Adaptive Function of Gastropod Larval Shell Features

Abstract

Similarities in features that occur commonly on the shells of many planktotrophic gastropod larvae may be interpreted primarily in terms of (a) shared ancestry, (b) shared adaptive solutions to problems of planktonic existence, or (c) shared artifacts of a basic pattern of early biomineralization. The Paradigm Method of functional inference, developed by paleontologists to analyze structures of fossil organisms, is used to analyze recurring features of unknown function on gastropod larval shells. Four recurring features are shown to fit paradigmatic adaptive solutions to preventing or retarding mechanical breakage at the growing apertural margin of the larval shell. These features are apertural beaks, velar notches, peripheral angulations, and continuous spiral elements of shell sculpture. A fifth common feature on larval shells, discrete crystalline microprotuberances, is inferred to be primarily a non-functional artifact of remote biomineralization and self-organized growth, although secondary functional advantage cannot be discounted. Protective functional interpretations are supported by patterns of shell breakage and repair on larvae from the plankton and by patterns of shell failure in trials subjecting larval shells to artificial mechanical attacks. The ability to withstand mechanical onslaughts of zooplankton predators and to repair damage to the larval shell may be a highly significant means of reducing natural larval mortality. Additional key words: Paradigm Method, analysis of form, biomineralization, predation,