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

The mating system of the simultaneously hermaphroditic polychaeteworm Ophryotrocha diadema consists of a regular egg exchangebetween partners of a pair. Such reciprocal egg exchange hasbeen considered a form of cooperation, where one partner cooperatesby offering its eggs to be fertilized and expects to receivepartner's eggs to fertilize. Frequency of cases in which hermaphroditescheated (i.e., failed to give up eggs at their turn) and responsesto cheating were estimated by analyzing the behavior of 38 triplets of ovigerous hermaphrodites over a 2-week period. Thepartner did not reciprocate 8% of egg layings. The cheatedpartner did not detect most cases of cheating (16 out of 25).Such a low frequency of cheating can explain why no retaliationmechanism evolved in this species. Sixty-eight percent of the individuals from the original pairs deserted even if their partnersnever cheated them; therefore, cheating cannot be consideredthe cause of desertion. Rather, desertion appeared to be aconsequence of availability of a new partner whose oocyteswere riper than those of the old partner. It occurred becausethe opportunity arose for an immediate reward, indicating thatO. diadema egg exchange differs from that originally describedin some serranid fish as egg trading. The relationship betweencosts of desertion and population size is discussed.