Excavation of boreholes by the gastropod Urosalpinx: an analysis by light and scanning electron microscopy

Penetration of shell by the muricid gastropod, Urosalpinx cinerea follyensis, is accomplished by successive alternating periods of (a) chemical activity by the accessory boring organ (ABO) , and (b) rasping by the radula. This paper reports on the functions of the radula and of the ABO in producing the characteristic geometry of the borehole, and on the effects of radular teeth and of the ABO secretion on the microscopic anatomy of the surface of the borehole during the process of shell-boring. Radulae of U. c. follyensis and the surfaces of incomplete boreholes in the shell of Crassoslrea virginica, Mytilus edulis, and Mya arenaria were examined by means of light and scanning electron microscopy. Hardness tests of radular teeth and shell of prey demonstrated that marginal teeth are harder than rachidian teeth, and that the range of hardness of rachidian teeth overlaps that of the three species of shell. Rasping is carried out by two, occasionally three, of the five rachidian cusps. Rasping patterns are shallow and asymmetric. Rachidian teeth are worn to the base with use; marginal teeth wear only slightly as they are employed mainly in feeding. The distance between the tips of rachidian cusps corresponds with the interval between the parallel cusp traces rasped by them in shell. During each rasping period, snails scrape off about 1/10 to 1/5 of the surface of the chemically treated area of the bottom of the borehole. Dissolution of shell is accomplished by secretion from the secretory disk of the ABO. With each application of the ABO, most or all of the radular marks of the previous rasping period are erased by solution of a thin layer of shell. The pattern of etching is specific for each of the species of shell studied. In oyster and mussel shell, initial solubilization occurs through the organic, non-mineralized, prism sheaths, exposing prismatic forms shown by other workers to be distinctive for these species, and then proceeds into the organic-calcareous structure of individual prisms. Etching of Mya shell revealed no fundamental prismatic form. Shell-penetration includes dissolution of both organic complexes and CaCO3 crystals. Shell-boring by this snail is principally a chemical process, and the geometry of the borehole is generally a reflection of the morphology of the ABO.