfish feeding is not rare or unusual. Researchers have noted qualitative observations of it across many species, though actual rate measurements remain scarce. The ecological logic is simple: if you are a large jellyfish with potent nematocysts, and a smaller, softer-bodied jellyfish drifts into your tentacles, it is food. Cyanea essentially treats Aurelia as just another gelatinous prey item. The young ephyrae of the mauve stinger have been found with siphonophores (colonial relatives of jellyfish) as their most numerous gut contents, showing that even juvenile jellyfish may lean heavily on other gelatinous organisms when those are what is available.
How Digestion Works Without a Gut
Jellyfish do not have a separate stomach and intestine. They have a gastrovascular cavity, a single internal space that serves as both stomach and circulatory system. Digestion happens in two stages. First, enzymes secreted by specialized gland cells break down the prey inside the cavity, dissolving it into smaller particles. Then, cells lining the cavity absorb these particles and finish digesting them internally using their own digestive machinery.6PubMed Central. A non-bilaterian perspective on the development and evolution of animal digestive systems Anything that cannot be digested gets expelled back out through the same opening it entered. There is no separate exit.
The speed of this process varies by species and water temperature. In Alaskan waters at 14°C, digestion of copepods took an average of three hours for moon jellyfish (Aurelia labiata) and about two hours for lion’s mane jellyfish (Cyanea capillata). Larvaceans, which are softer-bodied, were digested faster by both species, averaging around an hour and a half.2Marine Ecology Progress Series. Predation on zooplankton by large jellyfish, Aurelia labiata, Cyanea capillata and Aequorea aequorea, in Prince William Sound, Alaska These digestion times matter for researchers trying to estimate how much jellyfish eat. If you count the prey in a jellyfish’s gut and know how fast that gut empties, you can calculate a daily feeding rate. Faster digestion means the snapshot of gut contents underestimates total consumption.
What Polyps Eat Before Becoming Jellyfish
Most people think of jellyfish as the bell-shaped medusa drifting through open water, but that is only the adult stage. Many species spend months or years as tiny polyps, sessile forms attached to rocks, shells, or other hard surfaces on the sea floor. Polyps eat too, and their diet shifts with the seasons.
Research tracking the polyps of jellyfish in the Beaulieu River in England found that polyp diet changed markedly between summer and winter. In summer, when plankton productivity was high, polyps drew nutrition primarily from pelagic (open-water) sources like small zooplankton. In winter, when plankton production dropped, polyps shifted to feeding on material derived from the sea floor.7PubMed Central. Insights into the feeding of jellyfish polyps in wild and laboratory conditions: do experiments provide realistic estimates of natural functional rates? The summer zooplankton diet promoted rapid protein synthesis in the polyps, while the standard lab diet of brine shrimp (Artemia) actually appeared to impede protein growth by comparison. This finding is a reminder that what we feed jellyfish in captivity and what they eat in the wild are not always the same, which matters for interpreting laboratory experiments on jellyfish biology.
Dissolved Organic Matter as a Nutritional Supplement
Young jellyfish may not rely entirely on catching visible prey. Research on the ephyrae (juvenile stages) of two Pacific species showed that dissolved organic matter in seawater can serve as a supplementary food source. Ephyrae of Aurelia labiata that were given only dissolved organic molecules had higher carbon content than ephyrae that were simply starved, indicating they were absorbing nutrition directly from the water around them.8Journal of Plankton Research. Uptake of dissolved organic matter by ephyrae of two species of scyphomedusae Further evidence came from the uptake of fluorescently labeled amino acid compounds and increased ammonia production after exposure.
This does not mean dissolved organic matter can replace real food. The ephyrae fed only dissolved molecules developed abnormal shapes similar to those of starved individuals, and none of them matured into full medusae. So dissolved organic matter may act as a survival buffer during food-scarce periods, helping juvenile jellyfish stay alive a bit longer, but it is not a complete diet. For the jellyfish to grow, develop normally, and reproduce, it still needs to catch actual prey.
Competition With Fish for the Same Food
Because jellyfish eat many of the same things that small fish eat, the two groups are direct competitors. This competition is a growing concern in marine management, especially as jellyfish blooms appear to be intensifying in some regions. The overlap is not trivial. Research in Chinese coastal waters found that the trophic overlap between jellyfish and small omnivorous fish (under 10 cm) exceeded 65% on average, meaning those fish and jellyfish are drawing from essentially the same food pool.9PubMed. Trophic effects of jellyfish blooms on fish populations in ecosystems of the coastal waters of China For larger omnivorous fish, the overlap dropped below 55%, and those bigger fish appeared to shift their diets upward in the food web when jellyfish biomass was high, essentially switching to prey that jellyfish could not easily catch.
The competitive dynamic runs both ways. Jellyfish eat the larvae of fish that, as adults, would eat jellyfish or compete with them for zooplankton. This creates a potential feedback loop: when jellyfish populations boom, they eat more fish larvae, which reduces future fish populations, which removes a competitor for zooplankton, which may allow jellyfish populations to grow further.10PubMed Central. The global susceptibility of coastal forage fish to competition by large jellyfish Whether this cycle actually drives long-term jellyfish increases is still debated, but the competitive overlap is well documented and the concern is real for fisheries managers.
Microplastics as Unintentional Prey
Jellyfish do not choose to eat plastic, but their feeding mechanisms make them particularly vulnerable to ingesting it. Because many species capture food by trapping particles in mucus or sweeping water through oral arms, they cannot easily distinguish between a copepod and a plastic fiber of the same size. Research on the upside-down jellyfish (Cassiopea xamachana) in Florida estuaries found that 77% of the 115 specimens examined contained microplastics.11PubMed Central. Evidence of microplastics from benthic jellyfish (Cassiopea xamachana) in Florida estuaries Synthetic microfibers dominated across all three study sites.
Cassiopea is a bottom-dwelling species that sits upside down on the seafloor, pulsing gently to draw water over its oral arms. This lifestyle makes it especially exposed to microplastics that settle into sediments. But the finding has implications beyond one species. Any jellyfish that feeds by filtering or trapping small particles is likely ingesting microplastics in polluted waters. The ecological significance of this is still being studied. Jellyfish are eaten by sea turtles, ocean sunfish, and various other predators, so plastics accumulated in jellyfish tissue could move up the food chain. The jellyfish themselves may be acting as sinks for microplastic pollution, concentrating particles from the water column and either retaining them or passing them to whatever eats the jellyfish.
Why Feeding Rates Are Hard to Pin Down
Estimating how much a wild jellyfish eats is surprisingly tricky. The standard approach involves counting prey items in the gut, measuring digestion time, and combining those numbers with field data on how many jellyfish are in the water. Each step introduces uncertainty. Gut contents give you a snapshot of what was recently eaten, but digestion speed varies with temperature, prey type, and the jellyfish’s own metabolic state. A copepod might take three hours to digest at one temperature but much longer in colder water.
Lab experiments add another wrinkle. When researchers feed polyps or medusae in controlled conditions, the results may not reflect natural feeding. The polyp study from the Beaulieu River made this point directly: polyps in the wild switched between pelagic and benthic food sources with the seasons, while lab-reared polyps were stuck on a brine-shrimp diet that appeared to limit their protein production.7PubMed Central. Insights into the feeding of jellyfish polyps in wild and laboratory conditions: do experiments provide realistic estimates of natural functional rates? If lab diets change how polyps grow and develop, then lab-derived feeding rates may systematically under- or overestimate what happens in the ocean. Researchers working on this problem are increasingly turning to chemical tracers in jellyfish tissue to reconstruct diet over longer time windows, rather than relying solely on what happens to be in the stomach at the moment of collection.
The difficulty of measurement feeds into a bigger open question: are jellyfish really increasing globally, or are we just noticing them more? If feeding rates and population sizes are both uncertain, then claims about jellyfish taking over the oceans deserve some skepticism. What the evidence does clearly show is that where jellyfish are abundant, they eat a lot, and the consequences ripple through the food web in measurable ways.