Competing models of community structure in assemblages of coral reef fishes have suggested that (1) these assemblages are structured by deterministic interactions between species, or between species and resources, or (2) the composition of these assemblages are determined by highly variable settlement from planktonic larvae. We examined interactions among newly recruited juvenile fishes and between juvenile fishes and transplanted resident damslfish on artificial reefs in St. Croix, United States Virgin Island. Two kinds of priority effects occurred: (1) recruitment of three species of settling juveniles significantly decreased in the presence of the territorial damselfish, and (2) prior settlement of a juvenile predator lowered successful recruitment of two juvenile prey species. The first effect increases determinism in the structure of coral reef fish assemblages, while the second decreases their predictability.

Observations were made on the fishes trespassing in 50 territories of the reefpomacentrid Eupomacentrus leucostictus, and on the responses of the territory-holders to the trespassers. The extent to which each of the trespasser species elicited agonistic responses from territorial E. leucostictus individuals was quantified: number agonistic acts/total trespass time. Using data on feeding habits, the diet overlap of each trespassing species on E. leucostictus was computed according to the formula a = ""i.P;I!Pihf'i.P;h 2 • Following a log-log transformation to correct for heteroscedasticity, a Pearson product-moment correlation between number agonistic acts/total trespass time revealed a significant positive correlation between these 2 variables. An expression for Potential Competitive Impact that combines trespasser size with diet overlap, a(V 8 )· 15 , showed a highly significant correlation with number agonistic acts/total trespass time (again using a log-log transformation) and a higher correlation coefficient (r = .613, P < .01). When this correlation was recalculated with the exclusion of the 9 trespassing species known to eat eggs, the correlation coefficient improved tor = .744, P < .01. These 9 egg-eating species have a significantly greater tendency to elicit agonism beyond that expected on the basis of their Potential Competitive Impact than do the other trespassing species.(t = 2.940, p < .02).This pattern of agonism indicates that interspecific territoriality is an adaptive behavior that reduces the loss of food in the territory to competitors and protects the nest from egg predators.

Sex determination in fishes is often enigmatic, a situation that is often made even more complex by the fact that the process of sexual differentiation in many species may be influenced by environmental conditions. This situation is typified in zebrafish, a popular model organism. Despite the vast array of information available for the species, the genetic controls of sex are unknown. Further, environmental parameters, such as rearing densities, seem to exert an influence on the sex ratios of captive stocks. In an effort to dissect the genetic and environmental controls underlying the expression of sex in this species, we manipulated growth of pure-bred and out-crossed zebrafish by varying their food supply during development. Faster-growing zebrafish were more likely to be female than siblings that were fed less, and out-crossed broods had higher proportions of females than broods from purebred crosses. The dependence of sex ratio on feeding rate is readily understood in terms of adaptive sex allocation: zebrafish life history seems to confer the greater pay-off for large size on females. A similar male/female difference in the pay-off for hybrid vigor could similarly account for the female bias of out-crossed broods-and it could be a manifestation of Haldane's rule.

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