Skeletal extension, density and calcification rates of massive free-living coral Porites lobata Dana, 1846

Abstract

Hermatypic corals are an important sessile group in the benthic structure of tropical coral reef communities. Many organisms, in response to the environmental conditions, have developed different strategies of growth by modifying their morphologies and reproduction mode, as is the case of free-living colonies called coralliths. So far, important parameters such as calcification, growth rates and their relation with biotic and abiotic factors on coralliths have not been described. This study represents the first record of a corallith form of Porites lobata and provides information on its sclerochronology. Coral growth parameters of different coralliths were compared by measuring extension rate (cmy−1), skeletal density (gcm−3), calcification rate (gcm−2y−1), average age (yr) and sphericity (S). The influence of the environment on corallith growth was assessed using water temperature as an abiotic factor and bioturbation by fish as a biotic factor. Analysis of annual density bands using X-ray densitometry provided a mean extension rate of 0.47±0.23cmy−1, skeletal density of 1.08±0.14gcm−3, and calcification rate of 0.51±0.26gcm−2y−1. The results reveal differences in growth parameters between coralliths including a strong relationship of calcification rate with seawater temperature. In addition, direct and indirect bioturbations promoted the colony rotation resulting in a hemispherical form. Hence, the evidence suggests that scleractinian corals have developed an important growth strategy that allows the species to form new colonies and maintain successful coral reef communities through free-living corallith growth.