Marine opisthobranchs are mollusks scarcely protected by a shell which can be either reduced or completely absent. It is generally accepted that they possess chemical defenses (allelochemicals) against predators. These protective molecules can derive from dietary habits (algae, sponges, tunicates, soft corals, etc.) and can be used either in their intact form or as derived metabolites. Added to this, other compounds are biogenetically obtained. These defensive compounds are very often concentrated in the external parts of the animal and are also released into the mucous secretion to deter predators. The chemical defenses of sea hares (order Anaspidea) and sea slugs (order Nudibranchia) are well studied. This review highlights the allelochemicals reported from Japanese anaspideans and nudibranchs, with particular attention to the finding of our group.
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