Population Variability and Impact of Sea Urchin Anthocidaris crassispina in Hong Kong

The ecology of Anthocidaris crassispina in local waters was studied through determining how food and hyposalinity stressors may regulate the abundance and distribution of this species in a gradient from estuarine water and oceanic water, and elucidating the morphometric variation and reproductive parameters this sea urchin to such food stressors; comparing the population structure, growth and mortality of this sea urchin protected and non-protected shores, and determining whether this sea urchin can exert important top-down control of macroalgal development on rocky shores, and the importance of winter surge in macroalgal recruitment and growth on the fitness of this sea urchin.;Anthocidaris crassispina exhibited morphological responses and resources allocation in response to food and salinity stressors. This animal is able to withstand adverse environmental hyposalinity and food limitation stresses which may experience in the oceanic zone and transition zone in Hong Kong waters. With the food deprivation condition, this species is possibly too strong to allow the location of resource allocation between growth and reproduction. In contrast to a lack of response of the test diameter to food limitation, A. crassispina responded to hyposalinity conditions by increasing the index of feeding apparatus. This species displayed an unusual strategy to deal with food limitation and hyposalinity and able to show plastic morphological and physiological responses when facing the challenge of varying environmental factors. Such stressors, however, would reduce the fitness of the species as they dramatically reduced the gonad masses.;Mark and recapture study of the growth and survival rate of A. crassispina provided critical information for the assessment of fishing pressure for this commercially exploited species in Hong Kong. The populations of this sea urchin in areas open to fishing have very lower densities and higher mortality, and the populations consisted of a higher proportion of younger sea urchins than the protected population. The population of A. crassispina, however, in the marine reserve had a slower growth rate than the fished populations, indicating high density of the urchin in the reserve might have reduced resource availability for each individual. Notwithstanding, all individuals of A. crassispina at all study areas grew relatively quickly in the first and second years, reaching exceeding the mature size. This indicated that, high urchin density in reserve, the sea urchin can reproduce within relatively short period of time, and contribute to the sustainability of reserve and non-reserve populations by external fertilization and larval development. This study has demonstrated the effectiveness of the marine reserve in protecting A. crassispina from fishing. Although sea urchins are sedentary animals with limited mobility, their relatively high survivorship in the reserve let them reach high densities.;Caged experiment conducted in the field demonstrated that, at high density (20 indiv. m-2), A. crassispina grazing can dramatically reduce algal biomass in algal stands in one month, and a longer grazing period may lead to algal barrens. With low densities outside the reserve (< 3 indiv. m-2), however, A. crassispina can exert very little grazing pressure on macroalgae. The seasonally abundant macroalgae in winter elicits rapid somatic and gonadal growth in the sea urchin. Consequently, differences in accessing to the pulse of macroalgal regeneration and growth may explain the variability in growth and reproduction of A. crassispina in local waters.;Nevertheless, this study strongly provided the examples of how ecological studies can be used to help us evaluate the effectiveness of small marine protected areas in protecting a commercially exploited sessile species. Furthermore, the result of this thesis has contributed to a better understanding of the ecological role of this sea urchin, and the importance of sustainable use and maintaining the healthy populations of this species in local ecosystems.