GRAZING OF FRESHWATER RHODOPHYTA 1

ASTRACT Grazing of freshwater Rhodophyta was examined from three perspective: observation of algal fragments in gut contents of herbivores collecte from streams with populations of red algae, measurement of ignestion rates of three inveratebrates in single diet and choice experiments and analysis of potential food value of the rhodophytes by determining protein, carobohydrate, lipid and estimated caloric contents. The algal thallus forms studied were Audouinella violacea (Kütz.) Hamel, which consists of simple filaments growing in dense tufts, Batrachospermum virgatum (Kütz.) Sirod., which is a densely branched mucilaginous filament, and Tuomeya americanna (Harv.) Papenfuss, which is a branched cartilaginous psedoparenchyma. The thirteen invertebrate taxa containing pieces of one or more of these algae in theri guts included two amphipods, two mayfly larvae, three caddisfly larvae, one beetle larva, four chironomids and one snail. The frequencies with which the Rhodophyta were found in the gut contents were as follows: Audouinella 31%, Tuomeya 38%, Batrachospermum 46% and “chantransia stages” 54%. In all of the individual and choice experiments, Audouinella was ingested at the greates rate, followed by Batrachospermum and then Tuomeya, regardless, of the grazer. Audouinella had the highest protein and lipid contents while Tuomeya displayed the opposite trent. All three rhodophytes had very large calofic values (5.0–6.8 kcal μg −1 dry weight) due to their high lipid and low ash contents. Therefore, preferential grazing of Audouinella appears to be bases on its simple thallus and high protein contents rather than its calorific value.