Amphipod chemically defended seaweeds consumed

For most marine invertebrates that readily consume chemically defended seaweeds, it is not known whether they are actually resistant to, or simply tolerant of, algal secondary metabolites. In the case of specialist consumers (e.g., nudibranchs, ascoglossans, some amphipods or crabs see Section IV.B), a means of resistance to specific chemicals seems likely. However, for marine invertebrates that consume a diverse array of prey that produce different chemical defenses against a broad suite of predators,85,86 perhaps tolerance or less-specific mechanisms of resistance (i.e., gut pH) become more important. The actual mechanisms by which marine consumers avoid harmful effects of consuming chemical defenses (detoxification or dietary mixing) are even less well understood (see Section II.B.2). 

In contrast to generalist consumers, specialists that recruit to and feed only from certain hosts may be stimulated by unique metabolites produced only by that host. This is true for the crab Caphyra rotundifrons which feeds only on the chemically defended tropical seaweed Chlorodesmis fastigiata. The cytotoxic diterpenoid chlorodesmin (Fig. 3.2a), the major secondary metabolite of Chlorodesmis, deters fishes from consuming this alga, but stimulates feeding by the specialist crab when it is applied to a red alga (Fig. 3.3) that the crab will not normally consume (Hay et al. 1989). There are numerous similar cases where specialist crabs, amphipods, or... 

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