DEVELOPMENTAL PLASTICITY IN THE SHELL OF THE QUEEN CONCH STROMBUS-GIGAS

To evaluate the developmental plasticity of shell morphology in the queen conch Strombus gigas, we conducted transplant experiments at five sites in the Exuma Gays, Bahamas. Juvenile conchs from a sixth site were placed in cages at three sites, in an open cage at a fourth site, and in a naturally confined area at a fifth site. All animals spent 8 mo in the field. At the end of the experiments, the morphology of their shells was quantified by means of univariate and multivariate analyses of size and shape as advocated by Mosimann and James (1979). Comparisons were made within and among experimental groups at the first three sites, between groups at the fourth and fifth sites, and between experimental groups and naturally occurring animals at all six sites. Although the experiments extended over only 20% of the juvenile growth period, analyses showed that experimental animals at three of the sites differed significantly in the shapes of their shells. The shapes of these shells were also significantly different from those of the long-spined, bulky shells of naturally occurring animals of similar age at the source site. The greatest change in shape and growth rate occurred at the fifth site, where the rapidly growing uncaged experimental animals developed relatively slender, short-spined shells with large apertures and short spires. These changes were clearly in the direction of the size and shape of unmanipulated animals in the foster population. The results of these experiments, when combined with those of others, suggest that, although variation in the morphology of marine gastropods can have a genetic basis and can be influenced by natural selection, direct environmental induction can be the dominant process underlying patterns of local among-population differentiation. We recommend (1) that the morphometric methods used here to investigate the complex consequences of variation in the size and shape of organisms be applied in other taxa and (2) that the extent to which among-population patterns of geographic variation are directly induced by the environment be given more attention.