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

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 m 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.