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 fi lamentous cyanobacteria (Oscillatoria redekei, O. limnetica, Aphanizom enon flos-aquae, Anabaena PCC 7120) and a prochlorophyte (Prochlorothr ix hollandica). Except Anabaena, they are all commonly encountered in eutrophic Loosdrecht lakes, including Lake Loosdrecht, and their dimen sions ranged between 150 and 250 mum in length and 2 and 3.5 mum in wi dth. The small and large Euchlanis used as experimental animals had me an 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 f or 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, reachin g up to 690% for the small animals feeding on Aphanizomenon. The small animals generally appeared to assimilate the ingested food more effic iently (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 th e cyanobacteria and Prochlorothrix were generally somewhat lower.