EGG TRADING IN THE SIMULTANEOUSLY HERMAPHRODITIC POLYCHAETE WORM OPHRYOTROCHA-GRACILIS (HUTH)

Ophryotrocha gracilis (Huth 1934) is a simultaneously hermaphroditic p olychaete worm with external fertilization and a brief protandrous pha se. 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; s ex roles are sequentially alternated and self-fertilization is avoided . Egg reciprocation is kept evolutionarily stable by laying eggs in mu ltiple, small-sized egg clutches and reducing the reproductive success of pairs in which one of the two partners does not reciprocate egg ex change. The frequency of ovigerous hermaphrodites in mass cultures of O. gracilis is about 50%. Such a high mate availability preadapts herm aphrodites of O. gracilis to change partners very frequently and to re duce investment in parental care, contrary to what is observed in anot her egg-trading, simultaneously hermaphroditic species, O. diadema. La boratory populations of O. diadema have a frequency of only 17% oviger ous hermaphrodites.