Ctenophores are an ideal system from which to study the evolution of dioecy. These planktonic marine organisms are distinguished by their eight rows of cilia that run along their body (Fig. 2). All ctenophores except for those from the genus Ocyropsis are simultaneous hermaphrodites with eggs and sperm produced on opposite sides of the meridional canals, which run under these comb rows. Most ctenophores have the ability to self-fertilize; however, the degree to which self-fertilization occurs differs across ctenophores, suggesting that species have evolved a range of barriers to prevent self-fertilization. Such barriers may allow hermaphroditism to persist in these systems by avoiding or lessening the costs of self-fertilization. Ctenophores are also an ideal system in which to investigate dioecy from an evolutionary perspective. As the sister group to the rest of animals ctenophores occupy a pivotal position in the animal tree of life, which will allow us to make inferences about the reproductive ecology and genetics of monoecy in early animal evolution.