Positive and Host-Induced Negative Phototaxis of the Symbiotic Water Miteunionicola Formosa

The symbiotic mite Unionicola formosa is positively phototactic when free of any chemical influence of its molluscan host, Anodonta imbecilis, and negatively phototactic in the presence of host tissue homogenate. 2. Light-adapted mites exhibit maximal positive and negative phototaxis at 500 nm, while dark-adapted animals display maximal positive phototaxis between 500 and 540 nm and greatest negative responsiveness between 460 and 500 nm. 3. Dark-adaptation increases the sensitivity of U. formosa, resulting in a 1 to 2 log unit shift to lower intensities for maximal phototaxis and of the thresholds for both positive and negative responses. 4. Unionicola formosa tested in host homogenate exhibits negative phototaxis at high intensities and postive phototaxis at low intensities. 5. An apparent wavelength-dependent change in phototactic sign is more likely a function of differential sensitivity to intensity. 6. Although presumably a means by which host-symbiont contact is enhanced, the functional significance of the host-influenced behavior of U. formosa remains speculative.