How many times and why did (not) hermaphroditism evolve in Deuterostomia?

The Deuterostomia are large and highly diversified group that also includes vertebrates. Most species are gonochorists and only a small percentage are hermaphrodites. The predominantly hermaphroditic group is the tunicates, and among the vertebrates, hermaphrodites are known mainly in teleost fishes. In other groups, only a few hermaphroditic species are known. According to general ecological-evolutionary models, hermaphroditism arises in environments where there is a low abundance of individuals ("low density model") or when the reproductive value of one sex is favoured with larger size ("size advantage model"). Deuterostomia are an excellent model group for testing these models; teleost fishes, in particular, abounding in a variety of social and reproductive systems, are well suited for testing the "size advantage" model. The aim of this work is to determine from the available literature how many times and in which groups hermaphroditism has occurred in Deuterostomia, why it has evolved in these groups, and whether the occurrence of hermaphroditism fits general ecological-evolutionary models. Key words: hermaphroditism, deuterostomia, protandry, protogyny, simultaneous hermaphroditis