Associations between hydroid species assemblages and substrate types in the mangal at Twin Cays, Belize

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Hydroid species composition on various substrates in a mangrove ecosystem was investigated during the winter of 1987 at Twin Cays, Belize, Central America. Soft sediments, on which hydroids were either depauperate (peat) or not observed at all (silt, sand, mud), predominated in the study area. However, firm substrates, including submerged prop roots of red mangrove (Rhizophora mangle), turtle grass (Thalassia testudinum), benthic algae (Chlorophyta and Rhodophyta), floating algae (Turbinaria turbinata and Sargassum fluitans), epibenthic invertebrates (especially sponges, hydroids, molluscs, and crustaceans), wood, and rope, supported a moderately diverse hydroid fauna (48 species). More hydroid species (22) were found on mangrove prop roots than on any other substrate. Six substrate groups and 11 species groups were recognized in numerical analyses of hydroid–substrate frequency data. Constancy of species groups for substrate groups was mostly low or very low, reflecting the low frequency of occurrence of most hydroid species in collections from Twin Cays. Fidelity of some species groups for certain substrate groups was high, especially for those groups occurring on drifting algae and rope. Although most hydroid species were relatively facultative with respect to substrate, bottom type was an important factor influencing their distributions.