Influence of Lunar Periodicity on Medusae (Cnidaria) Composition in a Western Caribbean Reef: Community Structure Before Sargassum Blooms

The medusae of the Mahahual reef, in the Mexican Caribbean Sea, were studied to document changes in species composition and abundance over a lunar cycle in 2001–2002. Plankton was sampled during two months of the dry and rainy seasons, in the fore reef, channel, and reef lagoon. Fifty-two species were collected. The highest abundance and species richness occurred during the rainy season. Their composition and abundance were similar in the fore reef and channel, but different in the reef lagoon. Abundance and biomass changed among seasons, reef zones and lunar phases; the highest abundance and the lowest biomass were recorded during the full moon. The fore reef and channel were dominated by Liriope tetraphylla and Aglaura hemistoma, the reef lagoon by Cubaia aphrodite and Slabberia halterata. Pennaria disticha and Bougainvillia frondosa were exclusive to the new moon and Pelagia noctiluca and Aequorea macrodactyla to the full moon. The results suggest that the medusae assemblage do not change species composition during the lunar cycle of either season, and abundance increases during full moon. The oceanic influence and tide currents explain the presence of oceanic species and the similarities between localities, but they do not explain the increase in abundance during the full moon. This study was conducted prior to the arrival of Sargassum influxes in this region and can serve as a reference point for assessing its effects in recent years.