Sea stars eat an astonishingly wide range of foods, from mussels and clams to coral, sea urchins, algae, and even other sea stars. What makes their diet especially remarkable is not just what they eat but how they eat it: most species feed by pushing their stomach out through their mouth and digesting prey externally. This approach lets them consume animals larger than their own mouths and tackle hard-shelled prey that would stump other predators. The specifics of what ends up on the menu depend heavily on the species, the habitat, and what is available, and the ecological consequences of all that eating ripple through entire marine ecosystems.
The Stomach That Goes Outside the Body
Most sea stars have two stomachs. The cardiac stomach, the lower one, can be everted through the mouth and pressed against or into prey. Once in contact with food, the stomach secretes digestive enzymes that begin breaking down tissue on the spot. With bivalves like mussels or clams, a sea star wraps its arms around the shell, uses the suction of its tube feet to pry the valves apart just a fraction of a millimeter, then slips its paper-thin stomach into the gap. Digestion happens inside the shell, and the sea star retracts its stomach along with the liquefied meal.
This extra-oral feeding behavior is so central to sea star biology that it is regulated by ancient hormonal systems. Researchers working with the common European starfish found that injecting a neuropeptide called asterotocin, part of the same chemical family as oxytocin in mammals, triggered fictive feeding in which the animals everted their cardiac stomachs and adopted feeding postures even without prey present.1PubMed Central. Ancient role of vasopressin/oxytocin-type neuropeptides as regulators of feeding revealed in an echinoderm The fact that such a deeply conserved hormone drives this behavior suggests stomach eversion has been part of the sea star playbook for hundreds of millions of years.
How Sea Stars Find Their Next Meal
Sea stars have no eyes capable of forming images (a few species have simple light-sensing spots at their arm tips), so finding food is overwhelmingly a chemical affair. Sensory cells concentrated on the tube feet and along the arm undersides detect molecules dissolved in seawater, and sea stars can be extraordinarily sensitive to these signals. In laboratory experiments with the species Marthasterias glacialis, researchers found that feeding behaviors like stomach eversion and movement toward a source could be triggered by lactic acid at concentrations as low as about 30 trillionths of a mole per liter. Certain amino acids and even acetylcholine also provoked feeding responses at very low thresholds.2The Royal Society Publishing. Behavioral study of chemoreception in the sea star Marthasterias glacialis: structure-activity relationships of lactic acid, amino acids, and acetylcholine
In practical terms, this means a wounded mussel or a stressed clam leaking body fluids into the water is essentially broadcasting a dinner bell. Sea stars follow these chemical gradients upstream, sometimes traveling considerable distances to reach prey aggregations. This chemical sensitivity also explains why sea stars can locate buried bivalves beneath a layer of sediment.
A Menu That Varies by Species and Habitat
There are roughly 1,900 known sea star species, and their diets range from strict carnivory to near-herbivory. In the rocky subtidal waters of the Galápagos, field surveys found that a high proportion of sea stars (between 35% and 68% of individuals observed, depending on species) were actively feeding at any given time. Most of the species there turned out to be facultative herbivores, with diets dominated by crustose and turf algae supplemented by small amounts of sessile invertebrates. The two most common species had the narrowest diets, while one species, Pentaceraster cumingi, was identified as a size-selective predator of small pencil urchins.3Journal of Experimental Marine Biology and Ecology. Distribution and feeding ecology of sea stars in the Galápagos rocky subtidal zone This kind of niche partitioning, where closely related species living in the same area eat different things, reduces direct competition and lets multiple sea star species coexist.
In temperate waters, many familiar sea stars are dedicated predators of shellfish. The ochre sea star of the Pacific coast feeds heavily on mussels. The common European starfish Asterias rubens targets mussels and cockles. Tropical species cover even more ground: some eat sponges, others eat coral, and a few scavenge dead fish or organic debris on the seafloor.
Deep-Sea Diets
Sea stars living in the deep ocean face a very different food landscape. A study using fatty acid signatures to trace what deep-sea sea stars were eating sorted species into three distinct feeding categories: suspension feeders that filtered particles drifting down from the surface, predator-scavengers that hunted or ate dead organisms, and mud ingesters that consumed sediment for its bacterial and organic content.4Marine Ecology Progress Series. Feeding ecology of deep-sea seastars (Echinodermata: Asteroidea): a fatty-acid biomarker approach The suspension feeders showed the strongest link to carbon originally produced by photosynthetic plankton near the surface, while the mud ingesters relied heavily on bacterial carbon. This spectrum shows how adaptable sea stars are: even in the near-total darkness of the deep sea, different species have carved out distinct ways to make a living.
Crown of Thorns Starfish and Coral Destruction
No discussion of sea star diets is complete without the crown of thorns starfish, one of the few marine invertebrates capable of single-handedly reshaping an entire reef. These large, heavily spined sea stars feed almost exclusively on hard coral. They climb onto a coral colony, evert their stomach over the living tissue, and digest it in place, leaving behind bare white skeleton. A single adult can consume several square meters of coral per year.
The real damage happens during population outbreaks. Under normal conditions, crown of thorns starfish are relatively rare, perhaps fewer than one per hectare. But their populations can explode to densities of ten to over a thousand per hectare, stripping reefs of coral and drastically reducing the functional and species diversity of the ecosystem.5PubMed Central. Crown of thorns starfish life-history traits contribute to outbreaks, a continuing concern for coral reefs On Australia’s Great Barrier Reef, these outbreaks are responsible for close to half of total coral cover loss.6PubMed Central. Optimising crown-of-thorns starfish control effort on the Great Barrier Reef
Crown of thorns larvae eat something completely different from the adults. In their planktonic stage, the tiny larvae feed on single-celled algae. Under good food conditions, larvae begin feeding at two to three days after fertilization and can reach the settlement stage in as few as 18 days. Under poor food conditions, larval development stalls and the animals die before ever reaching the seafloor.7PubMed Central. Variable food alters responses of larval crown-of-thorns starfish to ocean warming but not acidification This food dependence during early life may explain why nutrient-rich coastal runoff, which fuels algal blooms, is suspected of contributing to outbreaks. More nutrients mean more food for larvae, more larvae survive, and more juvenile starfish settle onto the reef.
Keystone Predators on Rocky Shores
The ochre sea star, Pisaster ochraceus, became one of the most famous animals in ecology because of what happens to coastal communities when it disappears. On the rocky shores of the Pacific coast, this species feeds preferentially on California mussels. By keeping mussel populations in check, the sea star prevents mussels from monopolizing the rock surface and crowding out other species. Remove the predator, and the mussels take over.
That scenario played out on a massive scale beginning in 2013, when sea star wasting syndrome caused catastrophic die-offs along the North American Pacific coast. Declines of adults exceeded 75% at the vast majority of monitored sites, and at over half of southern sites the loss was 99% or more.8PubMed Central. Large-scale impacts of sea star wasting disease (SSWD) on intertidal sea stars and implications for recovery The consequences were visible: on the central California coast, the lower boundary of mussel beds shifted downward toward the water by an average of roughly 19 centimeters on the rock face over four years, colonizing space they had been kept out of for decades.9PubMed. Expansion of intertidal mussel beds following disease-driven reduction of a keystone predator
Even where sea star counts have started to recover, the animals coming back tend to be small juveniles. Because predation pressure depends on body mass rather than sheer numbers, the total predation potential on mussels has remained low despite improving head counts.10PubMed Central. Reduction and recovery of keystone predation pressure after disease-related mass mortality A population of small sea stars simply cannot eat what a population of large ones could. Full ecological recovery likely depends not just on the return of sea stars but on their survival long enough to grow to adult size.
Sunflower Sea Stars and the Kelp Forest Connection
The sunflower sea star, Pycnopodia helianthoides, is one of the largest and fastest sea stars in the world, with up to two dozen arms and a voracious appetite for sea urchins. That appetite matters enormously for kelp forests. Sea urchins graze on kelp, and without a major predator keeping urchin populations down, they can mow kelp forests to bare rock in formations known as urchin barrens.
Research has shown that sunflower sea stars regulate purple sea urchin populations and help maintain healthy kelp forests through top-down control. Their recovery, whether natural or through human-assisted reintroduction, may be a key step in restoring kelp forests at meaningful scales.11PubMed Central. Sunflower sea star predation on urchins can facilitate kelp forest recovery Sunflower sea stars were among the species hardest hit by sea star wasting syndrome, and their absence has been linked to urchin population explosions and kelp loss along the west coast of North America.
Interestingly, it is not just direct predation that matters. Even the chemical presence of a sunflower sea star suppresses urchin grazing. Experiments found that the scent of Pycnopodia triggers a flee response in red sea urchins, creating a localized “landscape of fear” covering about 15 square meters around the predator’s position. Within that zone, urchins stop eating kelp and focus on escaping.12PubMed Central. Sunflower sea star chemical cues locally reduce kelp consumption by eliciting a flee response in red sea urchins So even a modest number of roaming sunflower sea stars could protect patches of kelp simply by scaring urchins away.
Prey That Fight Back
Not everything a sea star targets goes quietly. Scallops, for instance, have evolved a dramatic escape response. When a scallop detects the chemical signature of a nearby sea star, it can execute a jump by rapidly snapping its shell valves together, or launch into a swimming escape consisting of repeated valve claps that propel it through the water. Where exactly the stimulus touches the scallop determines the response: contact along the lateral or ventral mantle edges triggers jumps, while a stimulus near the dorsal “ears” of the shell specifically triggers the swimming sequence.13The Royal Society Publishing. Neurobiology of the scallop. I. Starfish-mediated escape behaviours
Mussels use a different strategy. They can detect water-borne cues from approaching sea stars and respond by strengthening their byssal threads, the protein filaments that anchor them to rock. Some species also cluster more tightly together, making it harder for a sea star to isolate and pry open a single individual. Sea urchins, as noted above, flee from sunflower sea star scent. These defensive behaviors are all driven by the same basic chemistry the sea stars use to find prey: dissolved molecules in seawater. The arms race between predator chemosensing and prey chemosensing is constant.
How Sea Stars Store Nutrients Between Meals
Sea stars can go surprisingly long without food, partly because they maintain substantial energy reserves in their pyloric caeca, a pair of digestive glands that extend into each arm. These organs store carbohydrates, proteins, and lipids, drawing on them during periods of scarcity or when energy is needed for reproduction. In a study of the ochre sea star and the bat star, prolonged starvation caused the pyloric caeca to shrink and nutrient reserves to drop below levels needed for normal gonad development. Carbohydrate reserves were the first to go, followed by lipids, though even after twenty months without food, the ochre sea star still retained considerable lipid stores in its caeca.14PubMed. Histochemical study of gut nutrient reserves in relation to reproduction and nutrition in the sea stars, Pisaster ochraceus and Patiria miniata
This capacity for long-term nutrient storage means sea stars can survive seasonal food shortages and continue functioning at a basic level even when conditions are poor. It also means that reproductive output is closely tied to feeding success: a well-fed sea star produces more eggs or sperm, while a starving one redirects its dwindling reserves just to stay alive.
Temperature and the Limits of Feeding
Water temperature places hard limits on how, and whether, a sea star can eat. In the Antarctic species Odontaster validus, researchers found a clear hierarchy: the animals lost feeding competence at 6°C, could no longer move at 9°C, and died at 15°C.15Inter-Research (Marine Ecology Progress Series). Temperature limits to activity, feeding and metabolism in the Antarctic starfish Odontaster validus The ability to digest food shut down well before the animals actually died. For polar sea stars adapted to near-freezing water, even modest warming could mean starvation long before it means direct heat death. This has implications as ocean temperatures rise: the feeding window for cold-adapted species may narrow, reducing their ability to sustain themselves and reproduce.
Sea Stars as a Problem for Mussel Farmers
The efficiency of sea star predation, so ecologically valuable on wild rocky shores, is a headache for shellfish aquaculture. In areas of mussel cultivation like the Menai Straits in Wales, where sea star densities can reach about four per square meter, it has been estimated that sea stars remove at least 20,000 mussels per hectare per day. Individual sea stars typically eat about half a mussel per day, but larger individuals can consume much faster, and aggregations of sea stars have been observed advancing up to 200 meters per month, destroying mussel beds in their path.16Wiley Online Library. Efficiency of starfish mopping in reducing predation on cultivated benthic mussels (Mytilus edulis Linnaeus)
Mussel farmers have long used “starfish mops,” heavy rope tangles dragged across the seabed to snag sea stars by their spines, as a control method. The approach works to a degree but requires constant effort and rarely eliminates the problem entirely. In some regions, the economic losses from sea star predation on mussel beds run into millions of dollars annually.
Feeding Sea Stars in Aquariums
Keeping sea stars in captivity comes with dietary challenges that mirror their wild diversity. What works for one species may not work for another. Juvenile Brazilian sea stars (Echinaster brasiliensis), for example, showed a strong preference for biofilm-covered rocks over other food options in captivity, with biofilm accounting for about half of observed substrate preference. Sponges, which are often used as captive food for this genus, turned out to be the least preferred option for juveniles.17Wiley Online Library. Food-related substrate preference in juveniles seastar Echinaster (Othilia) brasiliensis (Müller & Troschel,1842) in captivity Sourcing sponges year-round also presents practical difficulties for commercial or laboratory operations, making biofilm-based diets an attractive alternative for early life stages.
For hobbyists maintaining sea stars in home aquariums, the same principle applies: research the specific species. Carnivorous species like the chocolate chip sea star will eat bits of shrimp, mussel, or clam placed near them. Sand-sifting species consume detritus and microorganisms in the substrate and will slowly starve in a tank without a deep, mature sand bed. Reef-safe species that graze on algae and biofilm need established live rock. Mismatching the diet to the species is the most common reason sea stars waste away in captivity.