Ontogeny of phototaxis and metamorphic competence in larvae of the nudibranch Phestilla sibogae Bergh (Gastropoda : Opisthobranchia)

Abstract

Temporal changes in phototaxis, metamorphic competence, and swimming patterns were studied in lecithotrophic larvae of the aeolid nudibranch Phestilla sibogae Bergh. Separate horizontal and vertical tests of phototaxis showed young larvae (Days 5–7 post-fertilization = Days 0–2 post-hatching) to be highly photopositive. Positive phototaxis decreased with age, showing greatest change in larvae 8–9 days post-fertilization (3–4 days post-hatching). Positive phototaxis also decreased with increasing exposure time to light (vertical tests, 60 min maximum exposure). Young larvae displayed a decreased photopositive response to high intensity vertical, but not horizontal, light. Older larvae (Days 9–11 post-fertilization Days = 4–6 post-hatching) showed increased photopositive swimming in horizontal, but not vertical, high intensity light regimes. Young larvae swam in mostly straight paths toward a light, and the frequency of stops and circling increased with age, while distance travelled and vertical swimming speed decreased. The data indicate that ontogenetic changes in photic behavior and the onset of metamorphic competence follow independent time courses, and may reflect a balance between selection pressure associated with dispersal and settlement, and developmental constraints associated with physiological and morphological development.