Individual clown anemonefish shrink to survive heat stress and social conflict.

Vertebrate growth is generally considered to be unidirectional, but challenging environmental conditions, such as heatwaves, may disrupt normal growth patterns and affect individual survival. Here, we investigate the growth of individual clown anemonefish, , during a marine heatwave. We measured the length of 134 wild clown anemonefish every month and monitored temperature at the scale of their anemone for five lunar months. Our results show that clown anemonefish shrink in response to heat stress and individuals that shrink also display relatively more catch-up growth. Further, shrinking is modulated by social rank and size, and individuals that shrink more often and in a coordinated fashion with their breeding partner have higher survival during the heat stress event. In conclusion, a plastic individual growth response to heat stress, constrained by the social environment, can lead to short-term survival benefits. If this plasticity were widespread in fishes, it may have marked consequences for populations and communities as heatwaves become more frequent.