Molecular phylogenetic relationships based on mitochondrial genomes of novel deep-sea corals (Octocorallia: Alcyonacea): Insights into slow evolution and adaptation to extreme deep-sea environments

A total of 10 specimens of Alcyonacea corals were collected at depths ranging from 905 m to 1 633 m by the manned submersible Shenhai Yongshi during two cruises in the South China Sea (SCS). Based on mitochondrial genomic characteristics, morphological examination, and sclerite scanning electron microscopy, the samples were categorized into four suborders (Calcaxonia, Holaxonia, Scleraxonia, and Stolonifera), and identified as 9 possible new cold-water coral species. Assessments of GC-skew dissimilarity, phylogenetic distance, and average nucleotide identity (ANI) revealed a slow evolutionary rate for the octocoral mitochondrial sequences. The nonsynonymous (Ka) to synonymous (Ks) substitution ratio (Ka/Ks) suggested that the 14 protein-coding genes (PCGs) were under purifying selection, likely due to specific deep-sea environmental pressures. Correlation analysis of the median Ka/Ks values of five gene families and environmental factors indicated that the genes encoding cytochrome b (cyt b) and DNA mismatch repair protein (mutS) may be influenced by environmental factors in the context of deep-sea species formation. This study highlights the slow evolutionary pace and adaptive mechanisms of deep-sea corals.