Daphnia are very sensitive to their surroundings and have the ability to react to environmental factors. For instance, the water fleas develop a protective helmet-like structure around their heads in reaction to kairomones, chemicals released into the water by predators. Moreover, some types of Daphnia can adopt dwarf forms that are small enough to pass through the gill rakers of certain fish. Scientists believe that the organisms are signaled to assume this form based upon the levels of waste materials excreted by the predators into the aquatic environment.