Habit and behaviour of the nestling bivalve Gregariella semigranata (Reeve, 1858) from the Mediterranean coralligenous

The minute bivalve Gregariella semigranata (Reeve, 1858) was first reported from the coralligenous algal reefs off of Marzamemi in southeastern Sicily. The species has a cryptic, nestling lifestyle within convolutions of calcareous algae and crevices, occurring also under the canopy of fleshy algae and inside the soft tissues of sponges. A peculiar cocoon of threads and agglutinated mud enveloping some Gregariella specimens was observed, partly filling the gaps between the shell and the walls of hosting crevices, with a possible function of protection and/or stabilization. The posterior half of the valves develops a hairy periostracum with a tuft of long branched barbules resembling the surrounding soft algae. The tuft is the only part protruding from the substratum, with a possible mimetic function against predators. The numerous specimens of G. semigranata from Marzamemi allowed us to better focus on the ecology and the life strategy of this bivalve and adds a further record from the Ionian coasts of Sicily to its known Mediterranean geographical distribution.