Egg trading in the simultaneously hermaphroditic polychaete worm Ophryotrocha gracilis (Huth)

Abstract

Ophryotrocha gracilis (Huth 1934) is a simultaneously hermaphroditic polychaete worm with external fertilization and a brief protandrous phase. The mating system of this species seems to meet conditions leading to the establishment of egg-trading behavior. Experiments showed that mating occurs in pairs composed of two simultaneous hermaphrodites; sex roles are sequentially alternated and self-fertilization is avoided. Egg reciprocation is kept evolutionarily stable by laying eggs in multiple, small-sized egg clutches and reducing the reproductive success of pairs in which one of the two partners does not reciprocate egg exchange. The frequency of ovigerous hermaphrodites in mass cultures of O. gracilis is about 50%. Such a high mate availability preadapts hermaphrodites of O. gracilis to change partners very frequently and to reduce investment in parental care, contrary to what is observed in another egg-trading, simultaneously hermaphroditic species, O. diadema. Laboratory populations of O. diadema have a frequency of only 17% ovigerous hermaphrodites.