Shell ornament in Pinna nobilis and Pinna rudis (Bivalvia: Pteriomorpha).

Shell sculpture of the Mediterranean bivalves Pinna rudis (L.) and Pinna nobilis (L.) have been compared by means of parametric and non-parametric univariate statistics, as well as techniques of joint multivariate classification and ordination. Furthermore, a quantitative definition of shell rugosity has been proposed, as an assessable factor potentially affecting anti-predatory defence and epizoic community settlement. Spine density, together with some derived measurements, which describe shell roughness well, best discriminated the two shell typologies and their variation as a function of individual growth. In this respect, P. nobilis showed greater individual variability, mainly due to progressive erosion in the largest specimens, in comparison with the noticeable morphologic uniformity of P. rudis.