The species composition, distribution, and abundance of the planktonic medusae collected during a plankton survey in Banco Chinchorro, an oceanic reef atoll of the western Caribbean were studied. Samples included the forereef (oceanic) zone and the reef lagoon. Highest mean medusan
abundance was observed over the forereef zone and in nighttime samples, lowest abundances occurred in the reef lagoon. A total of 16 species was identified, Aglaura hemistoma, Liriope tetraphylla, and Solmundella bitentaculata being the most abundant. Together, they comprised
up to 80% of the total numerical abundance of medusae in the sampled area. They belong to a group of medusae dominant in the oceanic and reef-related areas of the western Caribbean Sea and in the southern Gulf of Mexico. Cluster analysis revealed forereef and reef lagoon assemblages, the latter
with lowest density, diversity, and species richness. The reef lagoon showed a weak oceanic influence along and across the Banco Chinchorro reef-atoll system. Dominance of oceanic forms within the reef lagoon was attributed to the effect of the across-reef tidal currents. The community structure
of the reef-related medusan fauna appeared to be quite uniform despite the expected migratory behavior of these zooplankters, tidal exchange across the reef, input of oceanic species, and time of day. New surveys including the polyp hydroid together with the medusae are needed in Banco Chinchorro.
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