Development and metamorphosis of the planktotrophic larvae of Rostanga pulchra (Mollusca: Nudibranchia)

Abstract

Rostanga pulchra MacFarland, a small (1 to 2 cm) dorid nudibranch, lays an average of 7000 eggs in the laboratory during a period of 30 days in the summer. The veligers hatch 15 to 16 days after oviposition and it takes another 35 to 40 days to become competent for metamorphosis at a temperature of 10° to 15°C. Larval cultures were maintained initially at a concentration of 500 veligers per 100 ml of filtered sea water (antibiotics added). During the planktotrophic phase of development, the veliger grows from 150 to 300 μm in shell length. Although the veligers are generalists in their food preference, the best result (faster growth) was achieved by feeding them with a combination of Monochrysis lutheri and Isochrysis galbana. The concentration of food cells was kept at 104 cells per ml of culture media and was supplied every 2 to 3 days. A veliger which is competent to metamorphose is identifiable morphologically by its propodium, eyespots, rhinophores, and spiculated dorsal papillae. The entire metamorphic process lasts 24 h when a suitable substrate such as the food sponge Ophlitaspongia pennata is provided. The competent veliger is able to delay metamorphosis for at least 3 weeks. Juveniles were kept in the laboratory for 70 days and, during this period, grew to a length of 4.5 mm. © 1978 Springer-Verlag.