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.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 128.235.251.160 on TueAbstract. Two herbivorous, interspecifically territorial West Indian damselfishes, the beaugregory (Eupomacentrus leucostictus) and the three-spot damselfish (E. planifrons), both inhabit the back-reef area of coral reefs in a narrow depth band. A survey of coral heads in this habitat revealed that beaugregories utilize all types of heads, while three-spots are found only on branching or creviced heads with relatively complex internal structure. Translocations of individual fish and heterospecific pairs were performed to determine whether this differential microhabitat utilization is the result of differences in microhabitat preference between the two species, or aggressive interactions between them, or both. As indicated by duration of residence of translocated fish on experimental heads, threespot damselfishes showed significant preference for heads with complex internal structure, while beaugregories showed no preference. In translocations of heterospecific pairs, three-spots won aggressive contests more often on internally complex heads, and beaugregories won more often on heads without internal structure, than would be expected on a random basis. Since this difference fell just short of significance (P = .053), translocations of heterospecific pairs were also done in aquaria with artificial heads. In these more controlled conditions, the preponderance of wins by three-spots on complex heads and beaugregories on simple heads reappeared as a significant difference, with occasional co-occupation of the complex head by both species. The observed differences in microhabitat utilization are thus due both to different preferences and aggressive interaction. It is probable that the aggressive interactions, a form of interference competition, are responsible for the evolution of the different habitat preferences. However, for both the three-spot damselfish and the beaugregory, it has been shown that a principal function of aggressive interspecific territoriality is the protection of food resources from exploitation competition.

The fish comn~unity of a large artificial reef made In 1960 of 800 concrete blocks in a seagrass bed on the south coast of St. John, U.S. Virgin Islands, was studied by comparing a series of visual censuses (1975)(1976)(1977)(1978)(1979) with each other and with a 1962 total-poison station collection (Randall, 1963). Discounting the presence of some cryptic species in the poison station that would not normally be seen in visual censuses (N = 10), as well as discounting seagrass-bed residents and roaming species only transient on the reef (N = 13), all of the censuses and the poison station (range 31-40 spp.) were found to be similar (Spearman Rank Correlations: all p C .01). This stability stands in contrast to the apparent unpredictability reported for smaller artificial and natural reef structures on the Great Barrier Reef Dybdahl 1975, 1978;Talbot et al., 1978) which has been attributed to chance factors of recruitment. We interpret our results as evidence for orderly Interactions as important factors in the determination of community structure, and suggest that short-tern~ studies of small reefs lack the resolving power to detect this type of evidence.

No abstract

We visually censused fishes along transects on the back-reef and adjacent lagoons of bank-barrier reefs at 6 sites on St. Croix, US Virgin Islands, to determine the extent to which coral reef fishes use lagoon habitats as nurseries. Fishes were recorded by size class (small, < 3 cm; medium, 3 to 5 cm; large, > 5 cm) on the back-reef, and on 5 lagoon habitat types: patch-reef, rubble, seagrass, algal plain, and sand. We examined densities of 4 focal 'species' (Acanthurus spp. [A. bahianus and A. chirurgus], Haemulon spp. [all species of the Haemulon genus], Sparisoma aurofrenatum, and Scarus iserti), and densities of all species combined to determine spatial and temporal patterns of habitat use. Although there was a general tendency for coral reef fishes to use lagoon habitats as nurseries, we discerned 2 patterns of habitat use: 1 group, exemplified by Acanthurus spp. and Haemulon spp., use lagoon patch-reef and rubble as nurseries in preference to back-reef and other lagoon habitats; in contrast, Sparisoma aurofrenatum and Scarus iserti preferentially use back-reef and, to a lesser extent patch-reef, as nursery, juvenile, and adult habitat. Temporal variation was greatest in the small size class and least in the large size class. Most settlement occurred during summer, which is when settlement in lagoon habitats was greatest, with a little settlement in winter in the back-reef habitat. For species that use lagoons as nurseries, lagoon habitats must provide advantages that offset the additional energy expense and predation experienced by both incoming larvae, as they cross over the reef and search for patch-reef and rubble, and juveniles, as they return to the reef during the juvenile-to-adult transition. Given the use of lagoon habitats as nurseries, there is a need for inclusion of lagoons in coral reef reserves.

SynopsisRecent studies of recruitment dynamics in demersal fishes have placed major emphasis on presettlement mortality, and little on events bridging late larval and early juvenile periods. Observations on 68 taxa of Caribbean coral reef fishes before and during settlement revealed the existence of a distinct post-settlement life phase called the transition juvenile, associated with the act of recruitment. Transition juveniles were found as solitary individuals, in conspecific groups, or in heterospecific groups. The groups were either uniform or heterogenous in appearance. The complexity of the transition phase and its apparently widespread occurrence in coral reef fishes suggests that important aspects of population structure may be determined between settlement and first appearance as a full-fledged juvenile.