Living on the edge? State of vermetid crusts in the Maltese Islands (Central Mediterranean Sea)

Abstract

Gregarious vermetid molluscs act as bioconstructors, forming crusts or reefs in the intertidal or shallow sublittoral zones of sub-tropical and warm-temperate waters (Fig. 1). Vermetid reefs are considered valuable habitats as they protect coasts from erosion, regulate sediment transport and accumulation, serve as carbon sinks, and also augment the species diversity found in the littoral fringe (Milazzo et al., 2014). In the Mediterranean, vermetid reefs are built through the synergistic action of the vermetid gastropods of the Dendropoma petraeum s. l. species complex (Fig. 2) and the coralline alga Neogoniolithon brassica-florida that grows between the gastropod shells and cements the reef, and triggers vermetid settlement (Chemello & Silenzi, 2011). The largest and best developed reefs occur in the eastern parts of the Mediterranean, but such bioconstructions are also common in the central-southern area in places where the winter temperature of surface coastal waters does not fall below 14 °C. The Maltese Islands represent the southernmost point of occurrence of vermetid reefs in the central Mediterranean (Chemello & Silenzi, 2011) and here these bioconstructions take the form of thin to moderately thick crusts covering the calcareous rock at and just below mean sea level (Azzopardi & Schembri, 1997).