A peculiar coral formation in the Mediterranean Sea, and its associated biota

Zooxanthellate hermatypic corals grow mainly in the tropical belt, but flourishing coral communities may also occur in marginal areas such as subtropical or warm-temperate seas (Benzoni et al., 2003, and references therein). In the Mediterranean Sea, the world's largest warm-temperate sea, the endemic, colonial, and zooxanthellate scleractinian coral Cladocora caespitosa builds reefs, with growth rates exceeding 5 mm year⁻¹ and calcification rates reaching 1.7 kg CaCO₃ m⁻² year⁻¹, similar to many tropical reef corals (Anderson et al., 2017; Morri et al., 2000). Fossils of reefs formed by this species are known from warmer climatic phases of the Quaternary (Aguirre & Jiménez, 1998; Peirano et al., 1998). Today, colonies may occur in isolation or as free-living nodules (coralliths), but when abundant, they may form beds made up of numerous sub-spherical colonies in dense populations or banks, that is, large structures reaching several decimeters high and covering areas of several square meters (see Bianchi et al., 2021, for a recent synthesis; a list of key references on these formations and their ecology is provided in Appendix S1: Section S1).

Here, we report on a novel type of large living formation of C. caespitosa found at Torre delle Stelle (39.141143° N, 9.401533° E), off southeastern Sardinia (Italy, western Mediterranean Sea), about 1 km outside the western border of the Capo Carbonara Marine Protected Area. The bioconstruction occurs at the entrance of a bay characterized by a relatively flat sandy bottom, large granitic boulders, and Posidonia oceanica meadows. Unlike the four above-mentioned coral formation types (isolated colonies, free-living coralliths, beds, and banks), which all develop on horizontal to gently sloping substrates, the present formation has developed on a vertical wall, with colonies growing outward in parallel. Smaller similar bioconstructions on vertical substrata were described from the Columbretes Islands, Spain (Kersting & Linares, 2012).

This peculiar coral formation is located about 25 m from the shoreline, starting on the top of a massive granite rock at 7.7 m depth and developing down to 9.8 m depth. Its maximum horizontal and vertical axes reach ~2.7 and 2.1 m, respectively, with ~0.4 m maximum thickness. The formation covers >5 m² and features a complex three-dimensional structure (Figure 1). The bioconstruction health status is relatively good, except for old necrosis signs on the top of the formation (Figure 1a) and a few localized small areas with recent necrosis, dwindled polyps, budding, or encrusted dead corallites (Appendix S1: Figure S1).

The formation has a regular periodic pattern of subequal horizontal ledges of increasing width, formed by bracket-like sub-horizontal colonies that grow outward parallel to each other, overshadowing the area beneath them (Figure 1). Thus, colonies beneath the upper ones can only grow a certain distance apart, giving the whole formation the aspect of shelving, a kind of coral formation hitherto undescribed (Figure 1b). By shelving, we mean a unit made up of coral ledges resembling shelves. These shelves form when colonies expand and then fuse both horizontally and vertically and connect with each other due to vertical accretion of the upper portions by gravity. The lower portion of the shelving is more irregular with smaller individual colonies (Figure 1c), due to reduced development in increasingly shady conditions. Dead corallites detached from broken colonies remain entrapped among the coral ledges, thus contributing to the consolidation of the whole structure (Figure 1d). Additional satellite colonies develop within the coral formation via sexual reproduction, with larvae settling near parental corals, and asexually by lateral polyp budding (Figure 1e,f).

Potential factors that might have enhanced the growth of the C. caespitosa shelving may be: (1) shallow-water massive granitic rocks, like the one hosting the observed bioconstruction, are typically poorly colonized, minimizing competition with soft frondose algae (Peirano et al., 1998); (2) the clear waters at Torre delle Stelle may maximize the photosynthetic activity of coral-associated zooxanthellae, especially in summer, while heterotrophy plays a greater role during colder months, with shorter daylight hours (Hoogenboom et al., 2010); (3) the relatively sheltered site within the bay, exposed to S and SE winds, may limit the impact of storms (Kersting & Linares, 2012); (4) high self-seeding of the shelving is likely, since we observed recruits and juveniles near colonies (Figure 1d,f).

The formations of C. caespitosa are extraordinary biodiversity aggregators (Koukouras et al., 1998). Many species are cryptic and/or small, hiding in the interstices and/or feeding on coral mucus (Bianchi et al., 2021), and cannot be inventoried without destructive physical sampling. Conspicuous species, recognized visually underwater during a daytime survey conducted on 9 July 2024 (for about 1 h) or identified in photographs (about 200 images taken), numbered 76 in the shelving of Torre delle Stelle (Appendix S1: Section S2).

Although none of the species found associated with the C. caespitosa shelving of Torre delle Stelle can be considered strictly associated with such a biogenic habitat, they offer examples of all the typical functional groups that characterize biotic reefs (Bianchi & Morri, 1996) (Figure 2). The primary constructor guild is represented by the coral itself, while several serpulids, among which Serpula vermicularis, Protula intestinum (Figure 2a), and—to a lesser extent—bryozoans such as Myriapora truncata and others (Figure 2b), the barnacle Perforatus perforatus, solitary scleractinian corals (e.g., Balanophyllia regia, Caryophyllia inornata, and Leptopsammia pruvoti), and molluscs (Cerithium vulgatum and Patella caerulea) are secondary constructors, which add their calcareous tests to the bioconstruction. Erect species of foliose algae, such as Padina pavonica (Figure 2c), massive sponges (e.g., Chondrosia reniformis, Ircinia oros, and I. variabilis), and the ascidian Halocynthia papillosa (Figure 2d) act as bafflers, reducing current velocity across the bioconstruction surface, and thus enhancing sediment deposition and cavity filling within the framework. The only member of the binder guild is the crustose coralline alga Lithophyllum incrustans (Figure 2d), which encrusts living and dead corallites of C. caespitosa, adding strength, rigidity, and cohesion to the whole structure. The destroyer guild is represented by the excavating sponges Cliona viridis and C. celata (Figure 2e) and the date mussel Lithophaga lithophaga, a boring bivalve (Figure 2f). Sponges may compete for space with C. caespitosa (Figure 2g) or even overgrow it (Figure 2h). The brittle star Ophiothrix fragilis, hidden within the interstices of the bioconstruction, protrudes its arms for filter feeding (Figure 2g), thus contributing to baffling. The sea urchin Arbacia lixula (Figure 2i) not only grazes algal seedlings that compete with the coral but also contributes to the wearing of the concretion performed by binders when grazing on encrusting corallines, thus acting as a destroyer. Decapod crustaceans, such as the squat crab Galathea intermedia and the hermit crab Clibanarius erythropus (Figure 2j), are scavengers, helping to keep the bioconstruction clean.

Fishes were the most speciose macrofaunal dwellers. The structural complexity of the wide C. caespitosa shelving provides several microhabitats for fishes, which amounted to 14 species (Appendix S1: Section S2). Some sciaphilous species were strictly associated with the coral shelving, exploiting specifically shady spaces, such as the cardinal fish Apogon imberbis (Figure 2b,l) and Liechtenstein's goby Corcyrogobius liechtensteini. The former was detected inside crevices and holes, where it retreats when disturbed, which is likely a behavioral defense response. The latter was observed to move rapidly among the small cavities present on the lower side of the coral ledges. Both species were mostly found in the lower part of the shelving (i.e., the part less exposed to light and developing vertically). The damselfish Chromis chromis (Figure 2l) was observed close to the coral, in some cases while protecting its eggs, laid in nests within the three-dimensional structure of the bioconstruction, against predators chiefly represented by adult ornate wrasse, Thalassoma pavo. The Madeira rockfish Scorpaena maderensis (Figure 2f) and the small blenny Parablennius zvonimiri quickly hid under the ledges when approached by the diver. The painted comber, Serranus scriba (Figure 2k), a piscivorous ambush predator, hid in shady portions of the shelving, where it took on a dark livery, waiting to attack the juveniles of C. chromis passing close to its hiding place. Other fish species censused on the coral formation were the rainbow wrasse Coris julis, the East Atlantic peacock wrasse Symphodus tinca, the axillary wrasse Symphodus mediterraneus, the white sea bream Diplodus sargus, the common two-banded sea bream D. vulgaris, and the golden grouper Epinephelus costae (Figure 2m). Some fish species (e.g., C. chromis, S. tinca, and E. costae) were also represented by juveniles, which may find refuge in the shelving or visit it for feeding. Overall, the relationships between fishes and the C. caespitosa shelving can be categorized as follows: (1) fish seeking shady habitats (e.g., the cardinal fish, a typical cave-dwelling species); (2) fish hiding to ambush prey (e.g., the painted comber); (3) fish transferring matter/energy from the water column to the bottom (e.g., the damselfish, a planktivorous fish that feeds daily and deposits dejections during night-time); (4) occasional visitors.

The C. caespitosa shelving of Torre delle Stelle is a natural monument. Reports like this, along with others documenting new formations (e.g., Kersting et al., 2023; Kružić & Benković, 2008), are crucial for protecting the species. Targeted conservation measures are needed to prevent climate and local anthropogenic impacts (Kružić et al., 2025; Vergotti et al., 2025). The increasing documented events of coral bleaching and mortality due to thermal anomalies (Jiménez et al., 2016; Kersting et al., 2013; Rodolfo-Metalpa et al., 2005), in particular, are threatening the persistence of modern Cladocora formations and their associated fauna, with potential detrimental impacts on marine Mediterranean biodiversity.

The authors declare no conflicts of interest.