Seaweed chemically defended

Chemically defended macroalgae often dominate their environments (e.g., Marques et al. 2006). However, how these macroalgal species became dominant and what the mechanisms that maintain their dominance are, are questions that remain largely unanswered. Generally, heibivores that would consume more palatable, undefended seaweed species could be assumed to be the cause of chemically defended seaweed dominance, but experimental studies at the community level to... 

On the other hand, association with more palatable seaweeds may have a negative impact on the chemically defended partner. For example, Halimeda specimens from Conch Reef, Florida Keys, with more than 50% of their thalli covered by Dictyota grow significantly slower than unepiphytized thalli (Beach et al. 2003). This study also verified that epiphytic Dictyota negatively affects metabolic rates of Halimeda tuna in part by shading their thalli, but probably also by chemical means, because the exposure to Dictyota-conditioned water elevated respiration rates in a manner similar to when H. tuna is naturally epiphytized by Dictyota. 

Hay ME, Renaud PE, Fenical W (1988b) Large mobile versus small sedentary herbivores and their resistance to seaweed chemical defenses. Oecologia 75 246-252 Hay ME, Duffy JE, Paul VJ, Renaud PE, Fenical W (1990) Specialist herbivores reduce their susceptibility to predation by feeding on the chemically-defended seaweed Avrainvillea longicaulis. Limnol Oceanogr 35 1734-1743... 

For two decades, the role of lipophilic chemistry in the evolution of mesograzer host range was heavily influenced by the enemy-free space hypothesis by Hay et al. (1987). This theory predicts that small herbivores consume and inhabit seaweeds that are chemically defended against larger, more mobile herbivores such as fishes and urchins. Support for these patterns can be found in several contexts from temperate North Carolina (e.g., Duffy and Hay 1991, 1994) and New Zealand (Taylor and Steinberg 2005), and the tropical Caribbean (e.g., Hay et al. 1990) and Guam (e.g., Paul 1988 Cruz-Rivera and Paul 2006), and the Antarctic (Huang et al. 

Small sedentary grazers such as amphipods appear to select chemically defended seaweeds as host plants since they would otherwise be subject to intense predation by reef fishes however, they do not sequester metabolites as do other selective grazers such as sacoglossans and opistobranch molluscs.105 109 Grazing by amphipods induces increased concentrations of acutilol A acetate and acutilol B in Dictyopteris menstrualis and makes the seaweed less susceptible to attack by other predators 112 the same terpenes acted as antifoulants which prevented the settlement of bryozoan larvae.107 This evidence for multiple roles for algal metabolites may provide an explanation of previously documented differences in chemical composition in Dictyota.113... 

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). 

The spatial-variation-in-consumers model (SVICM) was proposed by marine ecologists to explain both interspecific and intraspecific patterns of chemical defenses in seaweeds.111 It states that chemically defended seaweeds will be more evolutionarily persistent than undefended seaweeds in areas subject to significant herbivore impact. Likewise, this model explains intraspecifc variation in chemical defenses, in that well-defended individuals or populations will be more persistent over ecological time scales than undefended seaweeds in areas subject to significant herbivore impact. This model borrows heavily from the optimal defense theory, in that it suggests that seaweeds that occur in areas with high herbivory pressure will be vulnerable to herbivore attack and should thus be heavily defended. 

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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