Flexible allocation to male and female functions in hermaphrodites: a comparison among three polychaete worm species

Simultaneous hermaphrodites express both the male and the female function. They are expected to partition their reproductive resources opportunistically between their sex functions depending on environmental conditions, e.g., mating opportunities. When mating opportunities are rare, hermaphrodites are expected to allocate most of their resources to the female function and the remaining to the male function. As mating opportunities increase, hermaphrodites are expected to increase their allocation to the male function by subtracting resources from the female function. Experimental evidence for these predictions are contradictory and the question remains which factors promote or constrain the ability to adjust resources opportunistically between sex functions. We compared the ability to adjust sex allocation to mating opportunities in three closely related species of simultaneous hermaphrodites. We found that, in the polychaete worms Ophryotrocha diadema, O. adherens and O. gracilis, adults plastically changed their sex allocation depending on mating opportunities. Generally, worms allocated large amounts of resources to their female function when mating opportunities were low. In contrast, when mating opportunities were high, worms allocated proportionally less resources to the female function, in favour of the male function. However, the degrees of plasticity differed between species and between sex function within species. The differences between species in sex allocation plasticity possibly depended on the relative costs of the male and female functions in each species and on ecological factors, such as habitat conditions and the prevailing population densities in the wild.