How outcrossing hermaphrodites sense the presence of conspecifics and suppress female allocation.

One of the greatest remaining problems in the field of sex allocation is the variation across species of the extent to which individuals adjust their offspring sex ratio or sex allocation according to environmental conditions. Particularly in the field of sex allocation, there is a lack of research on the mechanisms and cues involved. We address this problem in the outcrossing simultaneous hermaphroditic polychaete Ophryotrocha diadema, which shifts sex allocation by suppressing the female function in the presence of reproductive competitors or multiple partners. We document that O. diadema hermaphrodites can evaluate group size by means of a species-specific chemical released by mature individuals in water. The perception of such a water-borne signal is sufficient to trigger the observed shift in sex allocation, i.e., a suppression of the female function, irrespective of the frequency of encounters with conspecifics. Under natural conditions, such a pheromone will favor partners' encounters and the adjustment of individual sex allocation according to the numbers of reproductive competitors and/or potential partners.