Sex economy in benthic polychaetes

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Abstract
Among polychaetes sequential hermaphrodites are generally protandrous and their sex allocation strategy appears to fit well with the Ghiselin size advantage hypothesis. In Ophryotrocha puerilis male reproductive success drops with increasing size because females prefer to mate with small males to avoid a costly conflict over sex. Moreover both partners of a pair can simultaneously change sex several times after some spawnings. Females being soon drained of resources, due to the higher reproductive and social costs of their role, switch to the less costly male role and males thus become more apt to function as females. Also in the simultaneously hermaphroditic species so far studied, O. diadema and O. gracilis, the male profit-effort function for reproductive success shows a decreasing slope. Some 80% of the physiological reproductive effort is allocated to the female functions. Simultaneous hermaphroditism is stabilized by slow sperm production and regular egg trading between partners of a pair. Local mate competition (LMC) has probably been important in the evolution of facultative change of gender in many polychaetes. In the population of the labile gonochoric Capitella sp. I males which are unable to gain access to females may develop into hermaphrodites and function as females, thereby offsetting the loss of mating success as males. The LMC model could explain also the female biased sex ratios found in several polychaete populations. Due to the slow mobility of adults, many populations are subdivided into very small breeding groups or show sib mating (e.g., in Dinophilus gyrociliatus daughters are inseminated by their brothers and mothers can control the sex of their offspring). Finally, conditions that lead to the evolution of biparental, uniparental or paternal care of embryos in polychaetes that do not spawn freely into the sea are discussed.