Gammarus ornatus; Print; Gammarus is an amphipod crustacean genus in the family Gammaridae. It contains more than 200 described species; making it one of the most speciose genera of crustaceans.Different species have different optimal conditions; particularly in terms of salinity; and different tolerances; Gammarus pulex; for instance; is a purely freshwater species; while Gammarus locusta is estuarine; only living where the salinity is greater than 25鈥?