Are tropical herbivores more tolerant of chemically rich seaweeds than are temperate herbivores? A test of seaweed- herbivore coevolution

There is a greater quantity and diversity of lipophilic secondary metabolites produced by seaweeds within tropical relative to temperate regions. Coevolutionary arms-race theory predicts that tropical herbivores should more readily tolerate consuming chemically-rich seaweeds than do temperate herbivores, however, tests of this prediction are rare. We assessed the willingness to consume freeze-dried tissue from 10 species of chemically-rich seaweeds exhibited by multiple populations of the herbivorous amphipod Ampithoe longimana collected from coldtemperate, warm-temperate and subtropical estuaries along the east and Gulf coasts of North America. In contrast with the coevolutionary predictions, geographic patterns of consumption of chemically-rich seaweeds were not correlated with latitude. Rather, populations in North Carolina (34oN) consumed more tissue from the diterpeneproducing seaweed genus Dictyota than did New England (41oN) or Florida (27oN) populations. Amphipod populations did not differ in their feeding responses to tissue from chemically-rich seaweeds in the genera Amphiroa, Halimeda, Padina, Penicillus, nor Udotea. Thus, Floridean A. longimana have not locally evolved a feeding tolerance for chemical defenses produced by tropical seaweeds, as would be predicted by an arms-race hypothesis. Rather, the geographic patterns in feeding behaviors reflect the relative importance of these seaweeds to the ecology of local populations.