Fresh-water Shrimp (Gammarus pulex), magnified. Gammarus, a genus of Amphipod Crustaceans, including numerous fresh-water and marine species. One species (Gammarus pulex), sometimes called the 'fresh-water shrimp,' is extremely common in quickly-flowing brooks. It is a tiny creature, about half an inch long, but so abundant that few can have missed seeing it. It generally keeps near the bottom, swims on its side, with a kind of jerking motion, and feeds on dead fishes, &c. In quiet water G. fluviatilis is common, and G. locusta is very abundant among seaweeds along all European coasts. Blind species of the allied genus Niphargus are found in many caves and wells.