Perception and Choice of Substratum by Epifaunal Amphipods Associated with Seagrasses

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Abstract
Laboratory experiments showed that the 3 epifaunal amphipods Cymadusa compta, Grandidierella bonnieroides and Melita elongata, normally associated with seagrasses, are capable of detecting small differences in the density of seagrass and actively select areas of high blade density. When different seagrass species Thalassia testudinum, Syringodium filiforme and Halodule wrightii were offered with equal blade biomass, H. wrightii was selected due to its high ratio surface area:biomass. When offered equal surface area all 3 amphipod species were randomly distributed among the seagrass species, confirming the hypothesis that surface area is the habitat characteristic chosen. Blade surface area of macrophytes provides the best estimate of habitat complexity in seagrass meadows. The surface area relationship also held for C. compta when the highly branched red alga, Gracilaria foliifera, was used as a substratum. Degree of habitat selectivity by the 3 spp. of amphipods was related to known differences in vulnerability to predation by Lagodon rhomboides. Habitat preference and biological interactions must both be examined to understand distributional patterns in seagrass epifauna.