Chasmagnathus convexus is a common mud-flat crab of the family Varunidae, which is endemic to East Asia.

[2][3] 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.