Borehole site selection in Naticarius hebraeus (Chemnitz in Karsten, 1789) (Naticidae: Gastropoda)?

It is commonly accepted that in cold-water naticids, a direct relationship between predator and prey size and borehole diameter exists, the latter being frequently situated near the umbus of bivalve shells. Aquarium observations of Naticarius hebraelcs from NE-Spain show that this predator traps its prey at the ventral margin, so that its proboscis with the boring organs remain closer to the umbus, the larger the snail is. This agrees with the findings that borehole diameter is larger when closer to the umbus. Furthermore, this parameter is larger near the umbus in the larger bivalve Glycymeris glycymeris, than in the smaller Spisula subtruncata, indicating that specific selection could change in time as predators grow. Standard deviation of borehole diameter is larger in Spisula, further away from the umbus, indicating, in agreement with Vignali & Galleni (1986), that smaller predators probably bore more randomly on bivalve shells than larger ones. In agreement with Negus (1975), a learning process in small naticids could exist in order to produce more consistently defined or located boreholes in time. In any case, this site-selection seems not to be an active result of the snail's predation, as suggested in former papers, but passively due to the predatorlprey size ratio or the bivalve's shell shape.