Behavioral Fever in Lined Seahorse (Hippocampus erectu) Enhances the Immune Response to Vibrio harveyi Infection

Ectotherms can elevate their body temperature in response to infection by seeking warmer environments, a phenomenon known as behavioral fever. This adaptive response, widely documented in fish, activates immune defenses and improves survival. To explore an eco-friendly approach for managing Vibrio-induced enteritis in lined seahorse (Hippocampus erectus) aquaculture, we investigated whether Vibrio harveyi infection triggers behavioral fever and enhances immune function. Seahorses were intraperitoneally injected with V. harveyi (1 × 10⁷ cfu/fish) and placed in a thermal gradient tank (19-31 °C), allowing free movement between chambers. Challenged seahorses exhibited a significant preference (p < 0.05, 1.31-fold) for warmer zones compared to unchallenged controls, whereas no such difference (p > 0.05) was observed in a constant temperature (25 °C) tank, confirming behavioral fever. Furthermore, fevered seahorses showed significantly elevated plasma cytokine levels (PGE2, IL-1β, IL-6, and TNF-α; p < 0.05), which normalized (p > 0.05) to baseline levels, except for TNF-α, compared to unfevered individuals. In kidney tissue, challenged seahorses expressing behavioral fever exhibited gene expression levels (tnf-α, il-6, ifn-g, and il-10) similar to unchallenged controls (p > 0.05) but significantly lower (p < 0.05) than those kept at constant temperature. These findings suggest that behavioral fever in H. erectus modulates core temperature to regulate cytokine release and immune-related gene expression. This study provides foundational insights for developing practical, non-invasive strategies to mitigate enteritis in seahorse aquaculture through thermal behavior manipulation.