Many studies have shown that photic behavior of amphipods is subject to parasitic manipulation. However, all these investigations have focused on but one property of light (i.e., intensity). This study investigated the possibility that variable wavelength sensitivity, as a potentially important component of amphipod ecology, is subject to parasitic manipulation. The photic behavior of freshwater amphipods Hyalella azteca, infected with the duck acanthocephalan Corynosoma constrictum, was tested. The phototactic responses of infected and uninfected amphipods to various wavelengths in the visible spectrum were compared, and to delineate the effects of intensity and wavelength on behavior, the preferences of amphipods for environments characterized by various combinations of light intensity and wavelength were determined. Response to blue light (400–450 nm) was little affected by infection. Amphipod response to higher red region wavelengths (600–700 nm) was altered by infection. Infected amphipods were significantly less responsive to green region light (500–550 nm), which could lead to increased wandering throughout the water column, thereby facilitating increased parasite transmission through increased predation risk. This study reinforces the subtlety with which parasites can alter their host's behavior, presumably resulting in an increased probability of being transmitted from the intermediate host to a definitive host.

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