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 (<i>Hippocampus erectus</i>) aquaculture, we investigated whether <i>Vibrio harveyi</i> infection triggers behavioral fever and enhances immune function. Seahorses were intraperitoneally injected with <i>V. harveyi</i> (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 (<i>p</i> < 0.05, 1.31-fold) for warmer zones compared to unchallenged controls, whereas no such difference (<i>p</i> > 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; <i>p</i> < 0.05), which normalized (<i>p</i> > 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 (<i>tnf-α</i>, <i>il-6</i>, <i>ifn-g</i>, and <i>il-10</i>) similar to unchallenged controls (<i>p</i> > 0.05) but significantly lower (<i>p</i> < 0.05) than those kept at constant temperature. These findings suggest that behavioral fever in <i>H. erectus</i> 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.