Coralligenous habitat

Photo by Alessandro Tommasi

Coralligenous is described as "a typical Mediterranean underwater seascape comprising coralline algae frameworks that grow in dim light conditions and relatively calm waters”, (Ballesteros 2006; UNEP/MAP-RAC/SPA 2008). Thus, it exclusively occurs within the Mediterranean Sea, typically at depths ranging from 25 to 200 meters, under very specific environmental conditions characterized by clear, dimly lit, cool waters with a constant temperature and mild currents.

The coralligenous habitat arises through a process of "bioconstruction" orchestrated by plant and animal organisms, whose calcareous skeletons craft intricate and vibrant three-dimensional structures reminiscent of tropical coral reefs. This mesmerizing spectacle, cherished by divers, serves as a sanctuary for numerous vital species. Red coralline algae (Rhodophyta) from the order Corallinales, including Mesophyllum spp, Lithophyllum spp, and Negoniolithon spp, act as the primary bio-builders. They gradually form layers on the rocky seabed, creating a secondary hard substrate with their calcareous thalli. This substrate supports the settlement of other red algae (Peyssonnelia spp) and various animal species, predominantly polychaetes, scleractinia, and bryozoans.

The growth of these and other animal organisms, known as "secondary bioconstructors", expands and consolidates the three-dimensional structure, increasing the number of microhabitats that shelter a very large number of species: for this reason, calcareous bioconstructions can be considered true underwater cathedrals of biodiversity.

The growth of calcareous bioconstructions is offset over time by chemical and mechanical degradation processes caused by "biodemolishing" organisms, predominantly sponges and molluscs belonging to the genera Cliona and Lithophaga, which gradually perforate, crumble, and dissolve the calcium carbonate. This delicate equilibrium between bioconstruction and biodemolition organisms sustains the vitality of the coralligenous reef, but at the same time it renders it highly susceptible to environmental changes. Disruptions to this balance can lead to the dominance of demolition processes, resulting in the deterioration of calcareous bioconstructions

The morphology and structure of the coralligenous depends mainly on the depth, topography and bioconstruction types present. Many coralligenous types have been described, but the most widely used definition is the one that distinguishes coralligenous on a geomorphological basis between reefs or frames and platforms or shoals (Pérès & Picard 1964; Ballesteros, 2006; SPA/RAC-UN Environment/MAP, 2019).

Coralligenous reefs develop on rocky substrates in the coastal circalittoral, usually between 30 and 90 m, but under special conditions of turbidity, currents, exposure and slope of the seabed, they can be found between 15 and 130 m. They can be divided into two main formations: wall or cliff coralligenes, which develop on vertical or sub-vertical rocky substrates with bioconcretions of varying thickness (from 20 cm to 2 m), and biogenic clumps, which develop bioconcretions up to several metres thick on horizontal rocky substrates (EUNIS, 2019).

Coralligenous platforms are large tabular bioconstructions that develop on the more or less horizontal substrates of the continental shelf, mostly between 40 and 120 m depth. They are found on sedimentary bottoms and can develop from the coalescence of rhodoliths or grow on rocky outcrops.

Although both types of coralligenous can be found over a wide bathymetric range, in the first 40/50 m depth, coastal rock systems are mainly characterised by coralligenous cliffs, while coralligenous beds are generally found in the deeper environments of the continental shelf (Ballesteros, 2006; Cánovas-Molina et al. 2016). Due to this proximity to the coast, wall coralligenous is therefore the one most exposed to anthropogenic pressures acting along the coastal strip, such as chemical-biological pollution, excessive sedimentation, tourist and commercial activities, and it suffers particularly from all those environmental changes that affect the more superficial bathymetric belts, such as climate change. The shelf coralligenous, on the other hand, is particularly exposed to pressures acting at greater depths, such as the accumulation of solid waste and trawling.

Coralligenous ranks as the second most significant "hot spot" of marine biodiversity in the Mediterranean, following Posidonia oceanica meadows (Bouderesque, 2004). Although it lacks a precise estimate of species numbers, given the structural complexity of its populations, the coralligenous habitat is believed to host more species than any other Mediterranean community (Ballesteros, 2006).

The coralligenous habitat stands as one of the foremost coastal ecosystems in the Mediterranean, boasting significant distribution, biomass, and role in the carbon cycle (Ballesteros 2006). Apart from its ecological importance, it serves as a crucial economic asset due to the presence of valuable commercial species like red coral, fish, and crustaceans (e.g., spiny lobster and clawed lobster), and its immense appeal for underwater tourism.

Because of its distinctive features, coral stands as one of the most endangered habitats in the Mediterranean. Hence, European legislation has consistently focused on its conservation and protection through pivotal directives like the Habitats Directive 42/53/EEC and the Marine Strategy Framework Directive 2008/56/EC. Additionally, international treaties such as the Barcelona Convention (1976) also hold significance in safeguarding and conserving this invaluable asset of the Mare Nostrum.