ABSTRACT
The common sand dollar Echinarachnius parma from eastern Canada, in addition to showing marked changes in the camber of the aboral surface related to the strength of wave and current activity, shows sexual dimorphism in the shape of the oral surface. The oral surface is more concave in males than in females. This dimorphism develops as individuals become sexually mature and is pronounced for individuals measuring >40 mm in diameter. Associated with this shape difference, the volume of females is greater than for the males and this corresponds to the larger size attained by the ovaries compared to the testis. Shape differences between males and females are rare in echinoderms and E. parma is the first species for which the sexes can be separated with a high degree of accuracy (>80%) on the basis of shape. Whereas the radial symmetry of the sand dollar E. parma is striking, we show that the mass and thickness of the anterior portion is greater than for the posterior portion. By adding weights to either portion and quantifying the rising response, we demonstrate that the asymmetry in body mass is required for the sand dollar to right itself.
