The alien bivalve Anadara demiri (Arcidae): a new invader of the Adriatic Sea, Italy.

This paper reports the observations recorded on the arcid clam, Anadara demiri, from its first reporting in the Adriatic Sea three years ago, when its presence only was noted, to the present day when its invasive potential is confirmed. Repeated recruitment events were observed and confirmed by the co-existence of two or more cohorts within the population. Anadara demiri appears to be a year-round spawner with two spawning peaks (late winter and summer). Together with the congeneric invader, Anadara inaequivalvis, it has colonized the external belt of the fine well-sorted sand biocoenosis where they may numerically dominate the benthic community. Anadara demiri appears to have gained full status as a component of the Adriatic Sea coastal benthic community despite its very recent first entry. Numerous features make it a strong competitor among which is the ability, retained into adult life, of attaching, by means of byssus threads to all kinds of hard substrata, particularly to other live molluscs such as the gastropod Aporrhais pespelecani and the bivalves Anadara inaequivalvis and Chamelea gallina. The presence of species actively interfering with A. demiri in various ways (the polychaetes, Polydora sp. and Sabellaria spinulosa, that bore into and foul the shell, respectively, and the predatory asteroid Astropecten irregularis) is discussed.