Communities of Mediterranean circalittoral caves and overhangs

Caves and overhangs constitute typical features of the Mediterranean circalittoral rocky bottoms. They support a great variety of species and host a considerable proportion of the total Mediterranean diversity of certain phyla, such as Brachiopoda, Bryozoa, and Porifera. Several protected and rare species, including recently described species, as well as deep-sea species have been reported. Most of our knowledge comes from caves in the north-west Mediterranean as cave biodiversity is extremely understudied in the southern and eastern basins. They are characterised by high morphological complexity, reflected in abiotic gradients and marked biological zonation along the longitudinal cave axis, from the entrance to the inner cave sectors. Light availability and water confinement are acknowledged as the main driving forces shaping this zonation. Thus, circalittoral caves support a variety of sciaphilic assemblages, usually distributed according to the following scheme: (i) a sciaphilic algae-dominated community at the entrance zone, (ii) a semi-dark zone where sessile filter-feeding invertebrates (mainly sponges and anthozoans) dominate, favored by the disappearance of macroalgae, and (iii) a dark zone which is sparsely colonized by sponges, serpulid polychaetes, bryozoans and brachiopods.

The shift from semi-dark to dark cave communities is evidenced through a sharp decrease in biotic coverage, biomass, three-dimensional biotic complexity, species richness, and the appearance of a black mineral coating of Mn-Fe oxides on the substrate. Additional zones, such as a transitional zone between semi-dark and dark cave communities and an azoic zone at the aphotic cave edge, have been occasionally identified. The limits of these zones might vary among caves with different morphology (e.g. inner zones tend to appear closer to the cave entrance in deeper waters), while in some caves some specific zones might be absent. Semi-dark caves are community is typically dominated by sponges, bryozoans, brachiopods and polychaete species. Serpulid polychaetes (e.g. Protula spp.) can form aggregations, which in some cases constitute the basis for the creation of biogenic structures; these ‘biostalactites’ are constructed by invertebrates (serpulids, sponges, and bryozoans), foraminiferans and carbonate-forming microorganisms. A number of deep-sea species belonging to various taxonomic groups (e.g. sponges, anthozoans and bryozoans) have also been recorded in sublittoral dark caves, regardless of depth

Indicators of quality:

Marine caves are characterised by high levels of individuality and heterogeneity which, coupled to the poor understanding of the ecosystem functioning, make it difficult the assessment of their ecologic quality. However, a number of indicators of quality have been recently suggested for the marine cave ecosystem, such as:

- Presence of invertebrates offering three-dimensional complexity to the substrate, and particularly fragile slow-growing species (e.g. red coral, erect bryozoans)

- High spatial coverage of suspension feeders (i.e. anthozoans) and large filter feeders (e.g. massive sponges)

- Presence of large mysid swarms

- Presence of various omnivorous and carnivorous mobile species (e.g. fish and decapods)

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats