This species is an internal parasite of barnacles, specifically occurring in the mantle cavity of Semibalanus balanoides (Linnaeus) and Austrominius modestus (Darwin). Being protandrous hermaphrodites, males of the species infect the barnacle, becoming female and castrating the host. The adults females reach up to 8 mm, though they can be considerably smaller than this. During their development, they lose obvious isopod characteristics, becoming sac-like, enlarged and yellow with two or three lobes on each side. The head and front pereopods remain, though in a reduced form as a small conical projection towards one end of the individual, whilst a small conical posterior region with traces of somites may also be observable.
Males and, especially, females of the species may be found in barnacle scrappings and identification should be confirmed by close examination using a binocular microscope. Arnott (2001) recommends removal of the barnacles using a scalpel immediately into a preserving solution to immobilise the free-moving males.
Distribution and Habitat
Recorded mainly on western and south-western shores of Britain, though there are also records, at a lower density, from north-east England (Arnott 2001), as well as from northern and south-east Scotland, indicating a rather wider distribution of the species. It is found especially in larger barnacles towards the lower end of the barnacle zone. Infection rates in S. balanoides of up to 90% have been recorded in south-west England (Naylor & Brandt 2015:54), though the infection rate in north-east England is considerably lower than this (0.0-13.6% for females and 0.0-9.9% for males according to Arnott 2001).
Arnott, Stephen. 2001. Infection of intertidal barnacles by the parasitic isopod Hemioniscus balani in north-east England. Journal of the Marine Biological Association of the United Kingdom 81, 171-2.
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.