AN ACCOUNT OF A PRELIMINARY MECHANISTIC MODEL OF SWIMMING BEHAVIOR INDAPHNIA - ITS USE IN UNDERSTANDING DIEL VERTICAL MIGRATION

This preliminary mechanistic model of normal swimming and phototactic behaviour in individual Daphnia was constructed using data and assumpt ions based on experiments and observations. Swimming under constant li ght intensity is characterized by short periods of upward movements al ternating with equal periods of downward movements. Two oscillators ar e proposed that generate these phases in swimming. Unexpected shifts i n depth, as observed in D. magna and D. 'longispina', are also present in the swimming of the computer daphnid and thus seem to be inherent to the underlying mechanism. As in real daphnids, during relative decr eases in light intensity of low velocity, positive phototactic upward swimming is stepwise. With increasing velocity in the change in light, these steps disappear. When the model is triggered by a natural incre ase in light at dawn, a small downward movement results. Migration dis tance can be increased to commonly found depths of migrating Daphnia b y the introduction of a 'fish exudate factor' into the model, which en hances the phototactic response. Since attenuation of light in the wat er affects the phototactic swimming response, it also influences migra tion distance. The results of model calculations agree quite well with an empirical relationship between Secchi disc depth and amplitude of diel vertical migration in a number of lakes.