Variation in the intersetular distance on filter setae of Simocephalus vetulus (O. F. Miiiler) (Crustacea-Cladocera)

SUMMARY. 1. The intersetular distance (filter mesh-size) of adult Simocephalus showed a two-fold seasonal variation in each of two ponds. In one pond the maximum intersetular distance was found in March, while in the other pond the maximum intersetular distance was found in September.
2. These seasonal changes in intersetular distances was positively correlated with changes in phytoplankton biomass (estimated by chlorophyll a). As food concentration increased (estimated by chlorophyll a) the mean intersetular distance of adull Simocephalus also increased.
3. The intersetular distances on fillcring setae also vary with the size of animals. There is a linear relation between body length and intersetular distance.
4. Variation in the intersetular dimension with body size may decrease competition tor food between adults and young.