Influences of population density on reproductive output at food excess in the guppy, Poecilia reticulata (Peters).

Abstract

Experiments were performed to investigate if population density and social facilitation may influence fecundity (number of ova produced per female) and fertility (number of intrafollicular embryos produced by a female) in the presence of excess food in the guppy, Poecilia reticulate (Teleostei: Poeciliidae). No significant differences in total body length, standard length and total body weight were found between females exposed to different population densities in the presence of excess food. The frequencies of females with only intrafollicular embryos or with only ova or with both embryos and ova in the ovary were not significantly different between groups at different population levels. This was also true for the frequency of females with only previtellogenic oocytes. There were no significant differences concerning ovarian length, width, height and volume between females from different population densities. This clear-cut result persisted also when gonad weight was examined. The average gonadosomatic index was highest in the highest population density. Highly significant correlations occurred between ovarian volume and total body weight as well as between ovarian volume and gonad weight at each population level (P<0.001, P<0.01). This was also the case for gonad weight vs total body weight (P<0.001, P<0.01). A significant and inverse correlation between fecundity and ovum diameter appeared only at the highest population level (P<0.01). No significant differences of fecundity and ovum diameter in addition to fertility and embryo size were revealed between different population densities when feeding ad libitum. This result differed from an earlier study in which increased population density caused a decrease in fecundity and fertility under conditions of food scarcity.