PODIAL PARTICLE PICKING IN CASSIDULUS-CARIBAEARUM (ECHINODERMATA, ECHINOIDEA) AND THE PHYLOGENY OF SEA-URCHIN FEEDING MECHANISMS

Selective particle-picking mechanisms of clypeasteroid echinoids (sand dollars and related taxa) are well-known. Those of the extant outgrou p to clypeasteroids, the cassiduloids (lamp urchins), have not been an alyzed to determine the origins of this sophisticated feeding mechanis m. Cassidulus caribaearum Lamarck, 1801, is a small cassiduloid living in the coarse, carbonate sands of protected beaches. The total gut co ntents of 24 specimens of C. caribaearum, representing a full size ran ge, were studied. The distribution of particle sizes in this sample wa s not significantly different from that of beach sediment, indicating that C. caribaearum is probably not a selective deposit feeder. Juveni les with a test length of less than 3.5 mm do not feed, but all echino ids that are at least 5 mm long have full, or almost full, guts. The s ize of the mouth does not limit the sizes of particles eaten, regardle ss of the size of the animal. Allometric analyses suggest that podial size is also not a strong predictor of ingested particle size. In vivo and histological observations differentiate between the test cleansin g functions of the spines and ciliary currents and the feeding activit ies of specialized podia. The new role played by accessory podia in fo od-collection by C. caribaearum is a synapomorphy for cassiduloids and clypeasteroids, whereas the exclusive use of phyllopodia seen in earl ier irregular echinoids is plesiomorphic.