Mayfly Nymph (IMAGE) University of California - Santa Barbara Caption Mayfly nymphs graze on algae that grow on river rocks. They're particularly sensitive to dissolved metals, so their presence is an indication that a stream is recovering. Credit Bruce Medhurst Usage Restrictions For Use Only with Credit License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.