Nudibranchs that exclusively eat scleractinian corals provide a rare opportunity to study specialist predation in the marine environment. To measure the diet breadth of 4 Phestilla species on Guam, we offered the nudibranchs different corals in choice and no-choice feeding assays. Larval preferences were determined by measuring the percent metamorphosis in response to different coral species. We compared the specificity of larval metamorphosis to adult feeding preferences. Phestilla sibogae ate a range of Porites species (Poritidae) in the field and would not eat other coral genera in the laboratory no-choice assays. Metamorphosis was approximately 90% in response to 4 Porites spp. Phestilla minor was found on Porites lutea and Porites annae in the field. It preferred P. annae over Porites cylindrica and P. (Synaraea) rus during the choice and no-choice assays. The highest rates of metamorphosis (approx. 80%) were in response to P. lutea, P. annae, and P. cylindrica. Phestilla sp. 1 is morphologically similar to P. minor, but it eats different Porites species. It preferred P. (S.) rus, but would eat P. cylindrica during the no-choice assays. The highest rate of metamorphosis (approx. 80%) was in response to P. (S.) rus. Phestilla sp. 2 is distinct from the other Phestilla species studied, as it is a specialist on corals in the genus Goniopora (Poritidae). It preferred G. fruticosa and also ate G. minor and G. lobata during the feeding assays. The highest rates of metamorphosis (approx. 60%) were in response to G. fruticosa, G. minor, and G. lobata. This study documents a range of diet breadth among Phestilla species. Phestilla spp. larvae could distinguish between coral species within a host genus and showed a tendency to have high metamorphosis on their preferred hosts, but they also metamorphosed in response to non-food coral species.

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