Gammarus is a genus of small, shrimp-like crustaceans commonly found in freshwater and marine environments. They belong to the family Gammaridae and play a crucial role in aquatic ecosystems as scavengers, feeding on decaying organic matter and algae. These organisms typically range from 1 to 10 centimeters in length and have a segmented body with long antennae. Gammarus species are important food sources for various fish and birds, contributing to the food web in their habitats. Their presence can indicate the health of aquatic ecosystems, making them valuable for environmental monitoring.