Chemical defenses of the tropical ascidian Atapozoa sp. and its nudibranch predators Nembrotha spp.

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Abstract
The ascidian Atapozoa sp. is common in shallow reef habitats throughout the tropical IndoPacific. Collections from the central Philippine Islands, northern Sulawesi (Indonesia), Palau, Kwajalein, and Ant Atoll (near Ponape) were examined for the presence of secondary metabolites. In most cases, nudibranch molluscs Nembrotha spp. were found feeding on the ascidians. Like many dorid nudibranchs, these appear to obtain their defensive chemicals through their diet. Both ascidians and nudibranchs contained large quantitites of a series of bipyrrole secondary metabolites previously described as the tambjamines. Concentrations of the different tambjamines and a related, highly blue-pigmented tetrapyrrole varied among collections of the ascidians and nudibranchs. Overall, concentrations of tambjamines and tetrapyrrole were higher in the nudibranchs, and were the major components of the mucus exuded by Nembrotha spp. when irritated. Tambjamine A and the tetrapyrrole were present in the nudibranchs but did not occur in detectable amounts in Atapozoa. The ascidian and nudibranch extracts and isolated metabolites were tested as feeding deterrents toward a variety of carnivorous fishes in field assays on 2 Guam reefs that differed in the types of fishes that fed during the assays. The crude extract, mixtures of tambjamines, tambjamine C, tambjamine F, and the tetrapyrrole were all significant feeding deterrents at or below natural concentrations. Tambjamines A and E were not deterrent when tested alone at natural concentrations; however, a 1:1 mixture of tambjamines E and F was deterrent when tested below natural concentrations. Evidence is thus presented that both the ascidian and its nudibranch predators use the tambjamines as chemical defenses against predators.