Results of ecological niche analyses
Hypervolume analysis indicated that all clades diverge in their niche, having adapted to largely distinct environmental conditions across their distribution range (Figure 4). Clade YS had a relatively narrower ecological niche compared with clade ECS and SCS. The greater niche differentiation occurs between clade YS and SCS (Table 3), and this is due to two niche-based operating simultaneously: the most preponderant is a niche shift, with clade SCS experiencing a wider range of depths and climatic conditions, and a parallel niche expansion of SCS (or a contraction of clade YS). The niche of clade ECS, on the other hand, although disjunct, was more similar to that of the other two clades, as emphasize by a small distance between centroids. The differentiation of clade ECS with respect to clades YS and SCS was only due to niche expansion/contraction processes (Table 3).
TSS and AUC scores showed that all algorithms but surface range envelop had excellent performance (Figure S4); thus, these nine algorithms were used to develop the ensemble models. The ensemble SDMs had high predictive capacity with mean (± standard error) TSS = 0.911 (± 0.003) and mean AUC = 0.987 (± 0.001). Projections of ensemble SDM suggest that a large part of coastal areas of China, Japan, and Korea are suitable for S. schlegeli , which generally matches the current known distribution range of this species (Figure 5a, Figure S5a). Habitat suitability projections for this species derived from all three GCMs showed a clear geographical separation between south and north populations during the LGM (Figure 5b-d, Figure S5b-d). In addition, SDM projections suggest that suitable area of S. schlegeli was greatly reduced during the LGM: the projected distribution area during the LGM was 10–15% with respect to current depending on the different GCMs.