THE EFFECT OF AQUATIC PLANT CHEMISTRY AND MORPHOLOGY ON FEEDING SELECTIVITY BY THE CRAYFISH, ORCONECTES-VIRILIS

1. Feeding preference trials conducted with male and female crayfish (Orconectes virilis) showed that out of ten submersed aquatic plant species, Chara sp. and Lemna trisulca were the preferred food sources, followed by Myriophyllum exalbescens or Utricularia vulgaris. 2. Analysis of plant fibre, alkaloid, total phosphorus, total and available nitrogen, and organic content showed that crayfish feeding preferences were not related (P > 0.5) to plant fibre or alkaloid content but were negatively correlated (P < 0.05) with plant nutrient content such that species of low nutritional value (Chara and Lemna) were consistently preferred. 3. Despite this preference for low nutrient food, daily intake rates by crayfish for phosphorus, nitrogen and organic matter were greatest when feeding on Lemna or Chara due to the large quantity consumed. 4. We suggest that feeding preferences of O. virilis are not determined by plant chemistry but, rather, relate to ease of handling small, short, bottom-dwelling plants such as Chara and Lemna as compared with large erect, roseate or floating-leaved forms. Because crayfish are selective herbivores, their introduction into waters where they do not naturally occur could have a dramatic effect on both the species composition and biomass of submersed plants.