Tube-dwelling in Gastrochaena dubia (Bivalvia); ecological requirements, functional morphology and structure of the crypt

The rock boring bivalve Gastrochaena dubia (Pennant, 1777) can also live freely on soft substrata, within a claviform crypt consisting of a thick, coarsely agglutinated external layer and an inner lining. Examination of Pleistocene material from Monte Mario, Rome (Cerulli-Irelli collection), allows an understanding of the ecological requirements necessary for this life habit and provides new information about crypt functional morphology and construction. The crypt of G. dubia rests subhorizontally, partly buried and with the siphonal tip emerging from the substratum. A low-energy substratum, with a sandy-muddy texture rich in a coarse biogenic fraction, is the main ecological requirement. Through a process of dissolution and re-secretion, G. dubia enlarges the crypt during growth, particularly anteriorly. The agglutinated, blistered structure of the external layer seems to be formed by means of mucous "bubbles" which mineralise, incorporating sand grains and bioclasts. An elongate, thin appendage of the foot seems to be involved in this process. The same foot appendage produces thin tubules which penetrate the crypt wall. They have a probing function in the boring life-habit, but it is not known if they have any function in the free tube-dwelling habit.