Climate‐Driven Warming Disrupts the Symbiosis of Bobtail Squid Euprymna scolopes and the Luminous Bacterium Vibrio fischeri

ABSTRACT Under the current climate crisis, marine heatwaves (MHW) are expected to intensify and become more frequent in the future, leading to adverse effects on marine life. Here, we aimed to investigate the impact of environmental warming on the symbiotic relationship between the Hawaiian bobtail squid ( Euprymna scolopes ) and the bioluminescent bacterium Vibrio fischeri . We exposed eggs of E. scolopes to three different temperatures during embryogenesis, namely: (i) 25°C (yearly average), (ii) 27°C (summer maximum) or (iii) 30°C (category IV MHW), followed by a colonisation assay under the same conditions. Decreased hatching success and reduced developmental time were observed across warmer conditions compared to 25°C. Moreover, exposure to the category IV MHW led to a significant decrease in survival after 48 h. With increasing temperature, bobtail squids required more bacteria in the surrounding seawater for successful colonisation. When colonised, the regression of the light organ's appendages was not dependent on temperature, but the opposite was found in non‐colonised bobtail squids. Furthermore, the capacity for crypt 3 formation in the squid's light organ, which is crucial for enhancing resilience under stress, also declined with warming conditions. This study emphasises the critical need to study the dynamics of microbial symbiosis under the projected conditions for the ocean of tomorrow.