Partner fidelity and egg reciprocation in the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema

Abstract

The mating system of the simultaneously hermaphroditic polychaete worm Ophryotrocha diadema consists of a regular egg exchange between partners of a pair. Such reciprocal egg exchange has been considered a form of cooperation, where one partner cooperates by offering its eggs to be fertilized and expects to receive partner's eggs to fertilize. Frequency of cases in which hermaphrodites cheated (i.e., failed to give up eggs at their turn) and responses to cheating were estimated by analyzing the behavior of 38 triplets of ovigerous hermaphrodites over a 2-week period. The partner did not reciprocate 8% of egg layings. The cheated partner did not detect most cases of cheating (16 out of 25). Such a low frequency of cheating can explain why no retaliation mechanism evolved in this species. Sixty-eight percent of the individuals from the original pairs deserted even if their partners never cheated them; therefore, cheating cannot be considered the cause of desertion. Rather, desertion appeared to be a consequence of availability of a new partner whose oocytes were riper than those of the old partner. It occurred because the opportunity arose for an immediate reward, indicating that O. diadema egg exchange differs from that originally described in some serranid fish as egg trading. The relationship between costs of desertion and population size is discussed.