Distribution of macrobenthic crustaceans associated with Thalassia, Halodule and bare sand substrata

La repartition des crustaces parmi les microhabitas vegetaux parait etre un phenomene complexe impliquant le choix de l'habitat and the pression de predation, tous deux affectes par l'espece et the biomasse des vegetaux presents.

Abstract

The influence of macrophyte biomass and species composition on the distribution of macrobenthic crustaceans was examined in 2 vegetated and 2 unvegetated grass bed microhabitats under similar physical-chemical conditions.Vegetated habitats consistently supported higher numbers of individuals and species than either of the unvegetated zones.Correlations between plant biomass in each microhabitat and both animal numbers and diversity suggested that seagrass biomass was a key organizing factor in macrophyte-associated faunal assemblages.The thin-bladed Halodule wrightii supported similar crustacean abundance per core, density and species richness to those found on widebladed Thalassia testudinum, contradicting the predictions of Kikuchi and Peres (1977) that widebladed magnozosterids should have a more abundant and diverse fauna relative to thin-bladed parvozosterids.In addition, after standardization to numbers of individuals per unit plant biomass (or surface area), H. wrightii supported higher abundance than T. testudinum suggesting that seagrass species composition is more important to faunal organization than previously thought.Although similar suites of species inhabited each substratum, relative abundances of individual species were markedly different among the microhabitats.Greatest differences occurred between vegetated and unvegetated zones.Few of the 24 dominant crustaceans (those species with abundances of 1 % or more of the total number of individuals collected) had significantly greater abundances per core on one seagrass species relative to the other.Three species were more abundant on T. testudinum while only 1 occurred in greater numbers on H. wn'ghtii.However, when species abundances per unit plant weight (or surface area) were compared between the 2 vegetation forms, 7 species were significantly more abundant on H. wrightii; only 2 species occurred in higher numbers on T. testudinum.Distribution of crustaceans among seagrass microhabitats appeared to be a complex function involving faunal habitat choice and predation pressure, both of which are affected by the species and biomass of seagrass present.