Covarying Shell Growth Parameters and the Regulation of Shell Shape in Marine Bivalves: A Case Study on Tellinoidea

Jean Béguinot

doi:10.1155/2014/519510

Abstract

Specific parameters characterising shell shape may arguably have a significant role in the adaptation of bivalve molluscs to their particular environments. Yet, suchfunctionally relevantshape parameters (shell outline elongation, dissymmetry, and ventral convexity) are not those parameters that the animal may directly control. Rather than shell shape, the animal regulates shell growth. Accordingly, an alternative,growth-baseddescription of shell-shape is best fitted to understand how the animal may control the achieved shell shape. The key point is, in practice, to bring out the link between those two alternative modes of shell-shape descriptions, that is, to derive the set of equations which connects thegrowth-basedshell-shape parameters to thefunctionally relevantshell-shape parameters. Thus, a preliminary object of this note is toderivethis set of equations as a tool for further investigations. A second object of this work is to provide an illustrative example of implementation of this tool. I report on an unexpected negative covariance between growth-based parameters and show how this covariance results in a severe limitation of the range of interspecific variability of the degree of ventral convexity of the shell outline within the superfamily Tellinoidea. Hypotheses are proposed regarding the constraints possibly at the origin of this limitation of interspecific variability.

Highlights

Since Reaumur [1] at least, the shape of molluscs shells is relevantly understood, not as a geometrical figuration per se, defined at the outset, and as the result of a cumulative growth process
Molluscan shells are one among those “archetypal” examples favoured by Thompson [2], where growth process definitely rules the resulting form which, would not make any sense being separated from its generating pathway
When focus is put upon the functional role of shell shape, another kind of shape descriptors is preferentially required, typically including shell elongation, the degrees of dissymmetry, and of ventral convexity of shell outline [9, 10]

Summary

Introduction

Since Reaumur [1] at least, the shape of molluscs shells is relevantly understood, not as a geometrical figuration per se, defined at the outset, and as the result of a cumulative growth process (of biomineral accretion). An illustrative example is proposed, within the marine bivalves superfamily Tellinoidea (Blainville 1814), demonstrating how a strong negative covariance between two specific growthbased parameters (“α” and “ρ,” defined below) results, in particular, in a substantial limitation of the range of interspecific variation of one of the functionally relevant shellshape parameters, the degree of ventral convexity of the shell outline.

Results

Conclusion