FEEDING IN EUCHLANIS-DILATATA-LUCKSIANA HAUER ON FILAMENTOUS CYANOBACTERIA AND A PROCHLOROPHYTE

Ingestion and assimilation rates of Euchlanis dilatata lucksiana Hauer, isolated from Lake Loosdrecht (The Netherlands) and cultured on lake water (seston < 33 mum), were measured in the laboratory using the C-14-tracer technique. The five taxa used as tracer foods, together with 6-7 mg C l-1 of lake seston in each case, included four species of filamentous cyanobacteria (Oscillatoria redekei, O. limnetica, Aphanizomenon flos-aquae, Anabaena PCC 7120) and a prochlorophyte (Prochlorothrix hollandica). Except Anabaena, they are all commonly encountered in eutrophic Loosdrecht lakes, including Lake Loosdrecht, and their dimensions ranged between 150 and 250 mum in length and 2 and 3.5 mum in width. The small and large Euchlanis used as experimental animals had mean lengths of 217-223 mum and 276-305 mum, respectively. Euchlanis fed on all the taxa offered as food. Clearance rates ranged from 51 to 99 mul ind-1 d-1 for the large animals and from 22 to 41 mul ind-1 d-1 for the small animals. The highest ingestion rate observed, 1.7 mug ind-1 d-1, was for the large animals feeding on Aphanizomenon. The daily ration for both size classes far exceeded 100% of body weight, reaching up to 690% for the small animals feeding on Aphanizomenon. The small animals generally appeared to assimilate the ingested food more efficiently (assimilation efficiencies: 37-100%) than the large animals (34-77%). Compared with an earlier study in which only lake seston (< 33 mum) was used as food, the specific clearance rates of Euchlanis on the cyanobacteria and Prochlorothrix were generally somewhat lower.