How females of the rotifer Asplanchna brightwelli swim in darkness and light: an automated tracking study

Abstract

This work explores the effect of darkness on the swimming behaviour of females of the rotifer Asplanchna brightwelli. Females were filmed in flat dishes alternately in white light (WL, 5000 μW cm⁻²) and in infra-red light (IR, 155 μW cm⁻²), each female for a total of eight successive periods of fifteen minutes per period. An automated tracking system was used to describe the swimming path of each female: twenty five x and y coordinates of the center of gravity of the animal per second, in a discrete space of 512 × 512 pixels. Indices characterizing the swimming performances of the females were then calculated: linear speed, angular speed and other angular parameters of the tracks. A Principal Component Analysis of swimming characteristics discriminated between WL tracks and IR tracks. Females swam slower and turned more in darkness than in light. These results show that beside a positive phototactic response, there is a photokinesis which increases the dispersion of animals in the light.