DISTRIBUTION OF ZOOXANTHELLAE AND PHOTOSYNTHETIC CHLOROPLAST PIGMENTS OF THE REEF-BUILDING CORAL MONTASTREA ANNULARIS ELLIS AND SOLANDER IN RELATION TO DEPTH ON A WEST INDIAN CORAL REEF

Studies on the distribution and photosynthetic chloroplast pigments of symbiotic zooxanthellae in the reef-building coral MOil/as/rea allllularis Ellis and Solander strongly suggest that photoadaptation to decreasing light intensity occurs within a population inhabiting the fore-reef of a West Indian (Jamaican) coral reef. It is suggested that the photoadaptation allows for the extension of the depth range of the species. The responses of M. allllularis and its zooxanthellae to transplantation suggest that colonies have a certain capacity for modification when placed at different depths. The magnitude of these potential changes is small and colonies do not fare well in the transplant habitats. Such sub-optimal conditions are reflected in a decrease in algal density! Col' living coral tissue, a decrease in zooxanthellar intracellular photosynthetic pigment concentration, and significant decreases in coral skeletal extension rates. Responses such as these are reasonably clear data in support of ecotypic variation and suggest that there are sun and shade populations of zooxanthellae in M. allllularis. Hermatypic scleractinian corals are responsible for the construction of modern reefs. They provide the framework and a good deal of the infilling sediments that form the basic wave-resistant structure known as a coral reef. It is believed that the remarkable ecological role played by hermatypic corals is due to the evolution of the symbiotic relationship these coelenterates have with dinoflagellates, commonly referred to as zooxanthellae (Yonge, 1973). The processes of calcification and zooxanthcllar photosynthesis have been linked to cnablc corals to deposit skeletal calcium carbonatc fastcr than environmental erosion (Goreau and Goreau, 1973). The net resuit is the construction of a geological structure at a rate so fast that it can be measured within thc lifetime of a human (Land, 1974; Gorcau and Land, 1974). The ecological importance of zooxanthcllac is demonstrated by the observation that thc lower depth distribution of hermatypic corals is above the lower limit of the photic zone, suggesting that light is of primary importance for the growth and 79 maintenance of hermatypic coral colonies (Wells, 1957). Reef-building corals primarily live between the ocean's surface and 100 m and are, therefore, subject to a photic environment in which there is considerable variation in both intensity and spectral quality (Jerlov, 1970) . While individual "adult" colonies are sessile throughout their lives, members of the same species may inhabit depth ranges as great as 95 m (Goreau and Wells, 1967) suggesting that the zooxanthellae within the same species of coral are able to photosynthesize over a tremendous range of light intensities and spectral qualities. Corals that enjoy large vertical depth distributions exhibit the general, though not universal, trend of skeletal flattening with increasing depth and/or in shaded habitats (Kawaguti, 1937; Goreau, 1963). The limits of this general trend are a sphere and flat plate (Barnes, 1973). This change in skeletal morphology seems to be the result of a decreasing calcium carbonate deposition rate as a direct consequence of decrcasing zooxanthellar photosynthetic activity (Gor80 BULLETIN OF MARINE SCIENCE, VOL 29, NO. I, 1979 Figure 1. Jepth distribution of mean colony size of M. alllwiaris on Dancing Lady Reef, Jamaica. Each point represents data from a single line transect run parallel to the reef flat across Dancing Lady Reef. Transect length varied between 23 and 93 m corresponding to the width of the reef. See Dustan (1975a) for details of methodology. reef is part of the vast north coast fringing reef system first described in detail by Goreau 11959b), and later by Kinzic (1973) a ld Goreau and Goreau (1973). It runs al[1ost the whole length of the northern coast of Jamaica with few gaps except for harbOl's and river mouths. Montastrea annularis iVaschosen for the study as it is abundant wer a wide depth range-greater than 50 ill (Goreau and Wells, 1967; Dustan, 1975a) and is a primary frameworkbuilding oral on most West Indian reefs (Goreau, 959b; Goreau and Land, 1974). On DanciIlg Lady Reef M. annularis has a bimodal depth distribution with peaks in abundance at 10 and 30 m (Dustan, 1975a). The specie, exhibits changes in growth form that are correlated with its habitat (Gareau, 1959a; Milliman, 1969; MacIntyre and Smith, 1973; Barnes, 1973). The 10-m peak is comprised mostly of round colonies. At 30 m and below most of the colonies assume a flattened form (Barnes, 1973; Dustan, 1~'75a; 1975b). The distribution of mean colony size parallels abundance I