Ecotoxicological studies on giant clams (Tridacnidae) : Environmental problems and future concerns

Abstract: Giant clams (Tridacnidae; Bivalvia) are described as important framework builders on coral reefs in the Indo-Pacific region. They are phototrophic as they are associated with primary producing zooxanthellae. Populations are low due to over-exploitation and most species are now endangered in many areas. Also the general degradation of coral reef habitats and marine pollution related disturbances are threatening local giant clam populations. The aim of my thesis is to provide information of physiological and ecological requirements of giant clams to improve the management of these species. By using mainly physiological methods and lethality tests, the responses to anthropogenic stress and natural disturbance were studied. In Paper I, the effect of elevated temperature was tested for in three different species of clams in order to rank the relative sensitivity. Using the end-point production to respiration ratio (Pg/R), we concluded Hippopus hippopus to be the most sensitive species. Paper II compares two different methods measuring copper stress on the photosynthetic efficiency. It was evident that fluorescence measurements and whole animal physiology gave different results. In Paper III Tridacna gigas were transplanted to sites with increased turbidity due to human activities during 6 months. Clearly, the actual growth rate decreased compared to a “clean” site. The clams from turbid waters on the other hand developed a somewhat higher tolerance to copper exposure. Tridacna gigas larvae were exposed to a combination of copper and low salinity in Paper IV, and it was shown that reduced salinity increases mortality, and synergistic effects were obvious. In Paper V and Paper VI effects of oil pollution on juveniles and larvae were studied. Juvenile clams displayed a reduction in the Energy Balance Equation due to reduced filter feeding and absorption efficiency. The larval stage had low tolerance to benzo[a]pyrene, and there was a negative trend in settlement success. Overall, the studies acknowledge harmful effects of pollutants found in tropical coastal areas. The importance of taking this into account when discussing the future of wild giant clams is indisputable.