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.