Monobrachium parasitum (Cnidaria: Hydrozoa) epizoic on antarctic bivalves and its bipolarity

During antarctic macrozoobenthos research, colonies of Monobrachium were found on small bivalves. The species problem of the genus Monobrachium is discussed and it is stated that there are three species. However, the widespread species Monobrachium parasitum might be a species-complex comprising several species. This can only be confirmed by rearing experiments and/or molecular investigations. Host specificity and the factors influencing the infection of the bivalve species are shown. The impact of the infection is demonstrated to be harmful to the host bivalve, although not of a parasitic nature. The phenomenon of bipolarity within the genus Monobrachium is discussed.