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Bilateral symmetry of the petals in Melitta tennis, Encope micropora, and arachnoides placenta (Echinodermata: Clypeasteroida)

Abstract:

Differences between paired structures sometimes occur normally, but random variation can give insight into developmental stability and environmental stress. In clypeasteroid sand dollars, the modified ambulacra, or petals, are among the most conspicuous bilaterally paired structures. In this study, all petal pairs deviated from perfect bilateral symmetry. Analysis of the anterior and posterior petals of Encope micropora suggested that they were antisymmetric. The anterior petals of small and large Mellita tenuis, and the posterior petals of large M. tenuis and of Arachnoides placenta, showed directional asymmetry with the left side larger than the right. The variation in the posterior petals of small M. tenuis and the anterior petals of A. placenta did not differ from fluctuating asymmetry. The posterior petals of small M. tenuis collected two years apart from the same population had similar fluctuating asymmetry indices. Based on the relative indices scaled for size, the indices for the anterior petals of A. placenta were two to four times greater than the indices for the posterior petals of M. tenuis. The same set of petals could not be compared across species as problems with size, antisymmetry, and/or directional asymmetry occurred in a disjunct pattern across species. Differences in symmetry between the anterior and posterior areas could result from an uncoupling of their development. The effect of the offset stomach and intestine in the body, resulting from the flattened body-form, is the probable explanation of the directional asymmetry producing larger petals on the left side. It is not known whether this directional asymmetry has a genetic basis or is the result of pressure from the stomach contents.