Effects of eutrophication on reef-building corals

Abstract

The sexual reproduction of Porites porites (Pallas), a shallow water hermatypic coral, was studied over a one-year period (June, 1982 to June, 1983) on three fringing reef complexes lying along an eutrophication gradient on the west coast of Barbados, West Indies. The data suggest that P. porites is a gonochoric species with a brooding mode of reproduction, but a low incidence (2.7%) of hermaphroditism was detected in a population sampled from a reef subjected to urban and industrial pollution. Gonadal development occurs within the mesenteries between the retractor muscles and the mesenterial filaments. Gametogenesis occurs during nine to ten months of the year, with the peak reproductive activity occurring predominantly in the fall and winter (November to January). Gametogenesis was therefore loosely synchronized between colonies; however, gonads in all stages of development were present within colonies throughout the reproductive season. The reproductive season of two P. porites populations sampled from two polluted reefs began one to two months earlier than that of a P. porites population sampled from a less polluted reef. The simultaneous presence of ova and larvae within a colony between November and April suggests that larvae may be released repeatedly during an extended breeding season. No correlation was found between the average number of gonads and polyp size. However, the gonad index (average number of gonads based on the sum of male and female gonads) showed an inverse relationship with a number of environmental variables. It is suggested that zooxanthellae in the maturing ova may play an important role in the reproductive success of P. porites. The reduction of zooxanthellae photosynthesis through reduced light levels may significantly lower the energy available from photosynthates to the maturing ova and/or embryos, thus depressing larval development and maturation. Coral colonies sampled from two polluted reefs contained lower numbers of larvae than colonies sampled from a less polluted reef. The 2: 1 sex ratio observed in a P. porites population sampled from a polluted reef may result from rapid asexual reproduction (fragmentation), indicating that the mode of reproduction may be influenced by environmental conditions.