The genus Ptychognathus comprises 32 species of brackish-water Crabs found in estuaries and the lower parts of river systems across the Indo-Western Pacific region. They are known as Pom Pom Crabs in the aquarium industry because of the clusters of fine setae (hairs) on their claws (although they are widely traded as freshwater Crabs rather than brackish-water Crabs). Most species of Ptychognathus have very localised distributions, being found on a single island, estuary, or river system, although one species, Ptychognathus barbatus, is found over an extremely wide area, including Japan, Taiwan, China, the Philippines, Malaysia, Indonesia, and New Caledonia. However, as part of a PhD thesis completed in 2006, carcinologist Ngan Kee Ng, then a graduate student at the National University of Singapore, examined the systematics of Ptychognathus barbatus, concluding that the populations described under this name represented two, rather than a single, species. Ng went on the lead a highly successful research group, specialising in the study of Crabs, for many years, before passing away in 2022, but never formerly published her PhD thesis. This means that all taxonomic nomenclature presented in the thesis is considered unusable under the terms of the International Code of Zoological Nomenclature, even if specialists in the field believe it to be generally correct.
In a paper published in the journal ZooKeys on 27 June 2025, Jhih-Wei Hsu of the Department of Life Science at the National Chung Hsing University, Jose Christopher Mendoza, of the Lee Kong Chian Natural History Museum at the National University of Singapore, and Hsi-Te Shih, also of the Department of Life Science, and of the Global Change Biology Research Center at the National Chung Hsing University, build upon Ngan Kee Ng's work, to formally divide Ptychognathus barbatus into two, and describe a new species.
The new species is named Ptychognathus dajie, where 'dajie' means 'elder sister', a title often used for women in leadership roles in Chinese-speaking countries, in honour of Ngan Kee Ng. A genetic analysis of museum specimens suggests that this species is found in estuaries and tidally-influenced portions of rivers, in Malaysia, Japan, Taiwan, China, Philippines, Indonesia, and Thailand. Surprisingly, Ptychognathus dajie is not particularly closely-related to Ptychognathus barbatus, instead forming a sister species to Ptychognathus guijulugani, a species found on Negros and Mindanao islands in the Philippines, placing it close to the base of the Ptychognathus family tree.
Specimens of Ptychognathus dajie have almost square carapaces, slightly wider than they are long, with a glossy upper surface and a concave frontal margin. The lower part of the claw is covered by long, thin setae; claws are larger in males than in females. The largest male specimen found was 20.2 mm wide and 16.8 mm long, the largest female found was 16.8 mm wide and 14.5 mm long. Colour is extremeley variable, and tends to match the substrate upon which the Crabs live.
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