The objective of this thesis research was to understand how environmental factors affect the distribution pattern of Crepidula onyx by investigating the biological and ecological processes that intervene the larval and juvenile stages. Special attention has been paid to the influence of biofilms on metamorphosis and to the latent effects of the larval experience on the juvenile stage....[ Read more ]

The objective of this thesis research was to understand how environmental factors affect the distribution pattern of Crepidula onyx by investigating the biological and ecological processes that intervene the larval and juvenile stages. Special attention has been paid to the influence of biofilms on metamorphosis and to the latent effects of the larval experience on the juvenile stage.

The importance of biofilm bacterial and diatom community composition on larval metamorphosis in C. onyx was demonstrated using biofilms developed in the field. Bacterial and diatom communities were similar for biofilms developed at Victoria Harbour and Peng Chau, but different for those developed at Port Shelter. Concomitantly, biofilms from Victoria Harbour and Peng Chau induced a very similar degree of larval metamorphosis, while those from Port Shelter induced significantly fewer larvae to metamorphose. In fact, C. onyx has a stable population in the vicinity of Victoria Habour and Peng Chau, but has not been found in the more oceanic waters of Port Shelter. These results suggest that localized biofilm cues may be at work in inducing C. onyx larvae to metamorphose at Victoria Harbour and Peng Chua, and hence, affecting the distinct population pattern of C. onyx in Hong Kong. Although the biofilms developed at different locations within Hong Kong waters were different in composition, the differences in composition were not likely caused by salinity differences despite the salinity gradient from the east to the west found in Hong Kong waters. On the other hand, such differences in composition might be caused by differences in nutrient levels in the water column. Nutrient levels were effective in changing the biofilm composition in ways that alter their effectiveness in inducing C. onyx larvae to metamorphose.

Limiting food for C. onyx larvae reduced both juvenile growth and survival in the laboratory, even though the juveniles were reared at an optimal food concentration. In addition, when larvae were starved in the first or the last two days of the larval life, the juveniles developed have reduced growth. However, in contrast to the results of the laboratory experiment, limiting larval food did not hamper juvenile survival in the field, although it still reduced the mean juvenile growth rate. There are different phytoplankton concentrations and, hence, different amounts of food available to C. onyx larvae at different locations within Hong Kong waters. This thesis research has shown that larval food limitation or short-term starvation can continue to affect an individual at the juvenile stage by reducing at least its growth, if not survival. These results suggest that larvae and juveniles might experience nutritional stress in western Hong Kong waters, thereby, limiting distribution of C. onyx to Victoria Harbour.

Findings of this thesis research will not only allow us to understand the larval biology of Crepidula onyx, but also how exotic species may establish themselves in foreign habitats by thriving despite environmental constrains such as salinity and nutritional stress.