Hemiarthrus abdominalis (Krøyer, 1840)

H. abdominalis is a parasite of various prawns and shrimps in genera such as Eualus, Hippolyte, Pandalus and Spirontocaris. It is typically found enclosed between the plates of the first two segments of its host. The female is a large, irregular mass of oostegites twisted to one side, the legs of the inner side remaining but most of those of the outer side being absent. Its head is withdrawn under the oostegites into the first pereon segment. The male has a typical isopod shape, the unsegmented pleotelson having a broadly triangular shape. Female up to 11 mm, male to 3 mm.

H. abdominalis is not included in Naylor & Brandt (2015), probably because its hosts are primarily sublittoral. A detailed description of all life-stages of this species (designated as Phryxus abdominalis) with copious illustrations is given in Sars (1899).