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

Abstract

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 abundance of these organisms was measured. In order to study why the coral was distributed only within the medium-density stands of T. testudinum, but not 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, and chLorophyll a were evaluated for the transplanted colonies. High mortality of M. areolata was recorded in the low-density zone of T. testudinum, attributable to siltation and predation, whereas in the high-density zone of this seagrass, the coral was under stress, as indicated by a lowered zooxanthellae density. The fragile, arborescent algae Neogoniolithon sp. was found distributed only within the medium-density zone of T. testudinum, and M. areolata selectively recruited onto that algae. Experiments in which the coral and algae were separated in the medium-density seagrass zone demonstrated that Neogoniolithon sp. offers M, areolata adaptive and ecological advantages: its survivorship was significantly higher when attached to the algae than when this substrate was not available. Because of the natural distribution of Neogoniolithon sp., the coral did not recruit in zones that would cause it stress, or even death. M, areolata and Neogoniolithon sp, eventually separate because of the increase in size (and weight) of the coral, and the fragility of the algae. Thus, the distribution of Neogoniolithon sp. explained the distribution of M, areolata instead of the high negative correlation found between the density of T. testudinum and the coral.