Variation in sex allocation plasticity in three closely related flatworm species.

Sex allocation (SA) theory for simultaneous hermaphrodites predicts an influence of group size on SA. Since group size can vary within an individual's lifetime, this can favor the evolution of phenotypically plastic SA. In an emerging comparative context, we here report on SA plasticity in three closely related flatworm species, namely , , and . For each species, we experimentally raised worms in three group sizes (isolated, pairs, and octets) and two enclosure sizes (small and large) in all factorial combinations and studied the effects of these factors on different estimates of SA. In addition, we also evaluated whether isolated worms engage in self-fertilization. We found that all species have plastic SA, with being more plastic than the other two species, as assessed by comparing standardized effect sizes of (a) the presence/absence of mating partners and (b) the strength of sexual competition. Moreover, we found that sperm production rate-but not sperm morphology-is plastic in , and that only self-fertilized during our observation period. Our study suggests that both SA and SA plasticity can diverge even between closely related species.