For marine predators, the sea floor is a smorgasbord. It's covered in snacks that can't move out of the way—sponges, bryozoans, clams and a host of other sedentary animals. They might not be mobile but they're not defenceless. Many species (especially among the sponges) are chock-full of toxins that deter predators after one nibble.
Some sea slugs hijack the toxins used by their prey and use them for their own purposes. (This is not confined to opisthobranchs. It happens across a range of animal groups—insects, fish, frogs and even birds.) Researchers from the University of Guam and the Florida Museum of Natural History studied two closely related species that feed on the same sponge and found contrasting defensive strategies.
The Pacific sea slugs Sagaminopteron nigropunctatum and S. psychedelicum graze on Dysidea granulosa. Both species sequester the same toxin—secondary metabolites called polybrominated diphenyl ethers—in the mantle and skin folds. They also release it in their mucus.
Despite the similarity in their use of hijacked chemical defences, the two species of sea slug avoid predator attack in different ways. Sagaminopteron nigropunctatum is camouflaged, whereas S. psychedelicum is brightly-coloured (as the name suggests). One hides from its predators. The other sends out a signal.
Whatever works. It's the only rule in nature.
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Becerro, M.A., Starmer, J.A. & Paul, VJ. (2006). Chemical defences of cryptic and aposematic gastropterid molluscs feeding on their host sponge Dysidea granulosa. Journal of Chemical Ecology 32(7): 1491–1500.
