Putative long distance gene flow and evidence of secondary outbreaks of the Crown-of-Thorns Starfish in the Pacific Ocean

Population outbreaks of the corallivorous crown-of-thorns starfish (COTS), Acanthaster ‘planci’ L., are considered among the most important biological disturbances of tropical coral reefs. A local COTS outbreak, a “primary outbreak”, can lead to so-called “secondary outbreaks” in adjacent coral reefs due to increased larval release and subsequent dispersion. Previous analyses have shown that in the Pacific Ocean, this dispersion may be geographically restricted to certain regions. Guam, an island in the western Pacific region, suffered from several COTS outbreaks in the last 50 years, and in this study we test whether Guam is genetically connected with surrounding regions as a consequence of secondary outbreaks. We used microsatellites to measure gene flow and genetic structure among 14 localities in the Pacific Ocean. Our results show substantial genetic structure between geographical regions. There was, however, a lack of significant genetic differentiation between localities separated by large geographic distances (e.g., Guam, Kingman Reef and Johnston Atoll) – a finding consistent with the existence of contemporary long distance larval dispersion and the gradual erasing of ancestral signatures of divergence. Long distance larval dispersion suggests that secondary outbreaks occur across regions in the Pacific, and have occurred in the recent outbreaks in Guam. However, significant genetic differences among outbreak localities around the island of Guam were also detected, revealing the signature of both primary and secondary outbreaks. Our findings highlight the importance of addressing likely triggers of both primary and secondary outbreaks in conservation efforts and programs that strive to control the growth and spread of A. ‘planci’ in the Pacific Ocean.