Some life history responses of the cladoceran Ceriodaphnia cf. dubia to variations in population density at two different food concentrations

The combined effects of food concentration and population density on some life history characteristics of the small-bodied cladoceran Ceriodaphnia cf. dubia were studied by examining animals maintained at densities of 100-3000 individuals 1^-1 for 8 days that had been fed either abundant (30 × 10⁴ algal cells ml^-1 ind^-1) or limited (5 × 10⁴ algal cells ml^-1 ind^-1) food. The amount of food provided significantly (p<0.05) affected the density at which the cladocerans produced fewer neonates. Cladocerans maintained at 1000 ind 1^-1 produced significantly (p<0.05) fewer neonates than those at 100 or 200 ind 1^-1 when fed abundant food, while cladocerans fed limited food showed a statistically significant (p<0.05) reduction in fecundity at 500 ind 1^-1 compared to those at 100 or 200 ind 1^-1. There was a general trend towards decreased age at sexual maturity with increased cladoceran density, and a significant (p<0.05) decrease in the age at sexual maturity for cladocerans maintained at 3000 ind 1^-1 was observed in some experiments. Sexual reproduction, as indicated by the presence of resting eggs, was observed in animals maintained at 2000 and 3000 ind 1^-1 and fed limited food. This indicated that, for the range of densities and food concentrations tested, a combination of crowding and limited food was required to induce sexual reproduction. Increasing the food concentration while maintaining a constant cladoceran density lead to significantly (p<0.05) increased reproduction, and no significant (p>0.05) effect on the age at sexual maturity. This indicated that the effects observed in crowded animals were not the result of negative effects due to the increased food concentrations, but were an adaptive response to crowding.