Gnathia vorax (Lucas, 1849)



ID Difficulty


Gnathia are small (2-7 mm) marine isopods most typically found living benthically offshore. Unlike other isopods, they only have five pairs of legs. They also exhibit marked sexual dimorphism, with males having large heads and mandibles and females being inflated, with fused pereon somites 3-5 and reduced mouthparts. The young (praniza larvae) have a similar shape to the females and live as external parasites on fish in estuaries. Identification is based on characteristics of the adult male cephalon and mandibles.

In G. vorax (5.0-7.0 mm), the cephalon has a deep depression at the front with a pointed tooth at the centre. The lateral tooth on the mandible is closely appressed.

Distribution and Habitat

This species is usually found offshore around the British Isles.


Naylor, E. & A. Brandt. 2015. Intertidal Marine Isopods. Synopses of the British Fauna (New Series), No. 3. Field Studies Council, for The Linnean Society of London.

BRC code