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Abstract
The coral-eating crown-of-thorns starfish (COTS; Acanthaster spp.) play a major role in coral reef degradation in the Indo-Pacific region. However, the impacts of environmental factors on the phylogenetic and genetic characteristics of COTS in the northern Indo-Pacific convergence region remains unclear. We used mitochondrial DNA (mtDNA) and microsatellite markers to analyze the phylogenetic relationship, demographic history, genetic diversity and genetic structure of COTS in the South China Sea (SCS) and explored the impact of environmental factors on historical population expansion, genetic differentiation and larval dispersal. There was a clear signature of a population expansion in the SCS using the mtDNA marker. According to microsatellite loci analysis, COTS have high genetic diversity in the SCS. STRUCTURE analysis indicated that COTS in the Pacific Ocean can be divided into four subgroups: the SCS, Western Pacific, Pacific equatorial current affected zone, and Pacific insular atolls populations in the Pacific Ocean. Fst-statistical analysis revealed positive correlations between the Fst values and geographic isolation for all sampling sites. Additionally, there were no clear associations between the Fst values and chlorophyll a concentrations among coral reefs in the SCS; however, there were significant positive associations between the Fst values and particulate organic carbon (POC) concentrations within small geographic distances. These results suggest that COTS underwent historical population expansion after the Last Glacial Maximum, possibly followed by coral population expansion. The genetic structure of COTS populations may have been shaped by distinct nutrient concentrations, particularly those of POC, over small geographic distances. Moreover, ocean currents provide a potential dispersal mechanism for COTS larvae in the SCS. This study demonstrates that environmental and oceanographic factors play important roles in shaping the genetic characteristics and larval dispersal of COTS populations in the northern Indo-Pacific convergence region.
Highlights
Coral reefs are one of the most biologically diverse ecosystems in the oceans (Reaka-Kudla, 1997; Robert et al, 2002; Wilkinson, 2008)
For crown-of-thorns starfish (COTS) populations within the South China Sea (SCS), there were no significant distinct genetic clusters in the median-joining haplotype (Figure 2B; separated by at least 18 mutation steps; Vogler et al, 2013). This result showed no clear association between haplotypes and geographic factors, and two dominant haplotypes were widely shared among the populations of the coral reefs in the SCS (Figure 3B)
COTS may have undergone historical population expansion after the Last Glacial Maximum in the Pleistocene, which might have been followed by the expansion of coral populations in the SCS
Summary
Coral reefs are one of the most biologically diverse ecosystems in the oceans (Reaka-Kudla, 1997; Robert et al, 2002; Wilkinson, 2008). COTS are the most important biological threat to coral reef ecosystems in the Indo-Pacific region (Pratchett et al, 2017a), and COTS outbreaks lead to substantial loss of live coral cover, and impair the integrity and resilience of reef ecosystems (Death et al, 2012; Kayal et al, 2012; Mellin et al, 2019). Many COTS outbreak events have been observed, the causes of dramatic COTS population explosions remain equivocal. It is still unclear whether outbreaks start from a single reef or arise simultaneously from separate locations due to limited temporal and spatial resolution of monitoring (Pratchett, 2005)
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