Damping the Impact of Intraspecific Variability of Whorl Growth-rate on the Range of Intraspecific Variation of Adult Shell-size in Gastropods with Determinate Growth: Evidence of a Finely-tuned Regulating Process

ABSTRACT

Many species among shelled gastropods � either land snails or marine snails � exhibit determinate growth and, therefore, are expected to implement some regulating process aiming at limiting the impact on the final (adult) shell-size of the (possibly excessive) intraspecific variability in the rate of shell-development. Indeed, a usually more or less limited range of variation is allowed for adult shell-size, in those species having determinate growth. Mollusks are expected to be no exception in this respect and, up to now, the occurrence of such a regulating process has actually been reported systematically, despite a still too limited number of investigations. Yet, the question remained of the ability of this regulatory process to finely adjust the end of the final development of both body and shell (specifically in term of the final number of whorls), so as to limit the variations in adult shell-size, despite the amplitude of intraspecific variability in shell development rate. I provide here preliminary empirical evidence for such a 揻inely tuned� regulating process � the strength of which is mirrored by the degree of 揋ouldian� negative covariance between whorl growth-rate and the number of whorls at adulthood. As expected, the strength of the regulating process reveals systematically increasing (i) with the amplitude of the intraspecific variability in shell development-rate and (ii) with the narrowness of the range of variation allowed for the adult shell-size (narrowness which depends upon the identity of the particular species under consideration). In addition to the already reported usual occurrence of this regulating process among shelled gastropods, its 揻inely tuned� character provides here still further evidence as regards its major importance in controlling the species-specific adult body mass, and this even for typically soft-bodied animals.