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SynopsisCoral-reef fishes have been selected to produce propagules for dispersal because they live in a patchy environment, and the adults cannot migrate between patches. For large species ( > 100 mm SL) and widely separated patches, numerous propagules are needed, often with specialized pelagic intervals. Individuals of small species are confined to portions of the reef. They are unable to produce enough eggs for effective longrange dispersal, and so they keep their vulnerable eggs and young out of the plankton until they are well developed enough to seek out and settle onto the appropriate habitat before dispersing.Guarded demersal eggs, requiring a greater individual investment by the small short-lived species, further reduces their individual fecundity. These costs to fecundity, and the reduction in vagile young appear to account for the lack of postzygotic investment in larger longlived species.All coral-reef fishes are selected to disperse, usually with a young planktonic propagule interval. Both large and small species produce a mixed outcome, with some propagules returned to, or retained at the home reef, while others disperse more widely. The smaller the species the greater the proportion of propagules retained.

Sexual differentiation in teleost fishes is characteristically labile. The most dramatic form of sexual lability is postmaturational sex change, which is common among teleosts although rare In birds and mammals sex is determined at conception by a gene or genes residing on distinct sex chromosomes. Much research on other vertebrate taxa has been guided by the assumption that sex differentiation is initiated by similar genetic factors. An increased number of cases of environmental sex determination have been documented (1, 2), however, especially among reptiles (3-5). To date, studies of environmental sex determination in vertebrates have focused on the physical environment. In teleost fishes the social environment also plays an important role in sex differentiation (6), most notably in many sequential hermaphrodites in which size-mediated dominance interactions regulate sex change (7-9).The size-advantage model (10, 11) provides a functional explanation for this labile form of sex differentiation, based on a consideration of factors that could maximize lifetime fecundity. This model is particularly effective in accounting for postmaturational, protogynous sex change among teleosts in which a few large males can monopolize mating within a population. The model is less clear in its predictions about monogamous species. In this type of mating system, for reasons first elucidated by Fisher (12), we would expect selection for sex-determining mechanisms that would tend to generate 1:1 sex ratios. The surest mechanism would be a two-factor genetic system. However, postzygotic means of achieving this are also possible. Such a mechanism would be desirable if two conditions are met: (i) the size-fitness trajectories of the two sexes are sufficiently divergent; and (ii) size differences result from factors other than gender itself. We provide evidence for a postzygotic mechanism of sex determination in a monogamous fish, one in which social interactions play an important role.The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.The Midas cichlid, Cichlasoma citrinellum, is monogamous and biparental (13). Within a cohort of Midas cichlids, as in many teleosts, pronounced variation in growth rate is apparent from an early age. The larger fish differentiate as males, and the smaller fish differentiate as females (14). A longitudinal study demonstrated that size ranks within a group are stable from an early juvenile stage through sexual maturity (14). Moreover, size ranges of males and females reared together overlap little. Because adult sex ratios in this species approximate 1.0 both in nature (13) and in the laboratory, the sex of a Midas cichlid can be reliably predicted from its relative size as a juvenile.In a typical group of siblings, juveniles above the median size rank differentiate as males; those below the median size rank differentiate as females. The...