Behavioral and Polian vesicle responses to high temperature in juvenile sea cucumber Apostichopus japonicus

Abstract

The sea cucumber Apostichopus japonicus is one of the most valuable benthic species in coastal aquaculture. Elevated summer temperature is a recurring constraint in sea cucumber aquaculture, yet the physiological mechanisms associated with thermal stress-induced behavioral changes remain unclear. The Polian vesicle is a fluid-filled organ within the water vascular system that contributes to hydrostatic regulation and internal homeostasis, supporting various physiological processes underlying the activities of sea cucumbers. However, its structural and biochemical responses to sustained thermal stress remain poorly understood. In the present study, juvenile sea cucumbers were exposed to a sustained high temperature of 28 °C for 5 days, and behavioral performance, Polian vesicle morphology, and biochemical components were systematically evaluated. High temperature significantly reduced adhesion time, movement distance, movement time, and average velocity, and decreased foraging and feeding activities in sea cucumbers. Concurrently, the Polian vesicle exhibited significant atrophy, reflected by decreased organ weight and relative organ index. Histological analysis revealed structural alterations, including thinning of the connective tissue layer and detachment of muscular and ciliated epithelial components. Biochemically, potassium (K⁺) concentration within the Polian vesicle significantly decreased at high temperature, whereas acetylcholine (ACh) and serotonin (5-HT) concentrations significantly increased. These alterations suggest the changes in ionic and neuromodulatory homeostasis within the water vascular system. Collectively, the findings indicate that short-term sustained thermal stress induces structural and biochemical alterations in the Polian vesicle, which may be associated with behavioral changes in juvenile sea cucumbers. This study provides new insights into the multi-level responses of A. japonicus to elevated temperature, and contributes to understanding thermal stress responses in sea cucumber aquaculture. These findings raise the questions about the mechanisms linking Polian vesicle dysfunction to behavioral inhibition, which warrant further investigation.