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

 Aggregations of the planktonic mysid Mysidium columbiae use vision to help maintain position within mangrove prop-root habitats and to maintain spacing within schools. Laboratory studies show that mysids videotaped in darkness using infrared illumination did not form schools and could not hold position in currents. In daylight, mysids more effectively held position in a flow-through chamber with a high-contrast visual reference than in its absence. The compound eyes of mysids are thought to be good motion detectors, but little is known about their visual acuity or sensitivity. An optokinetic drum 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 the same speed and in the same direction as the moving stripes. Swimming speed 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 followed the stripes and that reversed direction when the rotation of the drum was 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 visually resolve nearby prop roots and other mysids within schools. The photosensitivity threshold for the optokinetic response was found to be 0.001 μm photons m⁻² s⁻¹, similar to light levels during moonlight. These mysids are potential prey to a wide range of planktivorous fish, and their survival may depend upon their ability to maintain their position within schools and within the safety of the prop-root habitat during daylight hours in spite of currents and turbulence that would tend to disperse them. Received: 3 December 1999 / Accepted: 22 May 2000