Adaptation in the Deep Sea: How Depth Affects Morphology and Protein Function in Coryphaenoides rupestris and its Congeners

Although the deep sea is the largest habitat on Earth, there is little understanding of how adaptation and speciation occur across depth gradients. The roundnose grenadier, Coryphaenoides rupestris, has demonstrated genetic segregation according to habitat depth across several functional loci. In this study, 139 C. rupestris individuals were sampled for morphological analysis, and the protein OBSL1 was modelled to explore the impact of non-synonymous SNPs across the species’ depth range. It was confirmed that the genetic segregation is mirrored by morphological differences in deeper-living individuals. C. rupestris sampled from deeper habitats were smaller and had more slender body forms, along with larger eyes and mouth gapes. Lipid stores in the liver and swimbladder both increased with habitat depth. OBSL1, the modelled muscle protein, was shown to have tighter intra-domain boning in fish from shallower habitats. These changes were linked to changes in foraging strategy and an increased demand for energy conservation in deeper habitats. Analyses were extended to explore adaptation to even deeper habitats. OBSL1 was modelled for 13 additional Coryphaenoides species from abyssal and non-abyssal habitats. These analyses suggested that hydrostatic pressure was the key selection pressure for this protein as habitat depths increase to reach the abyssal zone. This finding was contrasted with those from the single-species analysis of C. rupestris to illustrate how selection pressures for adaptation and speciation change across a depth gradient in the deep sea.