Intertidal distribution,symbiotic association and reproduction of Pinnotheres bidentatus (Brachyura: Pinnotheridae) from Taiwan

The intertidal distribution of the free-swimming stage of the pea crab Pinnotheres bidentatus Sakai, 1939 was examined on the Kaomei coastal flat of western central Taiwan. Most pea crabs were collected from tidal waters close to the shoreline regardless of the collecting treatments utilized. Results suggest that the pea crab population remains inshore during ebb tides. Many hard and post-hard stages of P. bidentatus were discovered, including ovigerous females. The host was found to be Laternula marilina (Valenciennes, 1860) (Mollusca: Bivalvia: Laternulidae). The infestation percentage of the bivalve was high (average 74.6%). However, this may be due to collections of the bivalve population in the present study being skewed towards large sizes. This biased size structure of the host population may be also responsible for lack of positive size correlation between number and size of male and female symbiotic crabs and the host. Multiple infestations of P. bidentatus appear to be uncommon for L. marilina. Single infestations might be a model for bivalves in tropical regions where the sea is relatively oligotrophic and food competition may be enhanced with multiple infestations. In comparison to other members of the genus Pinnotheres or related species (e.g. P. pisum (Linnaeus, 1767) and Zaops ostreum (Say, 1817)), the body size of sexually mature female P. bidentatus is relatively small, even though it produces similar size eggs (approximately 300 microns). The general rule for size correlation between fecundity and female size in brachyuran crabs holds for P. bidentatus.