Intertidal and Land Crabs survey of Dongsha Island

Project Duration	2011-12-31 ~ 2012-12-30
Research Title	Intertidal and Land Crabs survey of Dongsha Island
Contracted Organization	National Chung Hsing University
Principal Investigator	Shih Hsi-Te
Project Participants	Chen Min-Wan, Kuo Kuan-Chun, Hsu Pei-Yi, Ou Tzu-Hui, Li Jung-Hsiang, Li Wei-Chin, Hung Ho-Tien, Wang Yu-Hsi, Lin Hsiu-Yu
Keywords	Dongsha Island, hermit crabs, Cardisoma carnifex
Abstract	Since September 2011, surveys have recorded 59 crab species from 17 families on Dongsha Island, with the family Diogenidae (hermit crabs) being the most diverse (13 species). Species composition varied among habitats. Seagrass beds were dominated by hermit crabs; rocky reef areas supported hermit crabs along with xanthid, portunid, and grapsid crabs; and areas near the small lagoon were dominated by ocypodid crabs. Density estimates showed that ghost crab burrow density peaked in May, particularly along the southern shore. Among grapsid crabs, Grapsus albolineatus had higher densities than Metopograpsus thukuhar. Fiddler crab burrow density also peaked in May. Six biodiversity hotspots were identified: two estuary areas, the coast near Site 2, the western and eastern sides of the small lagoon, and the central rocky shore of the southern coast. The population structure of the giant land crab (Cardisoma carnifex) indicates continuous recruitment of juveniles and a stable population. Individuals generally remain near their burrows, with no evidence of long-distance movement except during reproductive migration. Their diet mainly consists of large plants, including Pandanus trees, which provide abundant food resources. Fecundity ranges from approximately 90,000 to 180,000 eggs, increasing with body size. Larval release typically occurs around spring tides at sandy shorelines along the island perimeter, including the northern estuary, Site 1, and near the national monument. Among terrestrial hermit crabs, Coenobita rugosus represents the largest population and frequently utilizes empty shells of the invasive African giant snail. The minimum size at sexual maturity is 9.4 mm carapace length. Fecundity ranges from 500 to 15,000 eggs, with larval release also occurring around spring tides, most frequently observed on beaches near the national monument. Five species of fiddler crabs were recorded on Dongsha Island: Uca crassipes, Uca tetragonon, Uca jocelynae, Uca perplexa, and Uca splendida. Uca jocelynae was the most abundant, while Uca splendida was the least abundant. All five species were observed at Site 4. Their diet primarily consists of sandy sediments and plant detritus, with minor proportions of algae (including green algae, colonial algae, filamentous algae, and diatoms). As deposit feeders, there was no significant difference in dietary composition among species. Sediment grain size analysis showed higher proportions of coarse sand at Site 1 compared to Sites 2 and 4. Fecundity among three species (Uca crassipes, Uca jocelynae, and Uca tetragonon) ranged from approximately 4,000 to 18,000 eggs. In terms of conservation, the mangroves in the small lagoon are non-native and should be considered for management or control. Driftwood along the coast should be retained where possible, as it provides diverse habitats for crabs and other benthic organisms. Several crab species found on Dongsha are endemic to the South China Sea. The increasing number of recorded species highlights Dongsha Island as an important biogeographic node for marine species dispersal between Taiwan and the South China Sea. Continued long-term monitoring, combined with genetic studies and seasonal current analysis, would further clarify the origins and dispersal pathways of coastal organisms around Dongsha Island.