The crown-of-thorns starfish is one of the most voracious coral predators on tropical reefs, feeding almost exclusively on the living tissue of hard (stony) corals. Adults evert their stomachs out through their mouths and press them directly onto a coral colony, digesting the soft tissue externally and leaving behind a white limestone skeleton. But this infamous coral diet is only part of the story. Before a crown-of-thorns starfish ever touches coral, it passes through two earlier life stages with completely different menus, and even as an adult its feeding choices are surprisingly selective in ways that reshape entire reef communities.
How They Find Food
Crown-of-thorns starfish have no eyes in any conventional sense. They navigate the reef and locate prey almost entirely through chemoreception, picking up chemical cues dissolved in the water. Researchers have tested this in Y-shaped maze setups, where water carrying scent from live coral flows down one arm and clean seawater flows down the other. Adult starfish reliably orient toward the coral-scented arm, confirming that dissolved chemicals from coral tissue are the primary trigger that draws them to a meal.
The strength of this chemical attraction is not constant. It ramps up before spawning season, when the animals have the greatest nutritional demand, and drops off afterward. A study using Y-maze behavioral assays found that starfish were significantly more attracted to coral-derived chemical cues during the pre-spawning period than during the post-spawning period, and that locomotor activity was also higher before reproduction.1Proceedings of the Japan Academy, Series B. Reproductive phase and seawater temperature modulate locomotion and chemosensory-guided orientation in crown-of-thorns starfish This makes intuitive sense: an animal about to invest enormous energy in producing millions of eggs or sperm needs to eat more, and the sensory system appears tuned accordingly.
Juveniles use the same chemosensory approach but respond to different cues. In laboratory choice experiments, juvenile crown-of-thorns starfish showed strong attraction to both live coral and coralline-algae-covered rubble, with roughly 60% of individuals moving toward these food cues when presented as single options. They also actively avoided chemical cues from adult crown-of-thorns starfish, which may help small juveniles steer clear of cannibalistic adults.2PubMed Central. Chemosensory behaviour of juvenile crown-of-thorns sea star (Acanthaster sp.), attraction to algal and coral food and avoidance of adult conspecifics
What Larvae Eat Before They Ever See a Reef
Crown-of-thorns starfish begin life as tiny, free-swimming larvae drifting in open water, and at this stage they eat nothing resembling coral. Scientists had long assumed the larvae fed on generic phytoplankton, but a striking laboratory study showed that larvae can complete their entire planktonic phase eating only nitrogen-fixing Trichodesmium cyanobacteria. Researchers confirmed the cyanobacteria in larval stomachs using fluorescence microscopy and traced nitrogen from labeled bacterial filaments into the larval tissues using stable isotopes. Some larvae fed solely on Trichodesmium were morphologically ready to settle and become bottom-dwelling juveniles after just 19 days.3PubMed Central. Crown-of-thorns starfish complete their larval phase eating only nitrogen-fixing Trichodesmium cyanobacteria
This finding has big implications. Trichodesmium blooms are fueled by nutrient runoff from agriculture and sewage, which means human activity could be inadvertently boosting the food supply for crown-of-thorns larvae. More food during the larval stage means more larvae survive to settle on the reef, potentially contributing to the massive population outbreaks that devastate coral.
The Long Herbivorous Juvenile Phase
Once a larva settles onto the reef and metamorphoses into a tiny juvenile starfish, it does not start eating coral right away. For months to years, juvenile crown-of-thorns starfish are herbivores, feeding on crustose coralline algae, the hard pinkish-purple algae that encrust rocks and dead coral surfaces. Field sampling of over 3,500 juvenile starfish showed that the switch from coralline algae to coral is a critical turning point: juveniles that make the transition experience exponential growth, while those that remain on algae stay small.4PubMed Central. Contrasting size and fate of juvenile crown-of-thorns starfish linked to ontogenetic diet shifts
What surprised researchers is just how long the herbivorous phase can last. In a set of rearing experiments, juvenile starfish kept on coralline algae without access to coral survived for over six and a half years, remaining small but alive. When these long-delayed juveniles were finally offered coral, they transitioned to the coral diet and began growing rapidly.5PubMed Central. The hidden army: corallivorous crown-of-thorns seastars can spend years as herbivorous juveniles This means reefs can harbor a hidden reservoir of tiny, algae-eating juveniles that could switch to coral-feeding whenever conditions allow. A reef that looks free of crown-of-thorns starfish might actually be sitting on a time-delayed population waiting for the right trigger.
Which Corals Adults Prefer
Once adults begin eating coral, they do not graze indiscriminately. Crown-of-thorns starfish show consistent preferences that hold across oceans. Across species and regions, they tend to favor fast-growing branching corals in the family Acroporidae when those corals are available. Pocilloporid corals are often the second choice. Massive, slow-growing Porites corals sit near the bottom of the preference list.6PubMed. Crown-of-thorns seastar (Acanthaster spp.) feeding ecology across species and regions
Field studies on individual reefs confirm this pattern in more detail. On reefs where researchers tracked feeding choices against the relative abundance of each coral type, acroporid corals were consumed far out of proportion to their availability, followed by pocilloporids. Faviid, merulinid, and poritid corals were eaten significantly less often than expected, even when they were common on the reef.7PubMed. Optimal Foraging Theory Explains Feeding Preferences in the Western Pacific Crown-of-Thorns Sea Star Acanthaster sp.
The preference goes beyond just choosing one coral over another. Laboratory experiments found that chemical extracts from Acropora and Pocillopora actually triggered stomach eversion in the starfish, the preparatory step for feeding. Extracts from Porites, by contrast, caused the starfish to withdraw entirely from the coral.8ScienceDirect (Elsevier). Factors affecting the behaviour of crown-of-thorns starfish (Acanthaster planci L.) on the Great Barrier Reef: 2: Feeding preferences Something in the chemical signature of Porites is actively repellent, not just unappealing.
Why Some Corals Are Avoided
The repellent quality of Porites and the attractiveness of Acropora may come down to venom. Corals are cnidarians, the same broad group as jellyfish, and they produce an array of toxins. A genomic comparison of preferred versus non-preferred crown-of-thorns prey species found that the two groups carry different toxic arsenals. Both groups had neurotoxins and pore-forming toxins, but Porites species uniquely possessed a class of jellyfish-like toxins called CfTX proteins. Meanwhile, Acropora species had their own unique pore-forming toxins with an aerolysin domain.9PubMed Central. Coral Venom and Toxins as Protection Against Crown-of-Thorns Sea Star Attack
The implication is that the jellyfish-like toxins in Porites may be effective at deterring a starfish that digests externally, since the everted stomach is essentially a thin, exposed membrane pressed against the coral surface. Toxins that damage cell membranes could make feeding painful or dangerous. Acropora’s toxins, despite being different, clearly do not deter feeding the same way, and the reasons probably involve the specific receptors and tissues the toxins target.
The transition to coral-feeding is itself a risky moment for juvenile starfish. Young starfish attempting to feed on coral for the first time often sustain arm damage, presumably from the coral’s nematocysts and chemical defenses. Damaged arms were reduced to roughly 65% of the length of an intact arm, though most injured juveniles could regenerate with an average predicted recovery time of about four months.10Marine Ecology Progress Series. Coral defences: the perilous transition of juvenile crown-of-thorns starfish to corallivory This regenerative ability means coral defenses slow the starfish down but rarely stop them permanently.
How Much Coral a Single Starfish Eats
Feeding rates vary enormously depending on the starfish’s size and how much coral is around. A study that tracked daily consumption for 565 individual starfish across a range of conditions on the Great Barrier Reef found that large individuals, over 40 centimeters in diameter, at sites with high coral cover consumed over ten times more coral tissue per day than smaller individuals at low-coral sites. The large starfish at coral-rich sites averaged about 222 square centimeters of coral tissue per day, while smaller ones at depleted sites averaged around 19 square centimeters.11Coral Reefs. Daily feeding rates of Pacific crown-of-thorns starfish (Acanthaster cf. solaris) vary with body size and coral cover across the Great Barrier Reef
That tenfold range matters for predicting reef damage. During outbreaks, population densities can jump from fewer than one starfish per hectare to hundreds or even thousands per hectare.12PubMed Central. Crown of thorns starfish life-history traits contribute to outbreaks, a continuing concern for coral reefs Multiply a few hundred large, hungry starfish by 222 square centimeters of coral per day, and the math turns grim quickly. Feeding-rate data like this feeds directly into the models reef managers use to decide when and where to intervene with manual removal or injection programs.
What They Eat When Coral Runs Out
Crown-of-thorns starfish are specialized coral predators, but they are not perfectly obligate ones. When coral becomes scarce, which typically happens toward the end of an outbreak after the starfish have eaten through most of the available reef, they have been documented feeding on soft corals, sponges, and even molluscs. Under certain conditions, they will consume members of their own species.13ScienceDirect (Elsevier). Factors affecting the behaviour of crown-of-thorns starfish (Acanthaster planci L.) on the Great Barrier Reef: 2: Feeding preferences These alternative food sources appear to be last-resort options rather than a regular part of the diet. On healthy reefs with abundant coral, crown-of-thorns starfish focus almost exclusively on hard corals.
The willingness to eat non-coral prey at extremes means that even a badly degraded reef is not completely free from feeding pressure. A post-outbreak reef dominated by algae and rubble still has soft corals, sponges, and other organisms that might sustain a residual starfish population, and the hidden bank of algae-feeding juveniles described earlier adds another layer of persistence. Complete starvation-driven die-off of a crown-of-thorns population is possible, but the species has enough dietary flexibility to hang on longer than you might expect.
How Selective Feeding Reshapes Reef Communities
Because crown-of-thorns starfish do not eat all corals equally, their outbreaks do not just reduce coral cover, they change which corals dominate the reef. A study in the South China Sea documented this process in real time. Montipora colonies, which tend to be heat-tolerant encrusting corals, suffered heavy predation, with feeding scars covering about 57% of their surfaces. Their relative abundance on the reef dropped from 40% to 14%. Meanwhile, branching Pocillopora, a less heat-tolerant genus, increased from 40% to 60% of the community, largely because it was eaten less aggressively. Rarer genera like Millepora and Astreopora were hit hardest of all, with scar coverage between 78% and nearly 100%.14Marine and Freshwater Research. Impacts of selective feeding of crown-of-thorns starfish on the coral community in the South China Sea
This creates an unsettling dynamic when you consider climate change. If starfish preferentially remove heat-tolerant coral species, the reef community left behind may be more vulnerable to bleaching during marine heatwaves. The one-two punch of crown-of-thorns predation followed by a bleaching event could be worse than either disturbance alone, because the starfish have already culled the corals best equipped to survive warm water. It is worth noting that feeding preferences can vary somewhat between regions and between coral communities with different compositions, so the South China Sea pattern will not replicate exactly everywhere. But the principle holds: selective predation shifts community structure, and the direction of that shift matters for reef resilience.
A Diet That Changes at Every Life Stage
Pulling the dietary story together across the entire life cycle reveals just how versatile this animal is. In the plankton, the larva feeds on microorganisms, potentially specializing on nitrogen-fixing cyanobacteria that bloom after nutrient enrichment events. After settling, the tiny juvenile spends months to years as an inconspicuous herbivore on coralline algae, essentially invisible to reef surveys. The switch to coral feeding unlocks rapid growth, and from that point on the starfish becomes a dedicated corallivore that uses chemical cues to locate preferred prey, favoring branching acroporids above all others.
Each of these dietary stages has a different relationship with human impacts on the ocean. Nutrient runoff may boost larval survival by fueling cyanobacterial blooms. Overfishing of natural predators like large wrasses and triggerfish may reduce mortality during the vulnerable juvenile phase. And the loss of reef structural complexity from bleaching or storm damage can concentrate adult starfish on remaining coral patches, accelerating local destruction. The feeding ecology of the crown-of-thorns starfish is not just a natural-history curiosity; it is tangled into some of the most pressing management questions facing tropical reefs today.
Cannibalism and Other Unusual Prey
The reports of cannibalism in crown-of-thorns starfish are more than lab oddities. In overcrowded conditions, such as the tail end of a major outbreak when coral is nearly gone, larger starfish will consume smaller ones. The juvenile avoidance of chemical cues from adults, documented in laboratory choice experiments, may have evolved as a direct response to this threat.2PubMed Central. Chemosensory behaviour of juvenile crown-of-thorns sea star (Acanthaster sp.), attraction to algal and coral food and avoidance of adult conspecifics A juvenile that can smell an adult nearby and actively move away from it has a better chance of surviving to maturity.
Beyond cannibalism, crown-of-thorns starfish have occasionally been observed feeding on sea anemones, encrusting bryozoans, and even biofilm-coated substrate. None of these food sources support the rapid growth that coral feeding does, and none appear to be common dietary components under normal reef conditions. They represent a survival strategy rather than a nutritional preference, allowing the starfish to persist through lean times when its primary food has been exhausted. For reef managers, this behavioral flexibility means that simply reducing local coral cover does not guarantee the starfish will starve out of an area. They are more persistent than a strict coral-specialist would be, which complicates control efforts.