Effects of Polymorphus (Acanthocephala) on Colour and Behaviour of Gammarus lacustris

IN 1969-70 1,420 adult specimens of the amphipod Gammarus lacustris were taken from 2 eutrophic ponds in the northern Zealand; 94.9% were normal in colour (grey to brown) and 5.1% were blue. Dissections showed that all the blue shrimps were infected with the final larval stage (cystacanth) of the acanthocephalan genus Polymorphus, whereas the brown shrimps were only 3.9% infected with cystacanths. This difference was statistically highly significant (χ² = 860.2, P<0.001).