Ancyroniscus bonnieri Caullery & Mesnil, 1919
This protandrous hermaphrodite species is an internal parasite of females of the marine isopod Dynaeme bidentata. A free-swimming male attaches to the gut before developing into a female, which grows in the visceral cavity and brood chamber of the host, in most cases castrating it.
The male (cryptoniscus stage) grows up to 0.7 mm in length, and several can be found in the visceral cavities of both male and female D. bidentata. They retain typical isopod characteristics (antennae and antennules, somites, pereopods and uropods). Females lose obvious isopod characteristics other than a few somites, becoming lobed and sac-like.
Identification is relatively straight-forward given the host-specific nature of the species, though examination using a binocular microscope is necessary for confirmation, extraction from the host, and detection of males.
Holdich, David. 1975. Ancyroniscus bonnieri (Isopoda, Epicaridea) infecting British populations of Dynamene bidentata (Isopoda, Sphaeromatidae). Crustaceana 28(2), 145-151.
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.