In the simultaneously hermaphroditic worm Ophryotrocha diadema, eggs are laid in nesting sites i.e. widenings of mucous trails built by parents. Nesting sites where eggs are present represent highly predictable places where other conspecifics, attracted by the mucous trails, tend to congregate. Care for developing embryos is associated with regular alternation of sex functions and reciprocal egg exchange between partners of the same pair. Generally both parents tend their eggs, but eggs develop equally well if they are cared for by a genetically unrelated individual (adopter). The aim of this study was to investigate the possible evolutionary significance of the adoption of neglected eggs by unpaired worms. Results of our experiments indicate that: 1. Apparently there are no direct cues (e.g. chemical) for kin recognition: cues are only indirect, i.e. only eggs located in the nesting sites elicit parental care by any worm; and 2. In a low-density laboratory population (1 individual per 500 ml) of O. diadema, by tending eggs of whatever provenance in the nesting sites, unpaired hermaphrodites increase by 1.23 times their mating opportunities, compared with hermaphrodites not caring for eggs and seeking for a mate outside the nesting sites. In this sense, in a low-density population, non-parental egg attendance can be considered adaptive.