Gnathia oxyuraea (Lilljeborg, 1855)
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. oxyuraea (males 2.4-5.4 mm, females 3.9 mm), the cephalon is wider than long and has square corners and a ridge over each eye. At the front, there is a medial tooth with semi-circular indents either side. The lateral tooth on the mandible is indistinct.
Distribution and Habitat
A common offshore species which has the potential to occur intertidally.
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.