Ontogenetic and diel vertical migration of a planktonic echinoid larva, Dendraster excentricus (Eschscholtz): Occurrence, causes, and probable consequences

The ontogenetic and diel vertical distributions of the planktonic larvae of Dendraster excentricus (Eschscholtz) were examined (1) in the field, (2) in a 2500-1 floating enclosure, and (3) in a 1500−1 outdoor aquarium. In the field, both early (four-armed) and advanced (competent) stage plutei were found predominantly less than 6 m deep. In the enclosure and aquarium, all pluteus stages were distributed similarly at a given time of day, though competent plutei were not included in these experiments. The plutei underwent diel vertical migration in both the enclosure and aquarium, rising towards the surface at night and descending during daytime. Laboratory experiments were conducted to determine (1) how plutei orient vertically, (2) what larval features might regulate ontogenetic depth distribution, and (3) what environmental features cause the diel migration. All pluteus stages oriented vertically and were geonegative because an asymmetrical distribution of body mass, presumably due to the developing larval skeleton, causes plutei to swim and sink anterior-upwards. Forward swimming thus results in geonegative movement. Eight-armed larvae and newly metamorphosed juveniles were also found to have lower specific gravities than younger plutei, probably resulting from an accumulation of lipid reserves prior to metamorphosis. This decrease in specific gravity may account for the shallow distribution of advanced stage plutei observed in the plankton. Experiments to examine causes of the diel vertical migration excluded temperature, thermal discontinuities, and visible light, but showed that ultraviolet light under 315 nm inhibits swimming of all pluteus stages, but not of pre-pluteus stages, and results in daytime sinking of plutei. Ecological consequences of the lack of ontogenetic migration are unknown, though D. excentricus plutei probably are recruited from surface waters into both intertidal and subtidal adult habitats. The diel vertical migration apparently removes plutei from harmful levels of ultraviolet light; when confined in shallow water and exposed to simulated solar ultraviolet light regimes, plutei were killed or their development was retarded.