Rising costs of care make spiny chromis discerning parents

When the costs of parental care do not scale with the number of offspring being cared for, inclusion of non-descendant young into broods can be advantageous, leading to systems of alloparental care. However, if the cost of care scales with the number of offspring, selection may act against misdirected parental care. The spiny chromis, Acanthochromis polyacanthus, is a marine fish with extended biparental care, and broods that increase in size over the care period strongly suggest that alloparental care occurs in this species. However, A. polyacanthus parents directly provision their offspring by producing ectodermal mucus for their fry to feed on. The costs of such provisioning may scale with brood size, potentially increasing the costs of parental care. Using wild A. polyacanthus pairs, we tested whether foreign offspring are accepted into established broods, and measured how brood defence effort and mucal feeding scale with brood size. We found that A. polyacanthus discriminate between their own and foreign young, vigorously expelling experimentally introduced foreign offspring. Although defensive effort did not scale with brood size, mucal feeding was strongly dependent on brood size, and this increasing cost of care likely acts as the primary selective force on parental discrimination and rejection of foreign fry in A. polyacanthus.