Wavelength-dependent polarization orientation in Daphnia

The ability to detect and use the polarization of light for orientation is widespread among invertebrates. Among terrestrial insects, the retinula cel ls that are responsible for polarization detection contain a single visual pigment, either ultraviolet or short (blue) wavelength sensitive. With the exception of a few aquatic insects, the visual pigments underlying polariza tion sensitivity in aquatic invertebrates have yet to be determined. Here w e report that polarotaxis in Daphnia pulex, a freshwater crustacean, is wav elength dependent and most likely mediated by two visual pigments with abso rbance maxima in the middle (green) and long wavelength (red) parts of the spectrum. This contrasts with the response of a closely related species, D. magna, in which polarotaxis is wavelength independent and based on a singl e middle wavelength visual pigment. The visual systems in Daphnia are the f irst among crustaceans shown to utilize a middle wavelength pigment for pol arization detection and, in the case of D. pulex, the first shown to use mo re than one visual Figment for such a purpose.