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

## TL;DR

Lined seahorses infected with Vibrio harveyi seek warmer temperatures, which boosts their immune response and could help prevent disease in aquaculture.

## Contribution

This study is the first to demonstrate behavioral fever in lined seahorses and its role in modulating immune responses to infection.

## Key findings

- Infected seahorses prefer warmer zones (31°C) in a thermal gradient tank, indicating behavioral fever.
- Fevered seahorses show elevated plasma cytokine levels and normalized immune gene expression.
- Behavioral fever may improve disease resistance in seahorse aquaculture without antibiotics.

## Abstract

Behavioral fever, a thermoregulatory strategy where ectotherms migrate to warmer environments to enhance immune and survival, is well documented in many fish species. However, its existence in the lined seahorse (Hippocampus erectus) remains unexplored. In this study, we observed that Vibrio harveyi-infected seahorses consistently preferred warmer zones (31 °C) in a temperature gradient tank (19–31 °C), a behavior absent in uninfected individuals. Infected seahorses exhibiting fever displayed elevated plasma cytokine levels and downregulated immune-related gene expression patterns resembling those of uninfected controls. Our findings highlight the potential of manipulating thermal environments in seahorse aquaculture to activate behavioral fever, thereby improving disease resistance without relying on antibiotics.

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.