THE BIOLOGY AND LIFE-CYCLE OF THE RHIZOCEPHALA (CIRRIPEDIA)

This review describes aspects of the Life cycle and ecology of the Rhizocephala emphasizing (i) comparison with more conventional Cirripedia, and (ii) evolutionary and phylogenetic perspectives. Despite numerous extreme specializations to living as parasites in other Crustacea, most features of the rhizocephalan life cycle resemble those seen in other Cirripedia, and only the process of host invasion and the ensuing redifferentiation of the adult parasite represent truly unique features. Larval biology of the Rhizocephala has developed under selection pressures induced by the difficulty of locating the substratum for settlement, host defences against parasite invasion, and the special demands imposed on the sexual system of a parasite. Thus rhizocephalans have (i) lecithotrophy coupled with very small-sized larvae, (ii) special sensory organs in the cyprid, (iii) a very rapid host invasion accomplished by minute, female stages, and (iv) dwarf males nourished by the adult female parasite, such that it emulates a true hermaphrodite. This review also surveys the nature and underlying causes of the numerous effects that rhizocephalan parasites can induce on their hosts in terms of morphology, physiology, and behaviour. The 'host control' induced by rhizocephalans ensures that both male and female hosts accept the parasite as their own brood and care for it accordingly. This, and other details of host-parasite co-evolutions are discussed.