Microsatellites Reveal Genetic Homogeneity among Outbreak Populations of Crown-of-Thorns Starfish (Acanthaster cf. solaris) on Australia’s Great Barrier Reef

Abstract

Specific patterns in the initiation and spread of reef-wide outbreaks of crown-of-thorns starfish are important, both to understand potential causes (or triggers) of outbreaks and to develop more effective and highly targeted management and containment responses. Using analyses of genetic diversity and structure (based on 17 microsatellite loci), this study attempted to resolve the specific origin for recent outbreaks of crown-of-thorns on Australia's Great Barrier Reef (GBR). We assessed the genetic structure amongst 2705 starfish collected from 13 coral reefs in four regions that spanned~1000 km of the GBR. Our results indicate that populations sampled across the full length of the GBR are genetically homogeneous (G' ST = −0.001; p = 0.948) with no apparent genetic structure between regions. Approximate Bayesian computational analyses suggest that all sampled populations had a common origin and that current outbreaking populations of crown-of-thorns starfish (CoTS) in the Swains are not independent of outbreak populations in the northern GBR. Despite hierarchical sampling and large numbers of CoTS genotyped from individual reefs and regions, limited genetic structure meant we were unable to determine a putative source population for the current outbreak of CoTS on the GBR. The very high genetic homogeneity of sampled populations and limited evidence of inbreeding indicate rapid expansion in population size from multiple, undifferentiated latent populations.

Larval Survivorship and Settlement of Crown-of-Thorns Starfish (Acanthaster cf. solaris) at Varying Algal Cell Densities

“…Timmers et al [51] investigated genetic connectivity among CoTS populations in the northern Pacific, and revealed extensive gene flow along the 2500 km length of the Hawaiian archipelago, as well as between Hawai'i and Johnston Atoll separated by 865km. Similarly, Harrison et al [53] found no genetic structure (using 27 optimized microsatellite loci) for outbreak populations of CoTS along the length of Australia's GBR. Distinct differences in levels of genetic structure within versus among regions, suggests that larvae are sufficiently long-lived so as to be effectively dispersed among reefs within regions, but largely incapable of dispersal among regions.…”

“…[49,50], which must be maintained by limited larval and genetic exchange at these ocean scales. Within regions however, there is often very limited genetic structure [47,[51][52][53], indicative of extensive genetic exchange via widespread dispersal of larvae. Timmers et al [51] investigated genetic connectivity among CoTS populations in the northern Pacific, and revealed extensive gene flow along the 2500 km length of the Hawaiian archipelago, as well as between Hawai'i and Johnston Atoll separated by 865km.…”

30 Years of Research on Crown-of-thorns Starfish (1986-2016): Scientific Advances and Emerging Opportunities

“…Alternatively, genetic approaches may be used to explicitly resolve actual connections among discrete populations to validate dispersal patterns. However, using genetics to track the spread of outbreaks has proved difficult for CoTS due to the low levels of genetic differentiation apparent when using existing markers (Benzie 1992;Benzie and Wakeford 1997;Harrison et al 2017). On the GBR, for example, genetic sampling during outbreaks has failed to resolve any structure (Benzie 1992;Harrison et al 2017), indicating rapid expansion in population size from multiple, undifferentiated latent populations.…”

“…For the most part, CoTS larvae are expected to be dispersed only 10s-100s km between reefs (Dight et al 1990a), if not entrained within the confines of their natal reef (Black and Moran 1991;Black 1993). Genetic sampling of CoTS populations demonstrated that there is effective connectivity (reflective of ecological significant levels of larval dispersal) between reefs separated by <1,000km (Timmers et al 2011;Yasuda et al 2015;Harrison et al 2017). However, there tends to be very strong genetic differentiation of CoTS populations among ocean basins and geographic provinces (Yasuda et al 2009;Timmers et al 2012), suggesting that there is extremely limited connectivity (and therefore, negligible larval dispersal) at distances of >1,000km.…”

“…However, using genetics to track the spread of outbreaks has proved difficult for CoTS due to the low levels of genetic differentiation apparent when using existing markers (Benzie 1992;Benzie and Wakeford 1997;Harrison et al 2017). On the GBR, for example, genetic sampling during outbreaks has failed to resolve any structure (Benzie 1992;Harrison et al 2017), indicating rapid expansion in population size from multiple, undifferentiated latent populations. Similarly, studies elsewhere in the Pacific have identified largely homogeneous populations within specific reef systems (Vogler et al 2013;Yasuda et al 2015b;Tusso et al 2016), though CoTS generally exhibit substantial regional, archipelagic genetic structuring (Timmers et al 2012), reflective of limited large-scale dispersal.…”