Illumination, Vision, and Schooling of Astyanax mexicanus (Fillipi)

Abstract

The schooling behavior of both light- and dark-adapted Astyanax mexicanus (Fillipi) was observed under sudden changes in illumination in the range 10⁻⁷ to 10⁰ ft-c at different times in the diel cycle. In darkness the fish were dispersed, swam slowly, and showed no signs of mutual recognition of one another. There was no simple relationship between the schooling response and the illumination, but in general the frequency, size, and durability of schools were proportional to the illumination. Schooling was maximal above 10⁻¹ ft-c and approached zero at 10⁻⁶ ft-c. The schooling at any level of illumination varied with the testing procedures. The lowest responses were obtained with a gradual reduction of illumination in the evening. At illumination below 10⁻² ft-c maximal schooling was obtained from dark-adapted fish. From 10⁻² ft-c upward, it was obtained from light-adapted fish. The schooling rate (number of fish in a school) reached 100% for a few seconds at 10⁻³ ft-c and for longer periods at higher illumination, but was never sustained at the 100% level throughout any 4-min period. Schooling was generally enhanced by any sudden change in illumination but was never sustained at a high rate under monotonous conditions. The decline with monotony was equated with reduction of the fright situation. The decline with reduced illumination was attributed to the reduction of visual acuity and visibility, and occurred at a greater rate among faster-swimming fish. Notemigonus crysoleucas (Mitchill), swimming very slowly, exhibited high schooling rates at 10⁻⁵ ft-c. Earlier work showed that the retina of Astyanax in continuous darkness undergoes a rhythmic diel shift. The behavior showed that this retinal shift is not the classical transformation from scotopic to photopic vision. Dark-adapted during the day, the fish demonstrated rapid visual perception under sudden illumination at any value from 10⁻⁶ to 10^−0.69 ft-c. Dark-adapted fish at night exhibited increasingly longer light-shock reactions with sudden illumination above 10⁻² ft-c. According to schooling responses the cone threshold was about 10⁻³ ft-c, and the rod threshold about 10⁻⁶ ft-c. The mesopic range extended from about 10⁻³ to 10⁻² ft-c.