Characters in context: molluscan shells and the forces that mold them
Authors: Vermeij Geerat J. 
Published: July 2006

The characters and body parts of organisms are shaped by mechanical forces at two temporal scales. At the ontogenetic scale, the relevant forces are those of every day, exerted by muscles, other metabolism-powered processes, and normal interactions between the body and the external environment. At the phylogenetic scale, forces are strong enough to kill some individuals or to cause reproductive failure. These forces act more intermittently.

I explore these ideas by examining the characters of molluscan shells, which grow by the addition of skeletal material along the rim of the open end of a hollow, conical tube that is closed at its narrow (apical) end. In the idealized case of a null shell, the skeleton is a right circular cone, in which the magnitude and direction of growth are the same at each point along the rim. The rate of expansion of the cone is determined by the shell-builder's metabolism. Real shell-builders are exposed to, and themselves exert, forces that affect shell shape. These forces are generated by contact between the shell-secreting mantle margin and the substratum, by local or temporary deformations of the mantle margin imposed by other parts of the body and previously formed parts of the shell, and by contraction of muscles that connect the soft tissues to the inner shell surface. Early mollusks whose shells more or less resemble the null shell were slow-moving, epifaunal animals that clamped the shell against the substratum. Evolutionary increases in metabolic rate, associated with greater mobility and faster growth, made some ontogenetically important forces stronger and introduced new forces. As a result, the range of available phenotypes expanded. Refinements in genetic regulation of form, perhaps including an increase in the number of semiautonomous regulatory regions, further added to the specification and range of variation of characters that were subject either to evolutionary conservation or to natural selection. For example, the mantle margin in plesiomorphic gastropods appears to comprise one such region, which produces a growing shell margin in the form of a logarithmic spiral; in more-derived gastropods, the mantle margin may comprise two or more regions, which together produce a growing shell margin that departs strikingly from the logarithmic form of the outer shell lip.

The morphospace occupied by accretionary shells can be described by (1) the number of semiautonomous developmental modules, (2) selective regimes observable as phenotypic adaptive evolution, and (3) metabolic rate. The perspective outlined here implies that shells initially occupied a limited morphospace encompassing one or two modules, adaptation as an epifaunal clamping animal, and slow growth (low expansion rates) and metabolism. Further compartmentalization, together with increased metabolic rates in ecologically dominant taxa, caused the morphospace to expand both in the number of independent descriptors and in the range of values that each parameter spans. These trends in morphospace may characterize all major multicellular clades.