Faunal communities of sheltered Mediterranean infralittoral muddy sands

This habitat is situated in sheltered environments, such as embayments, with low hydrodynamic regime, and thus in stable sedimentary systems, at depths between 1-15 m. The substrate consists of a muddy-sandy sediment where the sand fraction is usually composed by mollusc shells. Organic matter and silt-clay contents are the primary driving factors determining species composition. The very variable environmental conditions of these shallow environments in terms of salinity and water temperature determine that these habitats are colonized by euryhaline and eurytherm organisms.

This habitat can be naturally colonized by seaweeds or seagrasses. When seaweeds and seagrasses are absent, polychaetes dominate the invertebrate assemblages, mainly Neanthes caudata, Pseudomastus deltaicus, Notomastus latericeus, Ampharete finmarchica, Mediomastus fragilis, Aonides oxycephala and Heteromastus filiformis together with some crustaceans (Ampelisca brevicornis and Leucothoe incisa). The phoronid Phoronis psammophila is also very abundant. The bivalves Thracia papyracea and Cerastoderma glaucum dominate amongst filter-feeding invertebrates although the presence of Loripes lacteus is also frequent. Holothurians and gastropods Cyclope neritea and Nassarius reticulatus are common deposit-feeders, sliding on the substrate. The presence of sensitive species such as the mollusc Pinna nobilis is not very frequent but it can be occasionally observed. The eel Anguilla anguilla, the Sea Bass Dicentrarchus labrax, the gobids Gobius spp. and soles (Solea spp.) are amongst the most common fishes. When macroalgal species are present, Acetabularia calyculus, Ulva spp. and Cladophora spp. are the most abundant. The main seagrasses are Zostera noltii and Cymodocea nodosa.

Five different sub-habitats have been described for muddy sands habitat, with different dominant species and under slightly different conditions. These are the sub-habitat with Caulerpa prolifera on sheltered superficial muddy sands; and the sub-habitat with Pestarella (=Callianassa) thyrrena and Kellia sp. where the silt-clay fraction is >5% and the organic matter content reaches moderate to high values. There is a sub-habitat associated to freshwater discharge with Cerastoderma glaucum and Cyathura carinata in compact sediments which is characteristic of organically polluted environments in brackish waters; aub-habitat with Loripes lacteus and Ruditapes species in muddy sands on bays, estuaries, coastal lagoons and other sheltered environments, always at shallow zones that are highly influenced by seawater; and a sub-habitat of hydrothermal oozes with Cyclope neritea and nematodes which is only present in shallow waters (< 10m) with high sulfide concentrations, high sediment temperatures and high salinities. 

Indicators of quality:                                                                                                                   

Most of the species included in the habitat description are bioindicators of environmental quality. The majority of bivalves are very sensitive to eutrophication, and more tolerant and opportunistic species tend to dominate with increasing eutrophication. Changes in abundance and richness of fauna composition are good indicators of trends in habitat quality. Ruditapes decussatus has been proposed as a pollution bioindicator in areas where mussels are not available. The accumulation of pollutants in Ruditapes’ tissues has been used to assess environmental quality.

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats