General ecology of the cassiduloid urchin Cassidulus caribbearum

Cassidulus caribbearum, a representative of the echinoid order Cassiduloida, occurs in localized populations of high densities (up to 100 m^-2) among the islands of the Puerto Rico bank in the eastern Caribbean Sea. These small (up to 35 mm) urchins are burrowers, principally in shallow-water areas with coarse sand bottom. Locomotion is achieved by ditaxic waves, passing from the front to the rear of large movable spines on the lateral portions of the ventral surface; this mechanism is unique among echinoids. Digging was most effective in sand grain sizes most closely approximating those of the environment (0 to 1). These urchins are somewhat selective deposit feeders, which ingest substrate particles primarily in the size range 0 to 1. Individuals ingest sand more or less continuously; complete passage of sand through the gut takes an average of 6 h in the laboratory. Aristotle's lantern is present in young juveniles (test length <1.0 mm), but is resorbed before the urchins reach a test length of 5.0 mm. The gut shows similarities in gross morphology to the spatangoid gut. Oxygen consumption is comparable to that of other tropical echinoids of similar wet weight. C. caribbearum broods its large (350 m) yolky eggs among the aboral spines for 10 to 12 days, at which time young urchins crawl off into the sand. Successive, overlapping broods are produced, so that frequently two broods of different developmental stages are present on a single urchin. Single urchins may brood continuously for up to 4 months or more. Throughout the year, at least 50% of all individuals over 18 mm were found brooding, with a maximum of 85% of all individuals brooding in mid-summer. The sex ratio of the population in all areas sampled was greater than 5 females to 1 male. No evidence of protandric hermaphroditism was found. Growth rate, measured from tagged indivuduals and from increase in median size in population samples, was between 0.5 and 1.0 mm month^-1. Mortality was due primarily to predation by the gastropod Cassis tuberosa, although occasional, episodic mortality was caused by physical factors such as heavy storm swells.