Drupella snails are small muricid gastropods with an outsized impact on tropical reefs. Since outbreaks were first documented in the 1980s in Japan, the Philippines and Western Australia, these snails have become increasingly recognised as major drivers of coral loss. The scale of the damage can be staggering. In Ningaloo Marine Park in Western Australia, extensive grazing by Drupella was linked to mean reductions of 86 percent in coral cover on reef flats and back-reef areas between 1980 and 1987, along with a 47 percent decline on a monitored fore-reef slope. Beyond directly stripping coral tissue, the snails wound their prey in ways that allow pathogens to enter, facilitating the transmission of diseases such as brown band disease on the Great Barrier Reef. Outbreaks are often fueled by human pressures: overfishing removes natural predators such as wrasse, triggerfish, snapper and pufferfish, while climate-driven stressors like warming seas, nutrient enrichment, sedimentation and pollution weaken corals and create ideal conditions for snail populations to explode.

What makes the new observations particularly significant is that Drupella is typically thought of as a specialist predator of scleractinian corals, especially branching Acropora species. Fire corals, by contrast, are not true corals at all. They belong to the hydrozoan genus Millepora, order Anthoathecata, and possess stinging cells called nematocysts that deliver painful stings and can deter many would-be predators. Of the 16 Millepora species recognised worldwide, six occur in Indonesian waters. Milleporids grow quickly and reproduce prolifically, allowing them to compete aggressively for space, sometimes overgrowing and killing neighbouring stony corals. Yet they also play crucial ecological roles, providing nursery habitat for juvenile fish and shelter for a wide range of symbiotic invertebrates, including crabs, barnacles and scallops, some of which occur nowhere else.

The new study, led by Febrianne Sukiato of Luminocean and SeaChange Indonesia, with Kok Lynn Chew of Coralku Solutions and Tanya Toofany of the University of Bremen and the Leibniz Centre for Tropical Marine Research, documents the interaction at Gili Asahan, a site within the Gita Nada Marine Protected Area. The reef there ranges from a shallow flat at 1 to 4 metres depth down a slope to a sandy bottom at 14 to 16 metres, with roughly 40 to 60 percent coral cover and at least 136 morphologically identified scleractinian species. The site has a history of overfishing and pearl farming, and local conservation group SeaChange Indonesia has been running coral restoration work that includes targeted removal of Drupella from stony corals.

On 4 July 2025, during a scuba survey lasting 90 minutes, the team inspected mature Millepora colonies with a minimum diameter of 50 centimetres scattered across the shallow reef flat. Snails were found on all six colonies examined, representing three fire coral species: Millepora dichotoma, M. tenera and M. intricata. In total, the researchers removed 122 Drupella snails using scissors and long tweezers, carefully extracting the animals to avoid breaking the fragile fire coral branches. The snails were identified to genus based on shell morphology; species-level identification was deliberately not attempted, since several Drupella species have highly similar shells and reliable identification typically requires detailed taxonomic or molecular analysis.

The evidence of active feeding was compelling. Adult snails, defined as those exceeding 2 centimetres in shell length, were found tucked deep within the complex skeletal recesses of the fire coral colonies, and white feeding scars were visible directly beneath them, exposing the bare skeleton where tissue had been scraped away. In two cases, the researchers observed what appeared to be aggregations spilling over from the Millepora colonies onto adjacent Acropora colonies, where active feeding was documented on the corner of the stony coral facing the fire coral. The direction of movement was inferred from this spatial pattern, although continuous behavioural observations were not performed, so the researchers caution that movement direction could not be definitively confirmed.

Perhaps the most intriguing pattern was the age structure of the snail aggregations. Adults dominated all six colonies, with most measuring between 20 and 25 millimetres in shell length and the largest reaching 28 millimetres. Juveniles, ranging from 14 to 19 millimetres, were present in five of the six colonies but consistently in lower proportions. The number of snails per colony did not correlate with colony volume; the researchers calculated ellipsoidal colony volumes and found essentially no relationship with snail counts, with a coefficient of determination of just 0.0252. Given the limited sample size, occurrences were reported descriptively rather than subjected to species-level statistical comparisons.

The predominance of adults inside fire coral colonies suggests what the researchers describe as a possible ontogenetic shift in habitat use and prey choice, likely driven by feeding plasticity, refuge availability and energetic demands. The reticulate skeleton of Millepora is riddled with deep cavities that could offer adult snails shelter from predators and protection from waves and currents, meaning the fire coral may serve a dual role as both refuge and secondary food source. Juveniles, which face higher energetic demands for growth, may preferentially target more nutritious prey such as Acropora, consistent with earlier studies showing that young Drupella exhibit distinct prey preferences and occupy different coral hosts than adults. Body size is known to strongly influence foraging behaviour in gastropods under resource limitation, and Drupella are known to relocate frequently within and between colonies, sometimes within days, producing highly dynamic group sizes.

Chemical cues may also help explain the movement between coral genera. Stressed or overgrown Acropora are known to release compounds that attract corallivorous gastropods, and Drupella have been shown to move more than a metre overnight in response to such signals. At Gili Asahan, fire coral colonies form bushes 30 to 150 centimetres wide that grow right beside stony corals in the shallows, creating ideal staging conditions. The authors propose that adult Drupella may use Millepora as a refuge or staging habitat before exploiting nearby, chemically detectable stony coral prey. Millepora’s own competitive dominance could even reinforce this dynamic: by overgrowing and stressing adjacent acroporids, the fire coral may enhance the chemical signalling that draws snails to its neighbours, indirectly facilitating its own expansion, at least until sustained snail predation offsets those gains.

Interestingly, the researchers note that the Drupella sampling at Gili Asahan did not coincide with periods of siltation or thermal stress, suggesting that the use of Millepora in this case was not driven by an absence of preferred prey. This contrasts with earlier reports from the Gulf of Eilat in the Red Sea, where large aggregations of Drupella cornus initially fed on Acropora and Pocillopora damicornis before switching to Millepora dichotoma and other unusual prey following prey scarcity, and where snails were recorded on reefs dominated by Acropora–Millepora assemblages. Similar prey-switching in response to bleaching has been documented at Koh Tao in Thailand, where Drupella turned to previously avoided taxa such as fungiid corals and Pavona after bleaching wiped out Acropora. The Lombok observations suggest fire coral consumption may occur even when preferred prey remains available.

The ecological implications could be far-reaching. Millepora populations are already vulnerable to climate change. In the U.S. Virgin Islands, a marine heatwave eliminated 84 percent of a newly established milleporid community, and in the Maldives, fire coral zones lost during the 1998 bleaching event have only recently re-emerged in the country’s far north. At Gili Asahan itself, anecdotal observations suggest that after the 2016 mass bleaching event, reefs south of the study site shifted from stony coral dominance to Millepora. Added mortality from Drupella could therefore compound thermal stress on fire corals, reducing habitat availability for the fish and invertebrates that depend on them and further destabilising reef food webs already strained by warming temperatures and ocean acidification.

The authors stress that their findings, being based on limited, qualitative field observations, should be interpreted with caution. They call for research to quantify the prevalence, severity and feeding rates of Drupella on Millepora across broader spatial scales, and they recommend nocturnal surveys, since Drupella are primarily active at night. Genetic or morphometric identification of collected snails would also help clarify species-level patterns, as the taxonomy of Drupella remains under revision; recent work has even suggested that what has been called Drupella cornus may actually comprise two distinct species. Continued monitoring, the team argues, is essential for understanding these emerging trophic dynamics and for informing targeted removal efforts. As outbreaks of these voracious snails intensify across the Indo-Pacific, fire corals, long overlooked as snail prey, may need to be added to the watch list.