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RETRODISPLACEMENT OF THE ORAL AND ANAL OPENINGS IN DENDRASTERID SAND DOLLARS

In regular echinoids, the mouth opening (or peristome) and the anal opening (or periproct) are located centrally. In irregular echinoids, the peristome tends to shift toward the anterior end of the test, whereas the periproct typically shifts toward the posterior end. This produces an anatomically polarized morphology, which is consistent with functional expectations. In the dendrasterid sand dollars, however, the peristome and periproct have been displaced in the “wrong” directions; the peristome has shifted posteriorly, whereas the periproct has shifted anteriorly. These movements, which run counter to functional expectations, may be termed “retrodisplacements.” This study presents a new model for the development of the oral surface in dendrasterids. The model assumes that the “Dendraster pattern” of development (which occurs in dendrasterids) was derived from the older “Echinarachnius pattern” (which occurs in other northern Pacific sand dollars). The Echinarachnius pattern includes two successive phases of asymmetric growth: the first phase favors anterior growth, whereas the second phase favors posterior growth. These two phases are normally balanced, and at maturity, the test appears to be symmetrical. But if the second phase of unequal growth were suppressed through a heterochronic change in development, the effects on test development would be profound. On the aboral surface, the predicted effects include posterior displacement of the apical system. On the oral surface, the predicted effects include retrodisplacement of the peristome and periproct, as well as conspicuous changes in ambulacral and interambulacral development. In fact, all of the predicted effects are characteristic features of dendrasterids. The model assumes that the oral and aboral surfaces could be simultaneously affected by the same developmental processes. This assumption is supported by empirical evidence: in dendrasterids, there is a strong correlation between the displacement of the apical system (on the aboral surface) and the displacement of the peristome (on the oral surface). The displacement of the apical system (known as “apical eccentricity”) is regarded as a valuable adaptation, because it facilitates suspension feeding. But if oral and aboral development are linked, then selective pressure for apical eccentricity would simultaneously produce new oral characters as well. Thus, the retrodisplacement of the peristome and periproct in dendrasterids may be related to developmental constraints. These unusual characters may have little or no functional significance.