Podial Particle Picking in Cassidulus caribaearum (Echinodermata: Echinoidea) and the Phylogeny of Sea Urchin Feeding Mechanisms.

Abstract

Selective particle-picking mechanisms of clypeasteroid echinoids (sand dollars and related taxa) are well-known. Those of the extant outgroup to clypeasteroids, the cassiduloids (lamp urchins), have not been analyzed to determine the origins of this sophisticated feeding mechanism. Cassidulus caribaearum Lamarck, 1801, is a small cassiduloid living in the coarse, carbonate sands of protected beaches. The total gut contents of 24 specimens of C. caribaearum, representing a full size range, were studied. The distribution of particle sizes in this sample was not significantly different from that of beach sediment, indicating that C. caribaearum is probably not a selective deposit feeder. Juveniles with a test length of less than 3.5 mm do not feed, but all echinoids that are at least 5 mm long have full, or almost full, guts. The size of the mouth does not limit the sizes of particles eaten, regardless 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 cleansing functions of the spines and ciliary currents and the feeding activities of specialized podia. The new role played by accessory podia in food-collection by C. caribaearum is a synapomorphy for cassiduloids and clypeasteroids, whereas the exclusive use of phyllopodia seen in earlier irregular echinoids is plesiomorphic.