Functional morphology of vermetid feeding-tubes

Vermetidae are a small family of warm-water sessile gastropods capable of b uilding upright tubes (feeding-tubes) to take advantage of the water now. L aboratory and field experiments carried out on some Mediterranean species ( Vermetus triquetrus, Vermetus granulatus and Serpulorbis arenaria) suggest that these structures function as exploratory tubes built not only to recei ve a better water flow but mainly to avoid obstacles. In fact, vermetids ex perimentally exposed in situ to different hydrodynamic conditions do not pr oduce them, but do so in the presence of an obstacle, such as thalli during the massive spring algal growth. This strategy allows them to compete for a virtual space, not directly occupied by the vermetid itself but necessary to spread its mucous net. This interpretation may improve the inference of paleo-environmental events from shell morphology. When building a feeding- tube, vermetids first cut off a portion of the shell on the side towards wh ich they prepare to turn, using their radula, and then produce a new tube f ormed by short segments, at different angles; till they have reached the de sired direction. This process is confirmed by the presence of scars on the shell, composed by a succession of lamellae. The regular distribution of th ese scars on fossil Petaloconchus intortus, which lived in soft substrates, may be interpreted as a response to periodical anoxic crises or an increas e in the sedimentation rates, Their great morphological plasticity makes ve rmetids close to colonial or modular animals. Thanks to their capability of expressing more than one growth-form, and of re-moulding their shell, they successfully compete for substrate space and are key-stone species in frin ge habitats.