SEASONALITY OF REPRODUCTION BY LIVEBEARING FISHES IN TROPICAL RAIN-FOREST STREAMS

Reproductive ecology, population structure, and diets of three common livebearing poeciliid fishes (Alfaro cultratus, Phallichthys amates, P oecilia gilli) from rainforest streams in Costa Rica were investigated over ten continuous months. The region experiences little annual temp erature variation, and although monthly rainfall is continuous each ye ar, two brief dry seasons typically occur. Monthly changes in indices of ovarian condition, percentages of females with developing embryos, and population size structure revealed that reproductive output by fem ales of all three species varied seasonally. Based on testicular condi tion, males were reproductively active year-round, however the mean go nadal index for males of two algivorous species showed low levels of s easonal cycling that largely coincided with female variation in reprod uctive effort. All three species had seasonal differences in the femal e size-brood size relationship, whereby larger females tended to carry more embryos during the wet season. Several important adult and neona te food resources are more available in the flooded forest during the wet season, which is also the period when conspecifics and predators a re at their lowest per-area densities. Three hypotheses are discussed: (1) brood size in relation to conspecific density-mating frequency, ( 2) reproductive allocation in response to variation in adult food reso urces, and (3) selection for greater reproductive effort during condit ions optimal for juvenile growth and survival. Data for Alfaro were co nsistent with the latter two hypotheses. In Phallichthys and Poecilia, diets were poorer during wet seasons, indicating that reproductive ef fort does not coincide with availability of adult food resources, and that selection probably favors greater reproductive effort during peri ods optimal for juvenile growth and survival.