Mesophotic ecosystems occupy the dimly lit zone of the ocean, roughly between 30 and 150 meters in many subtropical regions, where light is too scarce for most photosynthetic reef builders but where life still flourishes in remarkable abundance. In many oceanic archipelagos, these communities are dominated by black corals of the order Antipatharia, a group of more than 300 species characterized by proteinaceous skeletons, an almost universal lack of symbiotic algae, slow growth and long lifespans. Whip black corals are a distinctive form within this group: they grow as a single unbranched stem that can extend several meters in length, spiraling and swaying in areas of strong current. Where they aggregate in high densities, they form what ecologists call Marine Animal Forests, three-dimensional structures built by suspension-feeding animals that alter water flow, trap sediments, generate shade and create an architectural complexity comparable to that of terrestrial forests.

These animal forests are far more than scenic curiosities. Scientists regard them as biodiversity hotspots, nursery grounds and refugia for a vast array of associated species, from crustaceans to fish, and as key regulators of biogeochemical cycling in the sediments beneath their canopies. Yet because mesophotic depths require expensive equipment to explore, black coral forests remain among the least studied and least protected ecosystems on the planet. The new research, conducted as part of the Ocean Citizen project, a marine restoration initiative funded through Horizon Europe, set out to map benthic habitats along a stretch of Tenerife’s rocky coastline between La Arena and the southern end of San Juan Beach, and in doing so captured an unexpected ecological relationship on camera.

Between June 16 and 25, 2024, the research team, which included scientists from the University of Salento in Italy and the non-profit organization Oceana, deployed a remotely operated vehicle to survey the seabed. The vehicle, a Sibiu Pro model from Nido Robotics rated to 300 meters, was equipped with 4K cameras, LED lights and eight thrusters. It crawled along the bottom at an average speed of just 0.2 to 0.3 knots, its camera field of view spanning roughly 185 centimeters. Of 61 planned dives, 55 were completed, covering a total surveyed area of 31,217 square meters at depths ranging from 7 to 157 meters. The resulting footage was cut into 1,502 one-minute sample units, yielding 26 hours of analyzable video from which the team identified 169 distinct taxa.

The statistical analysis revealed a striking pattern. The bearded fireworm co-occurred with whip coral forests in 40 percent of the transects, appearing in the algae at the base of the black corals or in their immediate vicinity. Peak abundances of both the fireworm and the two whip coral species studied, Stichopathes gracilis and Stichopathes setacea, fell between 80 and 120 meters depth. Pearson correlation analysis confirmed that the association was not coincidental: the relationship between fireworm occurrence and S. gracilis produced a correlation coefficient of 0.895 with a p-value of 0.0027, a strong and statistically significant positive association, while the relationship with S. setacea yielded a coefficient of 0.715 with a p-value of 0.0464, also significant. All three species showed unimodal distributions across depth, peaking in the 80 to 100 meter range, with polynomial trendlines suggesting a partial overlap of their ecological niches.

Understanding why a fireworm would seek out a black coral forest requires appreciating the peculiar biology of both partners. Hermodice carunculata is a large amphinomid polychaete armed with brittle, venom-filled bristles that deliver painful stings to anything that handles it. It is an omnivorous, opportunistic feeder and scavenger, consuming decaying organic matter, and preying on corals, sea urchins, sponges and ascidians. It can also act as a reservoir for coral pathogens. Crucially, it is thermophilic and highly tolerant of environmental variation, coping with swings in temperature, salinity and oxygen. As ocean waters warm, the species has been expanding northward through the Atlantic and the Mediterranean, attacking fish caught in nets, degrading catch quality and injuring fishers, swimmers and divers. In shallow water it favors structurally complex habitats, hiding in rock cavities and algal canopies to evade predators and reduce exposure to current and light.

The whip corals, meanwhile, are ecosystem engineers of a very particular kind. Antipatharians require strong currents to feed, extending their polyp-covered stems into the flow to capture passing particles. Those same currents that sustain the forest also create a hostile hydrodynamic environment for a soft-bodied worm. The study’s authors propose a dual function for the forest: the whip corals are a product of strong currents, yet their dense stems and algal understory simultaneously provide shelter from the current’s sheer force. At the base of the corals, accumulated algae offer both refuge and a foraging ground rich in decaying organic matter, precisely the kind of organically enriched, microbially active habitat to which fireworms are known to be attracted. In essence, the fireworm appears to exploit a contradiction engineered by the corals themselves, benefiting from the very conditions that made the forest possible while escaping the conditions that make it formidable.

Perhaps the most consequential observation is what the researchers did not see. In all the analyzed video, fireworms were detected only at the base of the coral colonies, never climbing the stems. No feeding or predation behavior against the whip corals was recorded, and the coral populations appeared healthy, with no signs of damage or necrosis, despite their high density throughout the study area. This stands in contrast to recent reports of fireworms actively feeding on branching black coral colonies, concentrating on the colony tips. The evidence from Tenerife instead suggests a commensal or opportunistic relationship, in which the fireworm uses the forest as shelter and a feeding ground on associated algae and detritus rather than as prey, although the authors caution that the possibility of occasional coral consumption cannot be ruled out without further observation.

The finding carries implications well beyond the natural history of a single worm. The detection of H. carunculata at mesophotic depths extends its known vertical distribution, echoing a similar recent discovery in the same study area in which the green alga Caulerpa prolifera was recorded down to 80 meters, far deeper than previously acknowledged. As climate change pushes thermophilic, adaptable species into new depths and latitudes, understanding which habitats will harbor them, and whether vulnerable ecosystem engineers such as black corals will face new pressures, becomes a matter of urgency. Black corals are already listed in Appendix II of CITES, which regulates their international trade, and are used as indicator species for ecosystem vulnerability by regional fishery management organizations.

The authors argue that expanding knowledge of these deep, dim ecosystems is essential for conservation and for effective monitoring guidelines. Marine Animal Forests are among the most extensively distributed ecosystems on Earth, spanning from shallow mussel beds to deep cold-water coral habitats in both tropical and polar seas, yet the mesophotic black coral forests remain largely unmapped and unprotected. Determining the foraging advantages, potential risks and patterns of association between vagile species like the fireworm and the ecosystem engineers that structure these communities is, the study concludes, a necessary step toward safeguarding them. For now, the image of a venomous worm coiled in the algal shadows at the foot of a swaying black coral whip serves as a reminder that the ocean’s twilight zone is neither empty nor simple, and that its most surprising residents are only now coming into view.

Subject of Research: Co-occurrence of the bearded fireworm Hermodice carunculata with whip black coral forests in mesophotic ecosystems off Tenerife, Spain

Article Title: Co-occurrence of Hermodice carunculata (Polychaeta, Annelida) and Stichopathes spp. (Hexacorallia, Cnidaria) off Punta Blanca (SW Tenerife, Spain)

Article References: Fraissinet, S., Aguilar, R., Blanco, J., Arduini, D., Anglano, M., & Rossi, S. (2026). Co-occurrence of Hermodice carunculata (Polychaeta, Annelida) and Stichopathes spp. (Hexacorallia, Cnidaria) off Punta Blanca (SW Tenerife, Spain). Discover Conservation, 3(1), Article 23. https://doi.org/10.1007/s44353-026-00092-0

Image Credits: AI Generated

DOI: 10.1007/s44353-026-00092-0

Keywords: Hermodice carunculata, bearded fireworm, Stichopathes, black corals, mesophotic ecosystems, Marine Animal Forests, Tenerife, Canary Islands, ROV survey, biodiversity hotspots, ecosystem engineers, climate change