ALGAL INGESTION AND DIGESTION BY TWO EPHEMEROPTERAN PHISIOLOGY, MORPHOLOGY & ULTRASTRUCTURE 385 Algal ingestion and digestion by two ephemeropteran larvae from a Patagonian Andean stream VERÓNICA DÍAZ VILLANUEVA RICARDO ALBARIÑO Lab. Limnología, Centro Regional Universitario Bariloche, Unidad Postal Universidad, (8400) Bariloche, Argentina vdiaz@crub.uncoma.edu.ar Abstract The ingestion and digestion of two co-existing ephemeropteran larvae, Meridialaris diguillina (Leptophlebiidae) and Baetis sp (Baetidae) were examined and their mouthpart morphologies were compared. M. diguillina bears conspicuous brushing- like maxillae and Baetis sp has toothed, scope-like lacinia-galeae and mandibles. Larvae were fed on periphyton and faeces were collected for examination. The proportions of dead, dying and live cells in faeces were compared between grazers and with periphyton. The two species showed differences in grazing efficiency. Baetis sp did not feed on prostrated diatoms while M. diguillina fed on the whole periphyton assemblage. In terms of digestion, M. diguillina presented apparent higher digestion ability than Baetis sp. These differences showed dissimilar resource exploitation and therefore the two species might influence the structure and dynamic of the algal assemblage in different ways. Keywords: grazing, mouthpart morphology, diatoms, Meridialaris, Baetis. Introduction Most mayfly larvae are common stream dwellers, adapted to consume the biofilm growing on stones (Edmunds and Waltz, 1996). Their ability of accessing to different algal habits depends on larval mouthpart morphology (Karouna and Fuller, 1992). Collector-gatherers are provided with several sets of setae that allow them to access to loose material while scrapers present some sharp structures that enable them to dislodge highly attached algae (McShaffrey and McCafferty, 1988). Therefore, herbivory usually induces changes in the periphytic assemblage related to the differential susceptibility of algal species to grazing (Sumner and McIntire, 1982; Colletti et al., 1987; Pringle, 1996). Grazers may depress benthic algal quality since living food sources generally have a higher nutritive and caloric content than the same but dead materials (Lamberti and Moore, 1984). A decrease in food quality, caused by repeated ingestion and egestion of algae, may induce a depletion of the grazer population (Grimm and Fisher, 1989). Depending on the herbivore´s capability of digesting of ingested food, the reduction of food quality will occur at different velocities (Peterson et al., 1998). This variation in digestion efficiencies among herbivores may produce different effects on grazer populations. In South Andean streams, the ephemeropteran Meridialaris diguillina (DEMOULIN, 1955) (Leptophlebiidae) and Baetis sp. (Baetidae) are widely distributed and abundant inhabitants of stony riffle areas (Pescador and Peters, 1987). As Leptophlebiids and Baetids have contrasting mouthpart morphologies (Palmer et al., 1993), we hypothesised that these herbivores would display different capabilities of accessing to algae. The aims of this study were to analyse their capability of ingesting different algal species and to determine the digestibility of the consumed algae through the analysis of faeces content. Materials and Methods Larvae were collected from Gutiérrez stream (41º 07' S; 71º 25' W) in February 2001 with a Surber sampler (0.09 m 2 and 200 µm mesh size). They were carried immediately to the laboratory, where twenty individuals of each species were put in containers with 250 ml of filtrated stream water (3 replicates for each species). Body length (not including cerci) of Baetis sp. and M. diguillina larvae were measured under a stereomicroscope. Baetis sp. larvae averaged 4.78 µm (±0.16 s.e.,