Demographic costs of sex allocation: hermaphrodites perform better in sparse populations

Ophryotrocha diadema is an outcrossing, simultaneous hermaphroditic polychaete with external fertilization. In isolated pairs, mature worms take turn contributing eggs upon the condition that their partners reciprocate egg donation. In dense populations, these worms do not reciprocate. Instead, they strongly compete for mating in their preferred male role and produce few eggs. This plastic sex allocation may result in an overall different reproductive performance: mean individual reproductive output will be larger in sparse than in dense populations. We tested this hypothesis by measuring the individual reproductive output (paternal and maternal offspring) of worms in sparse and dense replicated populations. In dense populations, mean individual reproductive output was fourfold lower than that in sparse populations. We hypothesise that such dramatic demographic costs are potentially widespread in outcrossing simultaneous hermaphrodites with external fertilization and plastic sex allocation. The reproductive output of hermaphroditic organisms is a function of population density (i.e., the number of conspecifics) and studies on population growth and reproductive performance should take this effect into account.