Crown of thorns starfish live on coral reefs throughout the Indo-Pacific, from the Red Sea and the east coast of Africa to the central Pacific islands. They’re found anywhere warm, shallow coral reefs exist in this vast range, spanning roughly from 30°N to 30°S latitude. Their distribution includes some of the world’s most well-known reef systems: Australia’s Great Barrier Reef, the reefs of Japan, Hawaii, Fiji, the Maldives, Guam, Palau, Micronesia, American Samoa, the Cook Islands, and East Malaysia.
Not One Species, but Five
For centuries, scientists treated all crown of thorns starfish as a single species, Acanthaster planci. DNA analysis has since revealed the group contains at least five distinct species spread across different regions. The species on Australia’s Great Barrier Reef, Acanthaster solaris, is primarily found in the western Pacific and eastern Indian Ocean. The species in the eastern Pacific, where different coral communities exist, has adapted to feed on coral types that other crown of thorns species tend to avoid. For most practical purposes, the different species share similar habits and habitat needs, but their ranges don’t fully overlap.
Where on the Reef They’re Found
Crown of thorns starfish don’t spread evenly across a reef. Adults concentrate in the protected back-reef zones, the calmer, shallower areas sheltered from direct wave action. Juveniles take a different approach: they feed at night on the more exposed front reef, where the risk of being spotted by predators is lower in the dark. During outbreaks, large aggregations often first appear at deeper locations near the base of the reef, around 18 meters deep, before spreading into shallower water.
Surveys around Moorea in French Polynesia tracked starfish across reef habitat spanning from the surface down to about 35 meters, where sand plains begin. Researchers commonly found them at depths between 6 and 18 meters, though they’ve been surveyed across a 10 to 30 meter depth band on outer reef slopes. They rarely venture beyond the reef structure itself, staying where living coral provides both food and shelter.
Why They Cluster Where They Do
The single biggest factor determining where crown of thorns starfish concentrate is what corals are available to eat. When fast-growing branching corals like Acropora are abundant, the starfish strongly prefer them. Slower-growing, mound-shaped corals like Porites are generally their least favored prey. This means reefs dominated by branching corals tend to attract and sustain larger populations. In the eastern Pacific, where Acropora is largely absent, the local crown of thorns species adapts and feeds on whatever coral is available, including Porites.
This feeding preference has a practical consequence for where outbreaks cause the most damage. Reefs rich in Acropora, which also happens to be the coral with the greatest recovery potential after mass die-offs, bear the heaviest predation pressure.
Water Conditions They Need
Crown of thorns starfish need warm, tropical water. Their larvae develop best at around 28°C with a salinity of about 30 parts per thousand, conditions typical of tropical reef waters during the wet season. The larvae can develop across a broader thermal window of 25°C to roughly 31.6°C, which is important because it means warming oceans are opening up new territory for them.
Nutrient levels in the water also play a critical role in where populations boom. The tiny, free-floating larvae feed on phytoplankton, and their survival jumps dramatically when nutrient-rich water fuels plankton growth. Research has shown that larvae grow significantly faster and larger when chlorophyll concentrations (a proxy for phytoplankton abundance) reach 0.8 micrograms per liter or above. In very nutrient-poor water, at around 0.1 micrograms per liter, larval survival hits a critical low point. This means areas near river mouths, agricultural runoff, or even seabird nesting colonies (which deposit natural fertilizer) can become hotspots for larval survival and, eventually, outbreaks.
Outbreak Hotspots Today
Australia’s Great Barrier Reef remains the most closely monitored population center. As of 2024, outbreaks continue to affect reefs across the region, with persistent problems at some southern reefs and a newer outbreak in the northern sector. The overall status has improved slightly from “very poor” in 2019 to “poor,” thanks in part to ramped-up control efforts that have reduced impacts at both local and regional scales. Control teams inject individual starfish with solutions that kill them, a labor-intensive process that works best at protecting specific high-value reefs.
Outbreaks have also been documented repeatedly in Japan, Guam, Fiji, and across Micronesia. These tend to follow cycles, with populations exploding over a few years and then collapsing once they’ve consumed much of the available coral.
Their Range Is Expanding
Crown of thorns starfish are now showing up in places they historically weren’t found. On Australia’s east coast, the geographic range of outbreaks has shifted toward the poles over the past 50 years, pushing onto subtropical reefs and into coastal areas in New South Wales. This southward creep is driven by the strengthening East Australian Current, which carries warmer water (and floating larvae) into higher latitudes.
The math behind this expansion is striking: a 2°C increase in water temperature can reduce larval development time by 30% and increase the probability of survival by 240%. Combined with the species’ enormous reproductive output and a larval stage that drifts in ocean currents for several weeks, warming waters are effectively turning subtropical reefs into viable habitat. These subtropical reefs sit at the boundary between tropical and temperate marine environments and host unique mixes of species from both zones, making them especially vulnerable to a new, voracious coral predator establishing itself.
Scientists expect outbreaks on subtropical reefs to become more frequent as ocean warming continues, with larvae arriving through episodic dispersal events riding strengthening poleward currents. Reefs that were once too cool for crown of thorns starfish are increasingly within their thermal range.