关键词: 圆尾鲎, 线粒体D-loop, 遗传多样性, 种群遗传结构, 保护遗传学

Abstract

Aims：The mangrove horseshoe crab (Carcinoscorpius rotundicauda) is a "living fossil" of great ecological and biomedical value, whose populations have undergone severe declines. The South China Sea constitutes a critical biogeographic transition zone for this species; nevertheless, systematic investigations into its population genetic structure and historical demography across this region remain scarce. Populations along the coasts of southern China occupy the northern range margin of its distribution, whereas populations from the Indonesian archipelago lie within its historical diversity centre. 

Methods： We analyzed mitochondrial DNA control region (D-loop) sequences from 108 individuals representing eight geographic populations: three populations (59 individuals) from southern China (Huguang, Qiaotou, and Wanwei) and five populations (49 individuals) from the Indonesian archipelago (Balikpapan, Bangka Belitung, Demak, Jambi, and Madura). Genetic diversity was calculated in DnaSP. Haplotype networks were constructed in POPART, principal coordinate analysis (PCoA) in GenAlEx, and analysis of molecular variance (AMOVA) in Arlequin. Historical demography was inferred using neutrality tests (Tajima's D and Fu's FS) and mismatch distribution analysis. Redundancy analysis (RDA) was used to explore associations between haplotype composition and 20 marine environmental variables from the GMED database. 

Results： Results showed that the Kimura 2-parameter (K2P) genetic distance was 0.021 between the southern China group and the Indonesian archipelago group, whereas within-group genetic distances were 0.006 for the southern China group and 0.012 for the Indonesian archipelago group, respectively. A total of 39 haplotypes were detected, with overall high haplotype diversity (Hd=0.882) and moderate nucleotide diversity (π=0.014). Populations from the Indonesian archipelago exhibited significantly higher genetic diversity (Hd=0.933, π=0.013) than those from southern China (Hd=0.649, π = 0.006), consistent with the central-marginal hypothesis. The long-term effective population size (Ne) was estimated at 2,125–10,625 for the southern China group and 4,668–23,339 for the Indonesian archipelago group. Haplotype network, PCoA, and AMOVA concordantly revealed two deeply divergent evolutionary lineages with no shared haplotypes between regions (overall Φ_ST=0.830, P<0.001); hierarchical AMOVA attributed 46.81% of the variation to between-group differences (Φ_CT=0.468, P=0.019). Neutrality tests and mismatch distributions supported historical population expansion in southern China (Tajima's D=−1.887, P<0.05; Fu's FS=−4.380, P<0.01; unimodal mismatch distribution), whereas Indonesian populations showed no signal of recent expansion. RDA identified silicate concentration and maximum sea surface temperature as descriptive correlates of haplotype composition, but the global model was not statistically significant (P = 0.439). 

Conclusion： Our results demonstrate strong geographic genetic structure of C. rotundicauda in the South China Sea, with two evolutionarily significant units corresponding to southern China and the Indonesian archipelago. The south-to-north decline in genetic diversity supports the Sunda Shelf as a historical diversity center and glacial refugium. The northern marginal populations likely experienced post-glacial recolonization. These findings provide critical baseline genetic data for region-specific conservation strategies of this endangered marine living fossil.

Key words: mangrove horseshoe crab, mitochondrial D-loop, genetic diversity, population genetic structure, conservation genetics