Article · Wikipedia archive · Last revised Aug 1, 2026

Chasmagnathus

Chasmagnathus convexus is a common mud-flat crab of the family Varunidae, which is endemic to East Asia. In Japan, this crab is commonly called hamagani. This crab has two forms that differ in color; one is olive green and the other is purple. Differences in diet are believed to be responsible for the color variation between the two forms. C. convexus is large, relative to related crabs, and can reach 4.5 to 5 centimetres wide across its carapace. It is predominantly nocturnal.

Last revised
Aug 1, 2026
Read time
≈ 1 min
Length
250 w
Citations
8
Source
Chasmagnathus
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Arthropoda
Clade: Pancrustacea
Class: Malacostraca
Order: Decapoda
Suborder: Pleocyemata
Infraorder: Brachyura
Family: Varunidae
Genus: Chasmagnathus
De Haan, 18331
Species:
C. convexus
Binomial name
Chasmagnathus convexus
(De Haan, 1835)1
Synonyms1
  • Ocypode (Chasmagnathus) convexus De Haan, 1833
  • Helice spinicarpa H. Milne-Edwards, 1853

Chasmagnathus convexus is a common mud-flat crab of the family Varunidae, which is endemic to East Asia.23 In Japan, this crab is commonly called hamagani. This crab has two forms that differ in color; one is olive green and the other is purple.2 Differences in diet are believed to be responsible for the color variation between the two forms. C. convexus is large, relative to related crabs, and can reach 4.5 to 5 centimetres (1.8 to 2.0 in) wide across its carapace.4 It is predominantly nocturnal.2

References

References

  1. Peter K. L. Ng; Danièle Guinot; Peter J. F. Davie (2008). "Systema Brachyurorum: Part I. An annotated checklist of extant Brachyuran crabs of the world" (PDF). Raffles Bulletin of Zoology. 17: 1–286. Archived from the original (PDF) on 2011-06-06.
  2. Hsi-Te Shih; Hiroshi Suzuki (2008). "Taxonomy, phylogeny, and biogeography of the endemic mudflat crab Helice/Chasmagnathus complex (Crustacea: Brachyura: Varunidae) from East Asia" (PDF). Zoological Studies. 47 (1): 114–125. Archived from the original (PDF) on 2017-08-09. Retrieved 2014-08-18.
  3. Katsushi Sakai (ed.). "Chasmagnathus convexus". Crabs of Japan. Retrieved January 16, 2009.
  4. Miyasaka, Hitoshi; Genkai-Kato, Motomi; Goda, Yukiko; Omori, Koji (2007). "Length-weight relationships of two varunid crab species, Helice tridens and Chasmagnathus convexus, in Japan". Limnology. 8 (1): 81–83. Bibcode:2007Limno...8...81M. doi:10.1007/s10201-006-0195-8. S2CID 23289023.