(b) Mitochondrial diversity
Globally, average mitochondrial genetic diversity was higher in the western Pacific Ocean and lower along North American and European coastlines (Fig. 1A & B, Fig. S2A & B). For bothHd and π, diversity peaked at low-to-mid latitudes and declined towards the poles, particularly in the Northern hemisphere (Fig. 2A & B, Fig. S4A & B). Diversity was also consistently higher in the Coral Triangle and elsewhere in the western Indo-Pacific (Fig. 3A & B). For mitochondrial genetic diversity (eitherHd or π), we found that all latitude and longitude models performed better than the baseline (null) model (Table 1). Latitude, absolute latitude, and longitude were all significant predictors of mitochondrial genetic diversity (Table 1, Fig. S5). As expected, Hd increased consistently with the length of the locus in basepairs (Fig. S6) and decreased towards species range edges (although π did not) (Fig. S7).
Except for SST range, all environmental drivers were significantly correlated with mitochondrial genetic diversity (bothHd and π) and performed better than the null model (Table 2, Table S2 & S3). Sea surface temperature was positively related with mitochondrial diversity (Fig. 4, Fig. S8), while chlorophyll followed a quadratic relationship with diversity highest at upper-range chlorophyll values (5-10 mg/m3) (Fig. 4, Fig. S9). Overall, the best-performing models for mitochondrial genetic diversity (both Hd and π) were mean SST and maximum chlorophyll.