ONTOGENY AND ORIGIN OF THE BROODING SYSTEM IN ANTARCTIC URECHINID SEA-URCHINS (ECHINODERMATA, HOLASTEROIDA)

Echinoids usually broadcast gametes, and do not generally engage in a high degree of parental care. However, when they do, juveniles are typically maintained among the spines, or in shallow, external depressions in the test itself. The brooding Antarctic holasteroids Urechinus mortenseni and Plexechinus nordenskjoldi are bizarre exceptions: females develop an elaborate brooding system in which a small number of direct developing young are protected. Ontogeny of post-natal brooding urechinids is marked by profound divergence in the growth trajectories of male and female apical systems. In females, this leads to dramatic departures from the patterns found in all other echinoids. Otherwise, coronal skeleton allometry of males and females is almost identical. Juveniles in brood pouches grow larger than the diameter of the apical aperture through which they must pass to reach the external environment. The apical plates, from which the brooding system is suspended, '' hinge '' downward to enlarge the aperture, allowing the young to emerge from the female. A possible origin for the brooding system suggests derivation by centripetal plate addition from the ocular plates in the coronal skeleton. We develop a contrasting model for the origin of the brooding system that relies on a proposed homology between genital and periproctal elements of the apical system of echinoids and the more highly developed dorsal skeleton of other echinoderm classes.