Relationships between Manicina areolata (Cnidaria : Scleractinia), Thalassia testudinum (Anthophyta) and Neogoniolithon sp (Rhodophyta)

The free-living coral Manicina areolata, the seagrass Thalassia testudinum, and the coralline algae Neogoniolithon sp, co-occur in the reef lagoon of Xahuayxol, Quintana Roo, in the Mexican Caribbean. The distribution and abu ndance of these organisms was measured. In order to study why the coral was distributed only within the medium-density stands of T. testudinum, but no t in the high- or low- density stands of the seagrass, 3 size classes of M. areolata were transplanted into 3 naturally occurring seagrass densities. Mortality, displacement, righting reaction, zooxanthellae, mitotic index, a nd chlorophyll a were evaluated for the transplanted colonies. High mortali ty of M. areolata was recorded in the low-density zone of T. testudinum, at tributable to siltation and predation, whereas in the high-density zone of this seagrass, the coral was under stress, as indicated by a lowered zooxan thellae density. The fragile, arborescent algae Neogoniolithon sp. was foun d distributed only within the medium-density zone of T. testudinum, and M. areolata selectively recruited onto that algae. Experiments in which the co ral and algae were separated in the medium-density seagrass zone demonstrat ed that Neogoniolithon sp. offers M, areolata adaptive and ecological advan tages: its survivorship was significantly higher when attached to the algae than when this substrate was not available. Because of the natural distrib ution of Neogoniolithon sp., the coral did not recruit in zones that would cause it stress, or even death. M, areolata and Neogoniolithon sp, eventual ly separate because of the increase in size (and weight) of the coral, and the fragility of the algae. Thus, the distribution of Neogoniolithon sp. ex plained the distribution of M, areolata instead of the high negative correl ation found between the density of T. testudinum and the coral.