Freshwater Shrimp Gammarus pulex grow to 20mm in length and are a member of the Gammaridae, a large family of crustaceans in the order Amphipoda. The body is laterally compressed and at rest it curls into a C-shape. They invariable wriggle about on their sides. The upper antennae is longer than the lower pair and has a small branch part way along. The freshwater Gammarus lives in streams and rivers from lowland areas up into faster flowing hill country. If the current is fast it will shelter in weed or under stones. It also occurs in large ponds.
They are detrital feeders, which means they feed on any organic matter that is fine and decaying. There are a variety of ciliates which attach to their bodies as epizooites. (an animal living on another living thing but not as a parasite is called an epizooites). The sex of the offspring is determined by temperature, for example, in the estuarine species G. duebeni below 5°C it becomes a male, above this, a female.
Males are larger than the females and carry the eggs after laying. Often the amle can be seen carrying the female prior to mating. A very important food source for predators, especially fish.