Full compensation of parental care after partner loss in an annelid

In species with biparental care, each parent should theoretically benefit by exploiting its partner and reduce its investment in offspring care. The conflict is especially apparent when one parent deserts or reduces its investment, forcing the remaining parent to either abandon the brood or compensate for the loss of partner-care effort. However, compensation is expected to be partial for the evolutionary stability of biparental care. We investigated the level of care compensation in the simultaneously hermaphroditic annelid worm, Ophryotrocha diadema, where both parents care for egg clutches. We removed one parent and found that widowed parents fully compensated for the loss of their partner as the number of scan observations widows spent in clutch attendance was statistically equivalent to that of paired parents. Consistently, individually, widowed parents attended their clutch significantly more often than focal worms among paired parents. We discuss how full compensation can be compatible with the evolutionary stability of biparental care even in a system where the cost and benefit functions are theoretically the same for both parents, as most models assume. This species is rare, has sparse populations and its adaptations suggest that finding a partner is challenging. In this condition, nests might function as meeting points and nest attendance might promote mate encounters, thus mitigating the conflict of interests over parental care.