ABSTRACT
Although the particle-picking, substrate-selecting behavior of clypeasteroids (sand dollars and sea biscuits) is relatively well-known, that of the purported outgroup to clypeasteroids, the Cassiduloida, has not been similarly analyzed. These analyses are necessary to determine the origins of the sophisticated podial particle-picking mechanisms seen in sand dollars. Cassidulus caribaearum is a small cassiduloid echinoid that burrows into coarse, carbonate sands at the water’s edge of protected beaches such as that at Loblolly Bay, Anegada, in the British Virgin Islands. Our analyses indicate that the particle size distributions of the total gut contents of 22 Cassidulus representing a full size range (4.3–26.8 mm test length) are not significantly different from those of Loblolly beach sediment. Overall, then, Cassidulus does not appear to be a selective deposit feeder. Small individuals (less than 10 mm test length) do tend to take smaller particles than adults, but this slight bias is swamped by the much larger number of particles in the guts of adults. Juveniles do not appear to feed until they reach a test length of approximately 3.5 mm or more. By the time the echinoid is 5 mm long, the alimentary tract is full—a condition that pertains throughout the rest of the animal’s life. Contrary to earlier studies, mouth size does not provide an upper bound to the size of particles taken, regardless of the size of the animal. In even the smallest specimens the size of the mouth greatly exceeds the dimensions of the largest particles ingested. Allometric studies of buccal podia, phyllopodia, and accessory podia suggest that podial size likewise is not a strong predictor of particle size distributions in the gut, and that the podia work well within the stress tolerances of the adhesive properties of the suckers. The enhanced role played by the accessory podia in food collection by Cassidulus seems to be a synapomorphy for cassiduloids and clypeasteroids, because in the plesiornorphic condition exemplified by irregular echinoids such as spatangoids, food collection is performed by the phyllopodia only.
