CONICET | Buscador de Institutos y Recursos Humanos

The freshwater snail Pomacea canaliculata shows highly variable growth patterns in laboratory cultures. Part of this variation is attributable to rearing conditions, but the influence of the source of laboratory stocks has not been studied yet. Egg masses from a permanent shallow lake (LA), a permanent stream (AC) and an intermittent rivulet (AH) were obtained. One hundred newborn from each source were reared in homogeneous conditions (10 by aquarium, 25ºC, 24 h light/day, CO3Ca saturation, ad libitum lettuce). After maturity only one randomly selected couple of each aquarium was retained. Survivorship, maximum shell length and oviposition were recorded weekly. Mean aquarium ages at first oviposition were notably short: 102 (AH), 105 (LA) and 114 days (AC). By this moment mean female shell lengths were 40.5, 39.5 and 42.6 mm respectively, males being a little smaller (37.1, 36.8 and 37.2 mm). Nevertheless, differences between populations and sexes were not significant. Notwithstanding the fact that age at first oviposition is highly variable (and dependent on temperature, density and food availability and quality), most authors agree about the size at maturity of females (ca. 25 mm). However, our results indicate that maturity depends not only on size but a minimum development time is also required. The three populations showed sexually dimorphic growth patterns: from 45 days on, females are invariably bigger than males. However, variation in dimorphism expression degree, probably genetic in origin, was observed. LA and AC stocks first showed significant size differences at 72 and 110 days of age respectively, which were minimum and never significant in AH. Opposite to previous reports, size dimorphism appeared in these stocks long before sexual maturity (at 10-15 mm).

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