Stomach microbiome simplification of a coral reef fish at its novel cold-range edge under climate change

Climate-driven range-extensions of animals into higher latitudes are often facilitated by phenotypic plasticity. Modifications to habitat preference, behaviour, and diet can increase the persistence of range-extending species in their novel high-latitude ecosystems. These strategies may be influenced by changes in in their gut and stomach microbial communities that are critical to host fitness, and potentially adaptive plasticity. Yet, it remains unknown if the gut and stomach microbiome of range-extending species is plastic in novel ranges to help facilitate these modifications. Here, we categorised stomach microbiome communities of a prevalent range-extending coral reef fish species along a 2,000-km latitudinal gradient in a global warming hotspot, extending from their tropical core-range to their novel temperate cold-range edge. At their cold-range edge, the coral reef fish’s stomach microbiome showed a 59% decrease in bacterial diversity, and a 164% increase in relative abundance of opportunistic bacteria (Vibrio) compared to their tropical core range. Microbiome diversity was unaffected by fish body size, water temperature, physiology (cellular defence and damage), and habitat type (turf, barren, oyster, kelp, and coral) across their range. The observed shifts in stomach microbiome composition suggests dysbiosis and low plasticity of tropical range-extending fishes to novel environmental conditions (e.g., temperate prey and lower seawater temperature) at their novel range edges, which may increase their susceptibility to disease in temperate ecosystems. We conclude that fishes extending their ranges to higher latitudes under ocean warming can experience a simplification (i.e. reduced diversity) of their stomach microbiome, which could restrict their current rate of range extensions or establishment in temperate ecosystems.