Within each of three adjacent monospecific beds of seagrasses (Halodule wrightii Aschersen, Syringodium filiforme Kutzing, and Thalassia testudinum Banks ex König) in the Indian River lagoon, Florida, the motile epifauna of seagrass and drift algae (Gracilaria
sp.) were quantitatively sampled. In spite of sometimes large and significant differences in abundance of many epifaunal species, the lists of species on all substrata were similar, with an average of 75% of 47 species in common, mostly gastropods and peracarid crustaceans. Similarity analysis
grouped faunas by substratum type (i.e., either algae or seagrass) rather than by site. At each site, the majority of the top 15 species had significant differences in abundance between seagrass and algae, both in terms of individuals per unit plant biomass and per unit plant surface area.
Gastropod species were usually significantly more abundant on seagrass than on algae, whereas crustaceans were more abundant on algae. Both plant biomass and plant surface area were poor predictors of epifaunal abundances across substratum types, contrary to earlier predictions. These strong
plant-animal associations may be due to differential survivorship and/or active habitat selection. The former may be particularly important in determining relative abundance of gastropods and crustaceans. When locally abundant, drift algae can be an important component of seagrass systems,
and may provide superior refuge, food and habitat over seagrasses for associated epifauna.
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