Gnathia dentata (Sars, 1872)



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. dentata (males 2.8-3.8 mm, females up to 4.3 mm), the cephalon, which is wider than long, has a tridentate process at the front, and the lateral tooth on the mandible projects outwards.

Distribution and Habitat

Mostly an offshore species. G. dentata has occasionally been recorded intertidally in southern Britain and in the Irish Sea in Laminaria holdfasts.


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