Carriage of dwarf males by adult female puelche oysters: The role of chitons

Ostrea puelchana (d'Orbigny) is the only ostreid for which carriage of epibiotic males by adult females (> 55 mm) has been reported. On the natural grounds of San Matias Gulf (Argentina), larvae settle on the shells of adult female oysters. Spat concentrate on an expansion of the anterior margin of the concave shell (the platform), where they mature as males at about 2 mm of shell size, their growth being severely retarded as a result of interaction with the carrying female oyster. High concentration of spat on the platform relative to their density on the surface of the shell has suggested that a chemical released by the female might trigger a localized settlement response by the larvae. In this study I explore a different hypothesis, namely that the pattern results from higher survival of the spat settled on the platform, which provides protection from foraging activities of chitons. Most recruits die during their first month of life, chitons being an important cause of mortality. The chiton populations on two oyster grounds were evaluated. Results showed that in one of the oyster grounds, when chiton densities are high, the density of oyster recruits on oyster shells is low. The inverse pattern was observed in the second oyster ground, where chiton density is low. The effect of grazing by chitons on oyster recruits was experimentally assessed. The density of oyster spat on oyster shells was three times higher in the absence of grazers than when grazers were experimentally added. Results thus support the hypothesis that differential survival may contribute to the carriage phenomenon. (C) 1997 Elsevier Science B.V.