The calcareous egg capsule of the Patagonian neogastropod Odontocymbiola magellanica: morphology, secretion and mineralogy

Odontocymbiola magellanica is the only known South American volutid gastropod that deposits calcareous egg capsules. The spawn is moulded and fixed to flat or convex surfaces by the female's ventral pedal gland, during an hours-long process in which the female adopts a stereotyped posture and appears nonreactive to most external stimuli. Microscopically, the different cells of the ventral pedal gland show features suggesting their participation in the secretion of both the organic matrix and the calcium component of the calcareous layer. The latter consists mainly of numerous spherspherulites that are packed together around cylindrical, septated spaces which traverse the spherspherulitic layer and attach to the membranous layers surrounding the capsule cavity. These septated spaces should ensure permeability of the capsule wall, which is necessary for gas exchange and excretion by the embryo. The calcareous layer is made of high-magnesium calcite, a calcium carbonate polymorph in which Ca is partially substituted by Mg in the calcite lattice. Mg substitution is thought to confer a greater crack resistance to the mineral; it is found in many invertebrates, but apparently has not been reported before in molluscs. Odontocymbiola magellanica is a long-lived species, investing heavily in its egg capsules.