Cnidarians, the group that includes jellyfish, corals, sea anemones, and hydroids, feed on everything from microscopic plankton to small fish, and many supplement or even replace hunting with energy harvested from symbiotic algae living inside their own tissues. Their feeding strategies span an extraordinary range, from passive drifting tentacle curtains to bioluminescent deep-sea lures, and in one extreme branch of the family tree, to absorbing nutrients directly from a fish host’s bloodstream. What a cnidarian eats depends heavily on which cnidarian you’re asking about.
How Stinging Cells Make It All Possible
The defining feature of every cnidarian is the nematocyst, a microscopic capsule packed under pressure inside specialized stinging cells. When triggered by contact or chemical cues, a nematocyst fires a barbed or sticky thread in microseconds, injecting toxins, entangling prey, or both. This is the shared toolkit that lets a group of animals with no brain, no blood, and no centralized gut compete as predators in nearly every marine habitat on the planet. Tentacles studded with nematocysts are the most familiar arrangement, but the architecture varies wildly. A sea anemone holds its tentacles in a ring around its mouth. A jellyfish trails them beneath a pulsing bell. A siphonophore, which is actually a colony of specialized individuals, can deploy curtains of tentacles stretching meters through the water column.
Prey items stick or become paralyzed on contact and are transported to the mouth, which in most cnidarians also serves as the exit for waste. Digestion happens in a simple gastrovascular cavity where enzymes break food down, and nutrients are distributed directly through the tissue. There’s no gut in the vertebrate sense, no intestine, no separate stomach and esophagus. The whole system is radically simple, yet effective enough to support creatures from the size of a pinhead to colonial siphonophores longer than a blue whale.
Selective Predators, Not Mindless Drifters
A common misconception is that jellyfish just drift around swallowing whatever bumps into them. While some large species are genuinely indiscriminate, most are selective about what they eat. A broad review of jellyfish feeding found that most species are selective predators, with some having diets restricted to fish larvae or soft-bodied prey only.1ICES Journal of Marine Science. Of jellyfish, fish, and humans That selectivity can come from the size and type of nematocysts, the behavior of the animal, or where it positions itself in the water column.
Modern molecular techniques have made it possible to catalog jellyfish diets with much greater precision than traditional methods like visual gut analysis, which often turns up only unrecognizable mush. Researchers have used DNA sequencing on gut contents and tentacle scrapings of the Atlantic sea nettle to identify prey items that would be invisible under a microscope.2PubMed. Diet assessment of the Atlantic Sea Nettle Chrysaora quinquecirrha in Barnegat Bay, New Jersey, using next-generation sequencing These DNA-based methods have reshaped our understanding of cnidarian diets, revealing them to be both more varied and more targeted than anyone assumed when the only tool was a dissecting scope.
Siphonophores and the Deep-Sea Feeding Spectrum
Siphonophores may be the most impressive predators in the cnidarian world, and direct observations of their feeding in the deep ocean reveal a fascinating split between specialists and generalists. The small siphonophore Nanomia bijuga was observed feeding on 72 occasions, and 68 of those were on krill, making it a highly specialized predator working within a relatively narrow depth band.3PubMed Central. Deep pelagic food web structure as revealed by in situ feeding observations By contrast, large siphonophores like Praya dubia and Apolemia uvaria are passive generalists whose drifting tentacle curtains capture everything from copepods and comb jellies to fish and other jellyfish.3PubMed Central. Deep pelagic food web structure as revealed by in situ feeding observations Their strategy is less like a hunter and more like a living gill net strung vertically through the water.
At least one deep-sea siphonophore has taken hunting a step further by evolving bioluminescent lures. This species twitches glowing appendages to attract fish, which swim toward the light and into the colony’s stinging tentacles.4PubMed. Bioluminescent and red-fluorescent lures in a deep-sea siphonophore It’s a strategy more commonly associated with anglerfish, but siphonophores got there independently.
Stinging Grenades in Jellyfish Mucus
The upside-down jellyfish Cassiopea spends most of its life resting bell-down on the seafloor in shallow tropical waters, pulsing gently to expose its oral arms to sunlight. That posture serves its symbiotic algae, but Cassiopea also hunts in one of the strangest ways known among cnidarians. It releases tiny cell masses called cassiosomes into the surrounding mucus. These structures, roughly 100 to 550 micrometers across, are covered in nematocytes and remain motile for up to ten days after release.5Nature. Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea xamachana When cassiosomes encountered brine shrimp larvae in laboratory experiments, they immediately immobilized and killed the prey on contact.5Nature. Cassiosomes are stinging-cell structures in the mucus of the upside-down jellyfish Cassiopea xamachana In effect, Cassiopea outsources predation to autonomous stinging grenades that patrol the mucus cloud around it. This is likely the mechanism behind “stinging water,” the well-known phenomenon where swimmers in Cassiopea habitat feel stings without touching a jellyfish.
Sunlight as Food Through Symbiotic Algae
Many cnidarians don’t rely on captured prey alone. Reef-building corals, some sea anemones, and jellyfish like Cassiopea harbor single-celled photosynthetic algae, commonly called zooxanthellae, in their tissues. These algae convert sunlight and carbon dioxide into organic carbon and oxygen, fueling coral growth and calcification and effectively turning the animal into a solar-powered organism.6PubMed Central. The engine of the reef: photobiology of the coral-algal symbiosis In well-lit tropical waters, photosynthesis can supply the majority of a coral’s daily energy needs.
The nutrient exchange between coral and algae goes beyond just carbon. Dinoflagellate symbionts can fix 14 to 23 times more nitrogen from seawater than the coral host cells can, making the algae essential for acquiring this critical building block of proteins and DNA from nutrient-poor tropical seas.7PubMed Central. A single-cell view of ammonium assimilation in coral-dinoflagellate symbiosis The coral provides the algae with shelter and metabolic waste products they need, while the algae return sugars, amino acids, and fatty acids. It’s one of the most productive partnerships in the ocean.
Cassiopea takes this further by acting as a nutrient pump. By pulsing against the seafloor, it draws nutrient-rich water out of the sand beneath it, supplying its symbiotic algae with ammonium that fuels higher rates of photosynthesis than they’d achieve from the water column alone.8Marine Ecology Progress Series. Enhanced pore-water nutrient fluxes by the upside-down jellyfish Cassiopea sp. in a Red Sea coral reef So Cassiopea simultaneously farms sunlight through its algae, mines the sediment for fertilizer, and deploys autonomous stinging weapons to catch animal prey. Not bad for a jellyfish that looks like a wilted lettuce.
This photosynthetic partnership has deep evolutionary roots. Coral lineages that hosted zooxanthellae diversified more successfully than those that didn’t, but only after a major shift during the Triassic period. Before that transition, non-symbiotic coral forms actually diversified faster. It was only with the rise of the modern coral group, the scleractinians, that photosymbiosis became the dominant evolutionary strategy it is today.9PubMed Central. The contingent advantage of photosymbiosis in coral evolution
When the Algae Leave and Corals Have to Eat
Coral bleaching, triggered by heat stress, expels the symbiotic algae and cuts off the coral’s main energy supply. What happens next reveals just how important heterotrophic feeding, the animal side of the coral’s diet, actually is. Bleached corals ramp up their capture of tiny plankton from the water column and shift their preferences toward nitrogen-rich prey. In one study, bleached corals preferentially fed on Synechococcus, a type of cyanobacterium packed with nitrogen, over other picoplankton options with lower nitrogen content.10PubMed Central. Bleaching forces coral’s heterotrophy on diazotrophs and Synechococcus Bleached corals also incorporated 30-fold more nitrogen from nitrogen-fixing plankton in the water compared to healthy corals, suggesting a deliberate shift in nutrient strategy rather than random scavenging.10PubMed Central. Bleaching forces coral’s heterotrophy on diazotrophs and Synechococcus
The quality of the food matters enormously. In experiments testing different diets on bleached corals, complex feeds containing diverse particulate and dissolved components doubled to tripled growth compared to simple dissolved feeds. Bleached fragments of one species fed only thawed plankton died, while those receiving richer, more varied diets survived.11Marine Biology. Complex food sources aid physiological compensation of bleached corals These findings have practical implications for coral restoration: supplementing corals with diverse food during bleaching events could improve survival rates during the increasingly frequent marine heat waves hitting reefs worldwide.
There’s also growing evidence that scientists have been underestimating how much corals rely on animal food even when they’re healthy. Standard methods for measuring heterotrophy track carbon uptake, but corals appear to be selective about what they keep. Researchers found that fatty acids and nitrogen from prey were effectively incorporated into coral tissue, while carbon was not.12PubMed Central. Selective nutrient incorporation may underestimate heterotrophy of a mixotrophic reef-building coral In other words, corals may eat prey primarily for specific nutrients like nitrogen and essential fats rather than for energy, and the conventional measurement approach misses this because it tracks the wrong molecule. The real contribution of hunting to a coral’s nutrition is likely larger than the textbook version suggests.
Chemosynthetic Food in the Deep Sea
Below the depth where sunlight penetrates, photosynthesis is off the table. Deep-sea corals rely entirely on capturing particles sinking from above, but some have found another option. Corals living near cold seeps, places where methane and hydrogen sulfide leak from the seafloor, appear to tap into chemosynthetically derived food. Stable isotope analysis showed that many coral colonies near seeps incorporated carbon from chemosynthetic sources, and their microbiomes included sulfur-oxidizing bacteria that likely facilitate this process.13PubMed Central. Capacity of deep-sea corals to obtain nutrition from cold seeps aligned with microbiome reorganization The feeding strategy differed between coral species, with some shifting their microbiomes substantially near seeps while others showed little change, suggesting this isn’t a universal deep-sea coral trait but rather something certain species can exploit when the opportunity is there.
Parasitic Cnidarians That Eat From the Inside
Perhaps the most radical cnidarian feeding strategy belongs to a group most people have never heard of. Myxozoans are microscopic parasites that live inside fish and other animals, and despite looking nothing like jellyfish or corals, they are cnidarians. Molecular evidence firmly places them within the phylum, making them the only cnidarians that have abandoned free-living predation entirely in favor of absorbing nutrients from a host.
The genome of one myxozoan species, Thelohanellus kitauei, reveals that it relies heavily on secreting enzymes called proteases to digest host tissue and uses specialized receptor-mediated transport to absorb lipids and other nutrients.14Genome Biology and Evolution. The Genome of the Myxosporean Thelohanellus kitauei Shows Adaptations to Nutrient Acquisition within Its Fish Host Another myxozoan, Sphaerospora molnari, shows stage-dependent adaptations including proteins for nutrient uptake, immune evasion, and adhesion to host cells, some of which may have been acquired through horizontal gene transfer from completely unrelated organisms.15PubMed Central. Comparative transcriptomics reveal stage-dependent parasitic adaptations in the myxozoan Sphaerospora molnari These animals have simplified their bodies to the point of losing most cnidarian features, including tentacles and a gut, while retaining one thing: nematocyst-like polar capsules that help them anchor to and infect their hosts. It’s the cnidarian stinging cell repurposed for parasitism rather than predation.
How Jellyfish Feeding Reshapes Ocean Food Webs
Cnidarian feeding doesn’t just matter to the cnidarians. When jellyfish populations bloom, the collective appetite of millions of individuals can restructure entire food webs. In Chinese coastal waters, researchers found a trophic overlap of more than 65% between jellyfish and small omnivorous fish under ten centimeters, meaning they compete heavily for the same zooplankton prey.16PubMed. Trophic effects of jellyfish blooms on fish populations in ecosystems of the coastal waters of China Massive jellyfish aggregations depleted zooplankton but did not efficiently transfer that energy up the food chain, creating what amounts to a trophic dead end where energy gets locked into gelatinous biomass instead of feeding fish populations.16PubMed. Trophic effects of jellyfish blooms on fish populations in ecosystems of the coastal waters of China
The consequences are measurable for commercial fisheries. In Korean coastal waters, jellyfish bloom intensity showed a significant negative correlation with anchovy populations, and the effect scaled with bloom magnitude.17Hydrobiologia. Jellyfish blooms challenge the provisioning of ecosystem services in the Korean coastal waters It was the intensity of the blooms, not how long they lasted, that had the strongest effect on fisheries production. As jellyfish blooms become more frequent in warming and overfished seas, their role as competitors with commercially important fish species has moved from ecological curiosity to management concern.
Animals That Eat Cnidarians and Steal Their Weapons
Cnidarians sit on both sides of the predation equation. While they’re formidable predators thanks to their nematocysts, a surprising number of other animals eat them and, in one of nature’s more creative tricks, steal the stinging cells for their own defense. This behavior, called nematocyst sequestration, has evolved independently an estimated 9 to 17 times across four separate animal groups: comb jellies, flatworms, a group of small worm-like creatures called acoels, and sea slugs.18Invertebrate Biology. Sequestration of nematocysts by divergent cnidarian predators: mechanism, function, and evolution The nudibranch sea slugs are the most famous example. They consume hydroids or anemones, pass the unfired nematocysts through their digestive tract intact, and store them in finger-like projections on their backs called cerata. Anything that tries to eat the nudibranch gets a face full of stolen cnidarian weaponry. The fact that this strategy evolved so many times independently says something about how effective nematocysts are: even the animals that have figured out how to eat through cnidarian defenses recognize a good weapon when they swallow one.