WHY ARE LARGER CONVICT CICHLID (CICHLASOMA-NIGROFASCIATUM) FRY SOMETIMES ADAPTED INTO BROODS OF SMALLER FRY

Convict cichlids (Cichlasoma nigrofasciatum) are substrate-brooding fi sh, native to Central America, with extended biparental care of young. Parents in the field and laboratory readily adopt foreign young of si milar size to or smaller than their own, but usually reject larger for eign young. Under certain circumstances, the adoption of unrelated you ng has been shown to be adaptive to both donating and foster parents t hrough (1) the dilution effect: the probability that their own young w ill be captured by a predator is reduced as the brood size increases; and (2) the differential predation effect: smaller young are more easi ly caught by brood predators than are larger young. These effects are not mutually exclusive. It has been demonstrated that parents of young with a standard length (SL) of 8 mm or bigger reject larger foreign y oung less often than do parents guarding smaller young, suggesting tha t the differential predation effect, and interfry aggression, may only be at work at smaller young sizes. In this study relative vulnerabili ty of young to small gape predators (sub-adult conspecifics) decreased with increasing size of young (Fig. 1). When young were most vulnerab le to predators (less than 8 mm SL) there was little or no benefit fro m dilution with larger young. However, as the young grew, the benefit of dilution with larger fry increased (Fig. 2).