Behavioural response spectra for phototaxis by four European species of water mites that are differently associated with freshwater mussels were determined. The wavelength for maximal stimulation of positive phototaxis and the corresponding energy threshold, were: 560nm and 3.6 × 10−7 μW cm−2 for the free-living species Unionicola aculeata, 560 nm and 2.1 × 10−6 μW cm−2 for the commensal U. bonzi, 600nm and 3.3 × 10−6μW cm−2 for the obligate symbiont U. ypsilophora, and 600 nm and 1.3 × 10−5 μW cm−2 for the parasitic species U. intermedia. Sensitivity of these water mites to yellow-orange light is consistent with the spectral transmission of their native stream, the water of which readily absorbs wavelengths shorter than 500 nm.
U. intermedia exhibited wavelength-dependent phototaxis, with maximal positive phototaxis occurring in response to 600 nm light and negative phototaxis occurring at 440 nm. This differential photo-behaviour was not attributable to intensity effects. However, the positive phototaxis of this species to 600 nm light became negative when the mite was exposed to the chemical influence of its molluscan host. The spectral sensitivities of this acarine genus suggest the presence of at least two visual pigments within the taxon.