Eight species from seven genera of crabs from two families are reported for the first time from the coastal areas of Taiwan, including estuaries, coral reefs and sandy beaches. These are Clistocoeloma balansae A. Milne-Edwards, 1873, C. villosum (A. Milne-Edwards, 1869) and Parasesarma ungulatum (H. Milne Edwards, 1853) of the Sesarmidae; and Cyclograpsus longipes Stimpson, 1858, Otognathon uru N. K. Ng, Komai & P. K. L. Ng, 2009, Ptychognathus pilosus De Man, 1892, Pyxidognathus granulosus A. Milne-Edwards, 1879 and Scutumara laniger (Tesch, 1918) of the Varunidae.

Intertidal mudflat crabs of the genus Metaplax H. Milne Edwards, 1852 (Crustacea: Brachyura: Varunidae) from China, Taiwan, and northern Vietnam are taxonomically revised by morphological and molecular evidence. These crabs show sexual dimorphism and morphological variation of a considerable range in the infraorbital ridge, one of the primary features previously used for species identification. In this study, four species were identified from the region: M. elegans De Man, 1888; M. longipes Stimpson, 1858; M. sheni Gordon, 1930; and M. tredecim Tweedie, 1950. Based on the results of the morphological examination, and as confirmed by molecular evidence from mitochondrial cytochrome oxidase subunit I (COI), taxonomic confusion surrounding M. longipes was resolved, and M. takahasii Sakai, 1939, is considered a junior synonym of M. longipes. The geographical distribution of Metaplax longipes extends along the shores of China, north to Jiangsu, whereas the Southeast Asian M. tredecim was newly recorded from northern Vietnam and Hong Kong.

The brackish-water crabs of the genus Ptychognathus Stimpson, 1858, the most diverse genus in the family Varunidae, inhabit the estuaries or seashores influenced by freshwater. Ptychognathus sakaii, a new species from Kenting, southern Taiwan, is described in this study, with a comprehensive key to the ten species of this genus from Taiwan. This new species is similar to its congeners, but can be distinguished mainly by the features of the carapace, ambulatory legs, and male first gonopods.

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