Noninvasive Epidermal Mucus RNA-Seq Analysis for Developing Methods to Evaluate Environmental Stress in the Japanese Eel (Anguilla japonica)

Environmental changes in rivers caused by fluctuations in water temperature and increasingly intense rainfall linked to climate change are likely to influence the physiological condition (environmental stress) of wild Japanese eels (Anguilla japonica). This study examined the epidermal mucus of A. japonica, which can be collected noninvasively in the field, and evaluated whether RNA-Seq analysis could characterize the physiological state of this species in response to such environmental stressors. We conducted controlled tank trials simulating (1) suspended solids (SS) stress resulting from sediment runoff during heavy rainfall (using kaolin clay addition) and (2) water temperature variations (10 °C, 20 °C, and 30 °C) as physical stresses experienced by wild A. japonica under climate-driven changes. Subsequently, we performed RNA-Seq analysis to determine genes in the epidermal mucus responsive to each experimental stressor. As a result, under (1) SS stress, genes associated primarily with cellular immunity and inflammatory pathways showed elevated expression. In the (2) water temperature trials, relative to the control (20 °C), the low-temperature group (10 °C) displayed increased expression of lipid-metabolism-related genes such as scd, prom2, and tmem64, whereas the high-temperature group (30 °C) exhibited heightened expression of genes linked to the heat-shock response. Thus, because this study enabled the identification of genes specifically expressed in reaction to each stressor from the epidermal mucus of A. japonica, these findings are expected to support the future development of noninvasive approaches for evaluating the physiological state (health condition) of wild A. japonica.