Androdioecy (males and hermaphrodites) is a rare breeding system in multicellular organisms, found mostly in barnacles and branchiopod crustaceans. The most speciose and longest-lived androdioecious clade is the genus Eulimnadia Packard, 1874 (Branchiopoda, Spinicaudata), the clam shrimps, consisting of over 50 species that have maintained androdioecy for an estimated 24–180 million years. Many populations of Eulimnadia nevertheless comprise entirely “monogenic” hermaphrodites. Hypotheses proposed to explain the relative stability of androdioecy (sexual conflict, overdominance, and metapopulation model) differ in their predictions of the resistance of existing all-hermaphrodite populations to invasion of males and hermaphrodites. We tested whether all-hermaphroditic populations of Eulimnadia texana Packard, 1871 may be resistant to male invasion by adding males and “amphigenic” hermaphrodites to all-hermaphrodite, monogenic populations that have been inbred for eight generations. All-hermaphrodite populations of E. texana that have been selfing for multiple generations are

easily invaded by males, both directly and indirectly. The addition of males also increased the productivity of these experimental treatments, suggesting a selective benefit to outcrossing and thus to males. These results do not align with the sexual conflict nor the overdominance models, but are consistent with the metapopulation model of the maintenance of androdioecy.