Effects of physical interference on life history shifts in Daphnia pulex

Daphnia pulex were reared in 50 ml flasks, each containing 1, 20 or 40 individuals, which were serially connected with a 20-mum mesh screens between, in order to examine the effect of physical interference due to crowding on shifts of life history traits throughout two consecutive generations. A flow-through system, designed to maintain a sufficient food supply and minimize the accumulation of metabolites, was used. To eliminate the effect of infochemicals from crowded animals, a single-individual treatment flask was connected to two crowded flasks. In the first generation, D. pulex reared under crowded conditions grew more slowly after day 4 when oogenesis normally starts, and produced less offspring after day 9, compared with an animal reared alone, even when supplied with sufficient food. Although second generation daphniids of each treatment matured faster than in the first generation, crowded females grew more slowly even after day 2 and produced less offspring than single females. Age to maturity was no different between treatments in both generations. Crowded females, therefore, matured to smaller sizes but produced larger neonates compared with single females. Weight-specific reproduction rates of the first clutch were not significantly different between the treatments. These results suggest that physical interference between neighboring individuals due to crowding negatively affects growth and reproduction in daphniids. Crowded daphniids may allocate more energy to reproduction in order to produce larger and more starvation-tolerant offspring in preparation for severe food shortages. Crowding also triggered ephippial egg production and reduced survival compared with the single-individual treatment.