Population structure and polymorphism of Ceriodaphnia cornuta in kotmale reservoir

Abstract

A study of zooplankton was carried out in Kotmale reservoir, from August1990 to December 1991. Vertical and horizontal sampling was done oncea month near the dam of the reservoir, between 1100 hours to 1200 hours.A closing type net with mesh size of 50p m was used for collection ofzooplankton. Ceriodaphnia cornuta is a cosmopolitan species and the dominant formin the cladoceran community of Kotmale reservoir. It represented 98% ofthe whole population at certain months. Its population structure comprisednymphs and adults throughout the year. Ceriodaphnia cornuta showed polymorphism. Two morphologicalforms were identified. One form (horned form) had a comparatively smallerbody (size range of 0.30-0.45 mm) with spines on the head region & theother (unhorned form) with a larger body (size range of 0.48-0.84 mm) withoutbearing spines. Abundance of morphological forms were different in 1990 and 1991. The spined form predominated in the epilimnetic waters where fish predationwas high and the other form was found at all depths. Existence of suchmorphological forms may be an adaptation to minimize selective predationby fish.