Capsaicin-activated AjTRPV contributes to thermotaxis of Apostichopus japonicus larvae.

Temperature is a crucial environmental factor determining the survival and growth of Apostichopus japonicus. Thermotaxis represents an innate behavioral strategy that enables poikilotherms to migrate toward thermally favorable microhabitats. However, the molecular mechanisms underlying temperature sensation and thermal behavioral responses in A. japonicus remain unclear. Here, we investigated the functional roles of transient receptor potential vanilloid (TRPV) channels in regulating the thermotactic behavior of A. japonicus larvae. Both AjTRPVA and AjTRPVB were highly expressed during the auricularia larval stage, with expression levels 4.05-fold and 1.46-fold higher than those at the gastrula stage, respectively. Whole mount in situ hybridization showed that both AjTRPVA and AjTRPVB were predominantly localized in the ciliary bands of auricularia larvae. Sequence alignment identified highly conserved residues potentially responsible for capsaicin and heat activation, which are clustered in intracellular S4-S5 linker and S6 transmembrane domain. Behavioral assays demonstrated that A. japonicus auricularia larvae exhibit positive thermotaxis toward warm temperatures (18-27 °C). Capsaicin (a canonical TRPV activator) significantly reduced larval aggregation in preferred high-temperature zones (H2: 21-24 °C, H3: 24-27 °C), decreasing the total occupancy rate by 22.08%. Application of RR (a TRPV inhibitor) fully rescued the impaired thermotactic performance and restored normal larval thermal preference when released from high-temperature site. Collectively, these pharmacological results indicate that TRPV channels may contribute to positive thermotaxis in A. japonicus larvae.