Abstract:
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 l−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 l−1 produced significantly
(p<0.05) fewer neonates than those at 100 or 200 ind l−1 when fed abundant food, while cladocerans fed limited
food showed a statistically significant (p<0.05) reduction in fecundity at 500 ind l−1 compared to those at 100
or 200 ind l−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
l−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 l−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.
