An investigation into the effects of Streptococcus agalactiae on the 5-HT system and the behavior of GIFT tilapia (Oreochromis niloticus)

•We investigated the impact of pathogen on fish from behavior response.•The critical swimming speed of fish challenged by pathogen were quantified.•The 5-HT changes caused by pathogen were studied synchronously.•The physiological mechanism underline the behavior change was discussed. Meningitis is a characteristic clinical manifestation in GIFT tilapia (Oreochromis niloticus) infected with Streptococcus agalactiae. Fish that are infected with Streptococcus agalactiae and survive often show neurological disorders, such as constant, aberrant swimming. In the current study, the critical swimming speed (Ucrit) of farmed fish was investigated. The Ucrit of tilapia infected with S. agalactiae decreased to 2.78 BL s−1, 3.05 BL s−1 and 2.94 BL s−1 at 4, 8 and 16 h after challenge, respectively (P