Feeding in Daphnia galeata on Oscillatoria limnetica and on detritus derived from it

Abstract

Oscillatoria spp. are generally very abundant in many shallow, eutrophic lakes in the Netherlands. However, this is less true for Daphnia galeata. The main aim of this study was to investigate whether the edibility of live Oscillatoria limnetica by Daphnia galeata, and of the detritus derived from this filamentous cyanobacterium, was, among others, an important limiting factor for the daphnids. We measured the consumption and assimilation rates of Daphnia using dual-labelling radio-tracer techniques (14C and 32P) to label separately the live Oscillatoria filaments and detritus prepared from these filaments. The two food types were mixed in different proportions, and both the food ingestion and food incorporation rates by daphnids were measured. The main findings of this study were that specific clearance rates of Daphnia on shorter Oscillatoria filaments were significantly higher than on the longer filaments, in other words the weight-specific ingestion rates were higher on the shorter Oscillatoria filaments than on the longer filaments from the batch cultures. The longer Oscillatoria filaments are more likely to clump and, therefore, are more liable to be rejected by Daphnia during the food collection and ingestion processes. The shorter filaments, in comparison, are apparently less prone to dumping and, therefore, are cleared by the daphnids at higher rates than the longer filaments. Feeding the daphnids on double-labelled (14C and 32P) shorter filaments revealed that the assimilation efficiency of shorter Oscillatoria filaments was generally higher for P than for C, probably because of a high C/P ratio of Oscillatoria. Daphnia (0. 75-1.85 mm in size) fed significantly better on dead (detritus) Oscillatoria filaments than on live Oscillatoria filaments, even if the relative proportion of detritus in the food was only about one-quarter that of the live filaments. This preference for detritus over live Oscillatoria, as indicated by Chesson's selectivity coefficient a, was apparently a passive process, rather than a case of active food selection. This 'selective' feeding on detritus was apparently facilitated by the relatively greater rejection of the live filaments than the detrital filaments. At very high food concentrations (15-25 mg C l-1), with the share of the live Oscillatoria filaments 2 to 2.5 times greater than that of the detritus, the ingestion rates increased proportionally less with increase in daphnid size than was expected on the basis of the allometric relationship between the length and weight of these animals. This implies that the larger animals had greater interfering effects of the Oscillatoria filaments on the food collection and ingestion processes.