| Summary: | 碩士 === 國立中山大學 === 海洋科學系研究所 === 104 === The body size is usually influenced by ecological factors. Reversely, we could study ecology from body sizes of organisms. In this study, we found that the body size of land hermit crabs (Coenobita rugosus) at Dongsha Island, South China Sea was bigger than that in other (eco) islands. To understand the underlying mechanisms, we tested a few hypotheses : (1) shell resource (2) parasites impact (3) competition (4) predation (5) food. We did not find supporting evidence for the first four hypotheses, as shells used by Dongsha were in poorer condition than elsewhere, very few individuals had external parasites at Dongsha or Siziwan, densities were highly variable within sites, and cohort numbers are the same between sites, i.e., Dongsha crabs were not larger due to older ages. The size differences between age 1 and 2, an index of growth rates, in both 2013 and 2014, was greater at Dongsha Island than at Siziwan. The condition index of C. rugosus is also significantly greater at Dongsha Isalnd than at Siziwan. These phenomena suggests that crabs at Dongsha may eat better. There are large amount of seagrass debris accumulated at the shore of Dongsha Island but none at Siziwan, thus we evaluated the possible role of seagrasses as food item of C. rugosus at both sites. Crabs at Dongsha prefer seagrasses to dicot leaves, but no such difference was found in crabs from Siziwan where the latter was the main food source. In feeding experiment, growth increment was faster in crabs from Dongsha feeding on seagrasss than dicot leaves; no such difference was found in Siziwan crabs. The above evidence is compatible with faster growth hypothesis that contributes to the larger size of Dongsha crabs, and abundance of seagrass debris must have played a significant role at Dongsha shores.
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