testudinaria consist of hermaphrodites, which attach directly to their host, and dwarf males (
complemental males sensu Darwin, 1854), which attach to hermaphrodites (Grisp, 1983; Zardus and Hadfield, 2004; Cheang et al., 2013; Zardus et al., 2014).
Darwin and his successors (e.g., Annandale, 1905; Broch, 1922; Batham, 1945) believed that the primary function of thoracican dwarf males was to complement the reproduction of hermaphrodites at low densities, and they often called such males "
complemental males." More recent theoretical studies (Charnov, 1982, 1987; Crisp, 1983; Yamaguchi et al., 2007, 2008; Urano et al., 2009) have stressed the role of reduced sperm competition in the evolution of dwarf males in barnacles.