(b) Mitochondrial diversity
Globally, average mitochondrial genetic diversity was higher in the western Pacific Ocean and lower along North American and European coastlines (Fig. 2A & B, S4A & B). For both Hdand π, diversity peaked at low-to-mid latitudes and declined towards the poles, particularly in the Northern hemisphere (Fig. 2A & B, S7A & B). Diversity was also consistently higher in the Coral Triangle and elsewhere in the western Indo-Pacific (Fig. 3A & B). For mitochondrial genetic diversity (either Hd 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 significant predictors of mitochondrial genetic diversity (Table 1, Fig. S8). As expected, Hd increased consistently with the length of the locus in base pairs (Fig. S9) and decreased towards species range edges (although π did not) (Fig. S10).
Both environmental drivers were significantly correlated with mitochondrial genetic diversity (Hd and π) and performed better than the null model (Table 2). Sea surface temperature was positively related with mitochondrial diversity (Fig. 4A & B), while chlorophyll-a concentration followed a quadratic relationship with diversity highest at mid-to-upper chlorophyll-a concentrations (5-10 mg/m3) (Fig. 4D & E).
When looking at how these global patterns differed across a subset of families represented in our dataset, we found substantial variation. While the majority of the 10 families followed the same overarching patterns (e.g. reduced mitochondrial genetic diversity at higher latitudes, increased diversity at elevated SST, and a quadratic relationship with chlorophyll-a concentration), several did not (Fig. S11-S13). Gadidae (cods) and Sebastidae (rockfishes) showed elevated mitochondrial diversity at higher latitudes and lower SST for bothHd and π, while the relationships between latitude and SST varied in Carcharhinidae (requiem sharks), Engraulidae (anchovies), and Rajidae (skates) by marker type.