How the 'forceps' of Lithophaga aristata (Bivalvia: Mytiloidea) are formed

The calcareous substratum borer, Lithopaga aristata Dillwyn, 1817, secretes posterior incrustations that take the form of interlocking 'forceps'. These are secreted, initially, as asymmetrical ridges by similarly asymmetrical dorsal and ventral glands in the left and right middle folds of the posterior mantle lobes. In the adult, the secretions are more uniformly spread over the posterior shell margins, concealing the juvenile ridges.
Opening and closing of the valves smooths the outer surfaces of the 'forceps' against the burrow wall, which also comes to be lined with calcium carbonate that is reciprocally smoothed, so that they occlude the borehole more effectively. Extension and retraction of the siphons probably smooths the inner surfaces of the 'forceps' which are sharpened by abrasion, one against the other, during valve opening and closing. The pair of inequilateral spikes so produced, project from the burrow aperture, occluding it, but probably, more importantly, distancing aperturally attacking predators from the true posterior shell margin. Interlocking of the 'forceps' and their sharpness may further deter would-be predators.