Mechanisms underlying thermally induced growth plasticity in juvenile Pacific halibut.

Growth plasticity in aquatic ectothermic vertebrates is an important factor driving somatic growth variation in natural populations in response to environmental change. In fish, growth plasticity is primarily due to changes in skeletal muscle growth, as this tissue is a major component of the body mass, with water temperature being a primary abiotic factor affecting growth. Investigating skeletal muscle growth plasticity is therefore key for understanding somatic growth variation. The Pacific halibut (Hippoglossus stenolepis) is an important fish species in the North Pacific Ocean ecosystem that has experienced marked changes in size-at-age over the last 100 years. Here, we investigated the molecular basis of growth plasticity in juvenile Pacific halibut acclimated to different temperature regimes under laboratory conditions. By integrating transcriptomic, proteomic and stable isotope analyses of skeletal muscle, we provide evidence for the activation of tiered molecular responses underlying thermally induced growth plasticity. Importantly, we demonstrate that growth plasticity involves plastic molecular responses at the gene, protein and metabolic levels in skeletal muscle that are finely tuned to regulate the synthesis of myofibrillar proteins, among other muscle-related processes. Furthermore, we have identified a set of growth biomarkers that, when tested under field conditions, characterize growth variation among wild individuals. These growth biomarkers, including known and novel growth-related genes, will be useful to elucidate the influence of factors driving somatic growth variation, including changes in size-at-age, in this and other teleost fish species. In summary, this study improves our mechanistic understanding of growth plastic responses to variable temperature regimes in teleost fish and highlights their potential for resilience and/or adaptability in the face of environmental variability.