Development of an ImageJ-Based Method for the Assessment of Diel Activity Patterns in Hydra oligactis: A Comparative Approach with Hydra vulgaris Toward Understanding Light-Driven Rhythms in Early-Diverging Metazoans

Abstract

This study aimed to develop a simple and reliable image-based method for quantifying movement in Hydra oligactis and to explore potential diel activity patterns. Using time-lapse recordings and a custom ImageJ macro, movement was measured as changes in %Area across 3000 frames. Image processing involved grayscale conversion, thresholding, and frame subtraction. Temporal trends were visualized using polynomial curve fitting and statistical analysis of six time-based batches. Results showed fluctuating movement patterns, with varying degrees of activity and estimated variability, suggesting non-random behavior that may reflect ultradian rhythms or partial diel regulation. While longer recordings are needed to confirm circadian patterns, this method provides a valuable framework for future studies on behavioral rhythms in early-diverging animals.