STUDIES ON THE LIFE HISTORY OF TWO CORAL-EATING NUDIBRANCHS OF THE GENUS PHESTILLA.

1. The complete life cycles of two coral-eating aeolid nudibranchs, Phestilla melanobranchia Bergh, 1874 and Phestilla sibogae Bergh, 1905, are described. Information on their life histories includes developmental stages and timing, duration of the veliger stage, veliger behavior, factors necessary for settling and metamorphosis, and adult growth rates, fecundity and longevity. The life cycles of the two Phestilla are similar and their physical and behavioral differences are related to the characteristics of their respective coral prey. 2. P. melanobranchia has planktotrophic development, is negatively phototactic when ready to settle, requires close proximity to living dendrophylliid coral tissue for metamorphosis and has a generation time from egg to egg of 60 days. The dendrophylliid corals on which P. melanobranchia feeds are small, patchy and photonegative in distribution. 3. P. sibogae which has now been under serial cultivation for four years, has lecithotrophic development, is positively phototactic when ready to settle, requires only a chemical factor from living Porites tissue for metamorphosing and has a generation time of 38 days. The Porites corals which P. sibogae feeds on are large, very common and photopositive in distribution. 4. Differences in predation pressure and prey tissue utilization efficiency are proposed as factors influencing the evolution of a significantly faster generation time in Phestilla sibogae than in the closely related P. melanobranchia.