Sex Allocation in the Simultaneously Hermaphroditic Polychaete Worm Ophryotrocha Diadema

Abstract

O. diadema is a simultaneously hermaphroditic polychaete worm with a brief protandrous phase. Pairs are formed preferentially between two simultaneous hermaphrodites, which reciprocally change sex roles and trade eggs. Allocation of reproductive effort during simultaneous hermaphroditic and protandrous phases was evaluated quantitatively, by microscopic examination of both fixed and living specimens. Ovarian tissue is four times as abundant as testicular tissue. The mean number of sperm counted in sectioned animals was taken as an approximation of the number of sperm offered per egg. As expected in species where the mating group always consists of two individuals, this number is low (50 sperm are offered per egg). Like eggs, sperm released at fertilization are replaced within 3 d. Such reduced allocation to testicular tissue allows saving in the physiological cost of sperm production. Efficiency of fertilization in the protandrous phase is low but positively correlated with body length (measured by the number of setigers). A fertilization ratio of 95% is obtained only at the simultaneous hermapohroditic stage. This explains why protandrous males are generally not selected as mates in the reproductive strategy of O. diadema. Self—fertilization is avoided by behavioral constraints. The observed female bias in reproductive effort in the mating system of O. diadema suggests that hermaphroditism should be evolutionarily stable, as the male investment in reproductive effort follows the law of diminishing returns.