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Are Echinoderm Egg Size Distributions Bimodal?

Mary A. Sewell,C. M. Young

Published 1997 in The Biological Bulletin

ABSTRACT

Marine invertebrates can be categorized into species that reproduce by producing either large numbers of small, energetically inexpensive eggs that become planktotrophic larvae, or fewer, larger eggs with more yolk and lecithotrophic development (1). The selective advantages of these alternative strategies were considered in a series of simple mathematical models by Vance (2, 3). These models predicted that intermediate egg sizes should have lower reproductive efficiency, and that only extreme egg sizes should be evolutionarily stable (2, 3). Specifytally, Vance's models (2, 3) and later modifications (4-7), predict that eggs of marine invertebrates should have bimodal size distributions, rejecting the contrast between small egg/feeding and large egg/nonfeeding modes of development and the selection against intermediate egg sizes. Evidence for bimodality in egg size distributions is, however, equivocal, with unimodal distributions seen in the majority of comparative studies that are appropriate tests of the hypothesis (8-13). Bimodal distributions have been described only in a few groups of molluscs (4) and asteroid and echinoid echinoderms (14). Here we test the prediction of bimodality in the holothuroid and ophiuroid echinoderms and show that although the natural log-transformed egg size distributions are visually unimodal, the holothurian egg size distribution is statistically composed of two discrete modes. Moreover, reexamination of the asteroid and echinoid egg size distributions (14) with the addition of data from more recent literature confirms that there are two statistical modes in the egg size distributions of these classes. Thus, in the phylum Echinodermata, there is a bimodal egg size distribution three of the four classes in which this prediction can tested.

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