Restricted host use by the herbivorous amphipod Peramphithoe tea is motivated by food quality and abiotic refuge

Abstract

There is a growing list of marine inverte- brate herbivores known to restrict their host choices to a subset of available species, yet the relative impor- tance of the evolutionary forces that select for special- ized feeding habits remain unclear. One such specialist is the gammaridean amphipod Peramphithoe tea (F. Ampithoidae) that restricts its distribution to the brown laminarian seaweed Egregia menziesii in Ore- gon. A Weld survey indicated that among available sea- weeds in the low intertidal zone of Boiler Bay, Oregon, Egregia housed greater than 90% of P. tea individuals. A set of laboratory-based habitat and feeding choice assays revealed that this specialized host distribution is likely the consequence of choices made by adult P. tea. The restricted host choice is apparently maintained by at least two evolutionary forces. First, a juvenile per- formance assay indicates that both Egregia and the co- occurring seaweed Alaria marginata, provide high food quality relative to other seaweeds available in the low- intertidal zone. Second, a Weld transplantation experi- ment revealed that Egregia protects adult amphipods from becoming dislodged with wave energy more read- ily than did Alaria. Thus, Egregia's value as good qual- ity food and refuge from abiotic stress together explain the restricted host use of P. tea. A comparison with previous studies suggests that use of Egregia is not con- sistent across the geographic range of P. tea, suggesting the possibility that the host preferences of local popu- lations may respond evolutionarily to geographic shifts in seaweed communities.