Sequestration and possible role of dietary alkaloids in the sponge-feeding mollusk Tylodina perversa.

Opisthobranchs of the genus Tylodina are found at exceedingly distant geographical regions in the marine environment but are always associated with sponges of the order Verongida (e.g., Aplysina species) which serve as prey for these gastropods. We investigated the chemical ecology of the Mediterranean species T. perversa that commonly feeds on A. aerophoba. The gastropod sequesters a set of sponge-derived brominated isoxazoline alkaloids which are accumulated in the mantle and egg masses and are furthermore exuded as part of the mucus when the animal is molested. Based on the documented feeding deterrent properties of the sponge alkaloids against fish, it is speculated that the sequestered sponge alkaloids serve also as a defense for T. perversa. Interestingly, specimens of T. perversa that were either collected while feeding on A. aerophoba or had been kept on these sponges under controlled conditions for several weeks almost always contained the brominated alkaloid aerothionin, which is not detected in A. aerophoba but occurs in the sibling species A. cavernicola instead. The latter sponge is also accepted as a food source by the gastropod, at least under experimental conditions. The possible origin of aerothionin in T. perversa is discussed.