The CoP Assay: A Robust Method for Quantitative Assessment of Food Intake in the Planarian Dugesia japonica

Planarians exhibit a variety of behaviors in response to external stimuli and have emerged as a promising animal model for studying the neural regulation of behavior. Although their feeding behavior has been well documented, robust experimental methods for quantifying their food intake remain limited. In this study, we developed a novel methodology for the quantitative assessment of feeding termed the "Complete-or- Partial (CoP) assay." This assay evaluates feeding responses as a binary outcome (complete consumption versus partial ingestion) in a volume-dependent manner at the individual level. Using this method, we examined the effects of food quality on feeding responses by comparing two distinct diets: a "basal diet" containing the minimum concentration of liver homogenate necessary to induce ingestion and support growth, and a "high-density diet" with a ten-fold higher concentration. The CoP assay enabled the robust estimation of food intake volume and successfully detected a significant overconsumption of the high-density diet compared to the basal diet, demonstrating that planarians regulate their food intake in a quality-dependent manner. Furthermore, individuals exhibiting partial ingestion did not immediately feed again even when presented with the same diet freshly prepared, indicating that they had reached satiety. Thus, we propose that the CoP assay offers a robust and scalable platform for evaluating both food intake volume and satiety status, providing a powerful tool to facilitate future molecular analyses of feeding regulation in planarians.