Swimming behavior characteristics of Manila clam (Ruditapes philippinarum) D-shaped larvae exposed to environmental stressors.

This study employed video analysis techniques to investigate the swimming behavior of D-shaped larvae of the Manila clam (Ruditapes philippinarum). The effects of environmental stressors on the growth, development, and swimming behavior of larvae were evaluated, including elevated temperature, hypoxia, and hydrogen sulfide, as well as their specific combinations: a binary combination (elevated temperature and hypoxia) and a ternary combination (elevated temperature, hypoxia, and hydrogen sulfide). The relationship between developmental abnormalities and aberrant larval behavior was also analyzed. The swimming trajectories of larvae exhibited four distinct behavioral modes: rectilinear, arc, circular, and motionless. Normally, the larvae exhibited an average swimming speed of 242.5∼315.7 μm/s and a maximum speed of 709.6∼1007.1 μm/s, with predominantly rectilinear movement trajectories (81.1∼85.7%). Under environmental stress conditions, however, hydrogen sulfide and multiple stressors had the most significant impact on larval survival and development. Compared to the control group, all experimental groups showed a significant decrease in the proportion of rectilinear trajectories and a marked reduction in swimming speeds. Rectilinear trajectories are the predominant motility pattern in normal larval movement, which is evident in our experimental analyses, whereas arc and circular trajectories were associated with developmental abnormalities. By simulating natural compound stress environments, this work systematically reveals the swimming behavioral trajectories of Manila clam D-shaped larvae. These findings indicate that behavioral indicators serve as sensitive biomarkers for environmental stress and larval health status. Such ethology-based analytical approaches can be utilized to evaluate the actual impacts of environmental factors on larvae, providing real-time and functional feedback on their physiological well-being. Consequently, this offers a valuable supplementary monitoring tool for both aquaculture management and environmental assessment.