Role of vision in the aggregative behavior of the planktonic mysid Mysidium columbiae

Aggregations of the planktonic mysid Mysidium columbine use vision to help maintain position within mangrove prop-root habitats and to maintain spacin g within schools. Laboratory studies show that mysids videotaped in darknes s using infrared illumination did not form schools and could not hold posit ion in currents. In daylight, mysids more effectively held position in a fl ow-through chamber with a high-contrast visual reference than in its absenc e. The compound eyes of mysids are thought to be good motion detectors, but little is known about their visual acuity or sensitivity. An optokinetic d rum was used to test the visual acuity and photosensitivity of mysids using the behavioral response of mysids to vertical black and white stripes that move past their field of view. When the drum rotates, the mysids swim at t he same speed and in the same direction as the moving stripes. Swimming spe ed of the mysids was measured to compare their speed to the turning rate of the drum using video-computer motion-analysis techniques. Detection of the moving stripes was also inferred from the proportion of mysids that follow ed the stripes and that reversed direction when the rotation of the drum wa s reversed. By varying the width of these stripes, the visual acuity of the mysids was determined. The ability of M. columbiae to follow stripes of 1 mm in width from a distance of 15 to 30 mm indicates that mysids can visual ly resolve nearby prop roots and other mysids within schools. The photosens itivity threshold for the optokinetic response was found to be 0.001 mu m p hotons m(-2) s(-1), similar to light levels during moonlight. These mysids are potential prey to a wide range of planktivorous fish, and their surviva l may depend upon their ability to maintain their position within schools a nd within the safety of the prop-root habitat during daylight hours in spit e of currents and turbulence that would tend to disperse them.