SPEISER, D. I.*; JOHNSEN, S.; Duke University; Duke University: Wavelength-dependent phototaxis as a depth regulating mechanism in a ctenophore

Opsins and photoreception well preceded the appearance of eyes in Metazoa. Our understanding of eye evolution may, therefore, be enriched by studying the photoreceptive systems present within basal phyla. Observations of Ocyropsis maculata, a lobate ctenophore, suggest a behavioral response to light, which this study examined further. O. maculata is common in near-surface oceanic waters during the day and feeds on copepods and a variety of planktonic larvae. Shallower depths may contain more food as well as higher levels of damaging UV radiation, making depth regulation of importance to O. maculata. Because blue and green light penetrates further than red light into water, O. maculata may control depth through a differential phototactic response to these wavelengths. This possibility was explored by placing animals in the center of a test chamber, allowing them to dark adapt for one minute, shining monochromatic light through one end of the chamber, and recording the position of the animal after two minutes of light exposure. The results from this experiment indicate that O. maculata preferentially swam toward deep-penetrating short wavelengths and away from shallow-penetrating long wavelengths. This suggests that O. maculata may indeed use wavelength-dependent phototaxis to regulate its depth.