Ontogenetic regulatory mechanisms, heterochrony, and eccentricity in dendrasterid sand dollars

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Posterior eccentricity has evolved independently in three lineages of northern Pacific sand dollars; it is best known in the genusDendraster. In these forms, the anterior areas of the aboral surface are much more highly developed than the posterior areas; consequently, the apical system is located posteriorly, rather than centrally. This morphology is linked to an unusual and highly successful mode of suspension-feeding. The evolution of eccentricity appears to be related to regulatory mechanisms found in many non-eccentric sand dollars, such asEchinarachnius parma. During the ontogeny of this form, the growth rates of the ambulacra and interambulacra are correlated with their position along the longitudinal axis. Early in ontogeny, the anterior areas develop at a faster rate than the posterior areas; later in ontogeny, this relationship is completely reversed. Normally, these two phases of unequal growth counterbalance each other, and the mature test appears symmetrical. However, if the balance between the two phases were upset, eccentricity would naturally ensue. In fact, aberrant Recent and fossilEchinarachniuswith the predicted anteriorly-and posteriorly-eccentric morphologies actually do exist. Posterior eccentricity is apparently produced by retention of the earlier unequal growth pattern, which favors anterior development. This represents trait neoteny; however, since the retained trait is a regulatory mechanism that controls growth rates over the entire aboral surface, the morphological effects are particularly profound. Thus, the seemingly bizarre morphology ofDendrastercan be derived by a change in the timing of an existing regulatory mechanism. This may help to explain the sudden appearance ofDendrasterin the fossil record and the absence of transitional forms. The unusual suspension-feeding behavior ofDendrastermay have been derived from a righting response that is common among other sand dollars.