Abundance, performance, and feeding preference of herbivorous amphipods associated with a host alga-epiphyte system

Abstract

Mesograzers can benefit their host macrophytes by selectively grazing on epiphytic algae, such as well documented for seagrass meadows and other systems where hosts seem to be less palatable than epiphytes. However, where both host and epiphytes are algae and are likely to be palatable, the predictions about the feeding preferences of mesograzers and, thus, their potential ability in controlling the epiphyte biomass are unclear. Also, the performance consequences of feeding on host algae versus epiphytes, what can explain the feeding preferences of mesograzers, have been rarely tested. The food value of the host alga Sargassum filipendula and two of its common epiphytic algae, Hypnea musciformis and Dictyota cervicornis, for mesograzers was contrasted by investigating the abundance, performance, feeding preference, and feeding rates of the herbivorous amphipods Cymadusa filosa and Sunamphitoe pelagica. Sampling was carried out seasonally between January and November 2012 in a subtropical shore from the Brazilian south-eastern coast (23°32′S, 45°10′W). The abundance of both herbivores was positively correlated with S. filipendula biomass. However, only C. filosa had a positive relationship with H. musciformis biomass during the season of lower S. filipendula abundance. Juveniles of S. pelagica were able to survive only when consuming S. filipendula. In contrast, C. filosa survived at similar rates on epiphytes and S. filipendula, but long-term feeding on epiphytes resulted in lower growth and reproduction. Both herbivores preferred to consume S. filipendula in choice assays, but only C. filosa was able to feed on epiphytes at similar rates as it did on S. filipendula in no-choice assays. These mesograzer species may differ from each other in the mechanisms determining their association with S. filipendula. For C. filosa, epiphytes should be an alternative source of food, mainly in periods of lower host availability. Thus, the role of epiphytes on mesograzer-Sargassum interaction seems to vary temporally and among herbivores. Overall, these results contrast with the general patterns about mesograzer preferences towards epiphytes reported for other systems. Therefore, the feeding preferences of mesograzers in host-epiphyte systems and, thus, their potential impact may depend on the identity of hosts and epiphytes.