Sex Change by a Polychaete: Effects of Social and Reproductive Costs

Abstract

In the protandric polychaete Ophryotrocha puerilis puerilis, in which an individual may repeatedly change sex, both members of a reproducing pair may simultaneously change sex after a number of spawning events. I investigated whether the simultaneous sex change can be explained by sex-based differences in growth rate due to differential reproductive costs. Dry mass and energy changes in members of reproducing pairs were monitored, as well as how interactions with other individuals affected foraging, growth, injury frequency, parental care, and fecundity. Males were found to gain mass and energy more rapidly than females, due to higher costs of reproduction in females. Such costs include a trade-off between initial and future reproductive rate, and a decreasing growth rate in reproducing females. In male-female pairs confined together, the male eventually grew larger than the female, and both changed sex in the majority of cases. Larger body size may be more important for females than for males since size is positively correlated with female fecundity. Furthermore, males did not incur larger behavioral costs than females due to intraspecific interactions. When single individuals were compared to groups of consexuals, average (per-individual) feeding rate, feces pro- duction rate, and growth rate were found to be lower in groups; this effect was found in males and females equally, except that feces production in females decreased more in the presence of consexuals than it did in males, suggesting a higher social cost in females. Also, in breeding units composed of three individuals (female, female, male; male, male, female), injury frequency, growth rate, and feeding time did not differ between sexes (presence/absence of consexual compared between sexes). However, time spent nursing eggs and variance in growth rate increased, and fecundity decreased, in females together with a consexual, as compared to when alone, constituting evidence of additional female costs; no such differences were found in males. After simultaneous sex reversal and some number of clutches, some pairs became simultaneous hermaphrodites, traded eggs, and, as a result, increased reproductive output. This counteracted senescence effects, which consisted of lower growth, lower fecundity, more nonviable clutches, and a lower mass-specific energy content in older animals. Thus, the low growth rate in females, due to higher reproductive and social costs, eventually makes males more apt to function as females and vice versa, which explains the simultaneous sex change.