C. hanseni is a parasite of Idotea marine isopods, including I. chelipes, I. emarginata, I. granulosa, I. linearis and I. pelagica. Like other members of the Cabiropidae family, it is a protandrous hermaphrodite. The small male cryptoniscus stage (about 0.5 mm in length) infects the host, becoming female, developing internally and losing obvious isopod characteristics. This male cryptoniscus larva is the only form of this isopod that is easily visible or identifiable. It can be distinguished from the cryptoniscus larva of C. cantacuzenei by its association with its hosts and by the presence of three (rather than two) teeth on its coxal plates. It is distinguished from C. meinerti (not so far recorded in Britain or Ireland but parasitising the same hosts) by being robust rather than slender, and by the basal segment of the antennule being short and rounded. Note that this species is not included in Naylor & Brandt (2015).
Distribution and Habitat
There are very few records of this species from Britain or Ireland, though it appears to be widespread. It has been recorded in the Moray Firth, the Firth of Clyde and the Firth of Forth (Scott 1895), in Idotea pelagica at Bournemouth (Cooper 1917), and has been found to be a parasite of Idotea balthica, I. emarginata and I. pelagica at Holy Island and Cullercoats in north-east England (Sheader 1977), and is likely the Clypeoniscus species found on Idotea granulosa on the southern shores of the Isle of Man (Salemaa 1986). The species has also recently (2021) been recorded (parasitising Idotea balthica) on the Berwickshire coast in south-east Scotland.
Jenkins, A. & R. Griffin. 2020. First record of the isopod parasite Clypeoniscus cantacuzenei (Crustacea: Isopoda) in the UK and the introduction of a key to the cryptoniscus stage of genus Clypeoniscus of UK Waters. Cahiers de Biologie Marine 61, 501-5.
Nielsen, S. 1969. Investigations on the genus Clypeoniscus (Crustacea Epicaridea) with notes on host-parasite relations and distribution. Sarsia 38, 31-70.
Salemaa, H. 1986. Breeding biology and microhabitat utilization if the intertidal isopod Idotea granulosa Rathke, in the Irish Sea. Estuarine, Coastal and Shelf Science 22, 335-55.
Sheader, M. 1977. The breeding biology of Idotea pelagica (Isopoda: Valvifera) with notes on the occurrence and biology of its parasite Clypeoniscus hanseni (Isopoda: Epicaridea). Journal of the Marine Biological Association UK 57, 659-74.
Scott T. 1895. Additions to the fauna of the Firth of Forth. Part VIII. Fourteenth Annual Report of the Fishery Board for Scotland, pp 158-166.