Mediterranean setoselliniforms and their exploitation of small-sized substrates

Specimens of the setoselliniform bryozoans Setosella vulnerata (Busk 1860) and Setosellina capriensis (Waters 1926) from shelf bottoms off SE Sicily in the 53-130 m depth range have been studied. Both species exhibit seemingly size-determinate, spirally budding colonies encrusting small sedimentary grains and show no particular selectiveness for the geological nature of their substrata. S. vulnerata slightly selects the size of its substratum, often developing colonies that remain smaller than their substrata or forming enveloping colonies which can be regarded as membraniporiform-like. In contrast, S. capriensis more actively selects smaller-sized grains, tending to develop cup-shaped colonies extending well beyond their substrata and forming up to four extra-grain astogenetic generations. Lacking extrazooidal basal calcification, colony extension is attained through disto-basal budding of sub-erect zooids with heavily mineralized basal walls, buttressing each other. Such colonies closely resemble lunulitiforms and are capable of performing complex activities.