Reproduction of Earthworms: Sexual Selection and Parthenogenesis

Abstract

Earthworms are generally cross-fertilization hermaphrodites, but up to 40% of the species can be parthenogenetic. In simultaneous hermaphrodites, a trade-off between male and female sexual functions is expected because the two sexes share limited resources from the same individual. In this chapter, several issues regarding sexual selection such as the role of spermathecae, copulatory behavior, allohormone injection, or adjustment of the donated sperm volume are reviewed. Parthenogenesis is present in some families as Lumbricidae, but is lacking in others. Parthenogenetic reproduction in earthworms is generally automictic and thelytokous, although apomixis and pseudogamy have been occasionally described. This kind of reproduction is poorly understood due to some background limitations such as the species concept in parthenogens or its possible origin, which are discussed in this chapter.