Local adaptation in the feeding preferences for chemically-rich seaweeds in the marine herbivore, Ampithoe longimana

Background/Question/Methods     Herbivorous insects with broad geographic ranges are exposed to locally distinct plant communities, and as a consequence tend to evolve higher affinity for and fitness on local plants.  Such local adaptation has been documented extensively among herbivorous insects, but largely ignored among marine herbivores.  We explored local evolution in feeding preference among populations of the marine amphipod, Ampithoe longimana, across 2,000 km of the Atlantic coast of the United States.  Our field surveys indicated that chemically defended seaweeds in the tropical brown genus Dictyota are an important host for A. longimana in North Carolina.  In contrast, amphipod populations in New England occur 100’s of kilometers north of the geographic range of Dictyota while populations in subtropical Florida occur within estuaries where Dictyota is rare to absent.  Populations of A. longimana from several locations within the three regions of New England, North Carolina and Florida were offered a feeding choice between Dictyota and the green seaweed Ulva intestinalis, which is abundant throughout the entire distribution of A. longimana and lacks known secondary metabolites.  Assays were repeated using fresh plant, lyophilized tissue and lipophilic extracts of D. ciliolata and D. menstrualis.  

Results/Conclusions     We predicted that if amphipods are responding evolutionarily to their local plant communities in a manner similar to that of herbivorous insects, then amphipods from North Carolina should display a stronger preference and tolerance for Dictyota and its secondary metabolites than amphipods from New England or Florida.  These predictions were supported by the data.  Both D. ciliolata and D. menstrualis and their lipophilic metabolites were preferentially consumed by A. longimana from North Carolina, but were consumed at lower rates by amphipods from the other two regions.  The lower preference exhibited by Floridean amphipods for these tropical seaweeds is particularly striking given that Floridean amphipods and Dictyota overlap in distribution in subtropical Florida.  Thus, for small marine herbivores and terrestrial insects, local patterns of host use readily predict the evolution of local feeding preferences.