HABITAT USE AND ECOLOGICAL SPECIALIZATION WITHIN LAKE DAPHNIA POPULATIONS

Many species of planktonic cladocerans display substantial variation i n habitat use (mean depth and diel vertical migration), both among and within populations. We examined whether clonal segregation and specia lization contributes to such behavioral variation within several lake populations of the cladoceran, Daphnia pulicaria. Electrophoretic and quantitative genetic analysis of clonal lines isolated from different depths at night revealed that clonal habitat specialization was common . Clones that utilized shallow water at night were genetically smaller at maturity and lower fecundity under standard laboratory conditions than the deep-water clones. The magnitude of this clonal habitat speci alization varied among lakes: populations displaying broad use of dept h habitats contained greater genetic variance than populations with mo re constrained habitat use. These results are consistent with known di fferences in selective factors in different depth habitats and suggest that substantial clonal specialization can occur within single popula tions. Since previous work has discovered a heritable basis to habitat selection in several Daphnia species, including D. pulicaria in our s tudy lakes, it is likely that clonal/depth specialization is an import ant factor affecting the trophic ecology of Daphnia.