Light intensity mediated polarotaxis in Pontella karachiensis (Pontellidae, Copepoda).

Polarization sensitivity provides animals with information not available in the intensity or spectral domains. We examined the polarotaxis reactions in the epiplanktonic copepod Pontella karachiensis. Polarotaxis reactions were intensity dependent. At intensities corresponding to ambient daylight, P. karachiensis showed an attraction to a polarized light field; while at low intensities, corresponding to nighttime illumination, it showed negative polarotaxis. P. karachiensis's eye contained two classes of photoreceptors, each class with microvilli at orthogonal orientation to the other. P. karachiensis' eye structure can provide information regarding the polarization percentage but is not sufficient to calculate the exact e-vector orientation. The threshold for polarotoxisis response was 20-30%. Animals responded similarly to horizontal and vertical polarization; and also showed negative phototaxis, affected by light polarization. Results suggest that P. karachiensis responds to polarized light analogously to changes in brightness. The dynamic pattern of polarotaxis responses suggests that polarization sensitivity may enable P. karachiensis to detect other planktonic animals.