Reciprocal egg trading and brood care in a hermaphroditic polychaete worm

Ophryotrocha diadema is a simultaneous hermaphrodite polychaete worm with a brief protandrous phase. Its mating behaviour was investigated in order to elucidate the relationships between mating system and reproductive biology. A genetically determined yellow or white coloration of the eggs and body walls made it possible to distinguish the egg releaser from the fertilizer. The following main features of the mating system were established. (1) Pairs are formed preferentially between simultaneous hermaphrodites, one partner releasing eggs and the other fertilizing them. There is no selfing. (2) The partners attain spawning synchronization by means of close mutual contact during a fairly time-consuming courtship. (3) Partners regularly alternate sex roles, usually with the same partner more than once in succession. (4) Both partners care for their eggs and protect cocoons of neglected eggs spawned by other pairs.