Light and diel vertical migration: spectral sensitivity and light avoidance by Mysis relicta

Ambient light levels determine the extent of diel vertical migration of man y species including mysid shrimps. Light levels perceived by an organism de pend on the intensity of light at the surface, the extinction of light thro ugh the water, and the sensitivity of the organism's light receptors. Each of these processes has spectral characteristics that should be taken into a ccount when measuring perceived light levels. We used microspectrophotometr y to determine that Mysis relicta has a single pigment with the characteris tics of rhodopsin based on vitamin A(1) and a peak sensitivity of 520 nm. S imilar to the use of the lux (scaled to human vision), we give ambient ligh t levels scaled to the mysid's visual spectrum in mylux units. Mysid distri butions were observed with acoustics around two artificial light sources in Cayuga Lake, New York. Mysids avoided light levels of 3.4 x 10(-7) to 2.1 x 10(-6) mylux. Similar light levels limited their vertical distributions d uring the night in Lake Ontario and during the day in Cayuga Lake. Of stand ard light sensors available, lux meters are more appropriate than photosynt hetically active radiation meters for determining light levels perceived by mysids.