There's No Place Like Home: Crown-of-Thorns Outbreaks in the Central Pacific Are Regionally Derived and Independent Events

One of the most significant biological disturbances on a tropical coral reef is a population outbreak of the fecund, corallivorous crown-of-thorns sea star, Acanthaster planci. Although the factors that trigger an initial outbreak may vary, successive outbreaks within and across regions are assumed to spread via the planktonic larvae released from a primary outbreak. This secondary outbreak hypothesis is predominantly based on the high dispersal potential of A. planci and the assertion that outbreak populations (a rogue subset of the larger population) are genetically more similar to each other than they are to low-density non-outbreak populations. Here we use molecular techniques to evaluate the spatial scale at which A. planci outbreaks can propagate via larval dispersal in the central Pacific Ocean by inferring the location and severity of gene flow restrictions from the analysis of mtDNA control region sequence (656 specimens, 17 non-outbreak and six outbreak locations, six archipelagos, and three regions). Substantial regional, archipelagic, and subarchipelagic-scale genetic structuring of A. planci populations indicate that larvae rarely realize their dispersal potential and outbreaks in the central Pacific do not spread across the expanses of open ocean. On a finer scale, genetic partitioning was detected within two of three islands with multiple sampling sites. The finest spatial structure was detected at Pearl & Hermes Atoll, between the lagoon and forereef habitats (<10 km). Despite using a genetic marker capable of revealing subtle partitioning, we found no evidence that outbreaks were a rogue genetic subset of a greater population. Overall, outbreaks that occur at similar times across population partitions are genetically independent and likely due to nutrient inputs and similar climatic and ecological conditions that conspire to fuel plankton blooms.

热带珊瑚礁上最严重的生物扰动之一，便是多产的食珊瑚棘冠海星（Acanthaster planci）种群暴发。尽管引发初始暴发的因素存在差异，但学界普遍认为，区域内外的后续暴发会通过初级暴发种群释放的浮游幼虫进行传播。该次级暴发假说主要基于棘冠海星极强的扩散潜力，以及一个核心论断：暴发种群（即大种群中的异常亚群）彼此间的遗传相似度，高于其与低密度非暴发种群的遗传相似度。本研究借助分子技术，通过分析线粒体DNA（mtDNA）控制区序列并推断基因流限制的位置与强度，评估了太平洋中部棘冠海星暴发种群借助幼虫扩散进行传播的空间尺度；本次研究共采集656份标本，涵盖3个海域、6个群岛的17个非暴发点位与6个暴发点位。棘冠海星种群存在显著的区域、群岛及群岛亚尺度遗传结构，这表明其幼虫极少实现理论上的扩散潜能，太平洋中部的暴发种群无法跨越开阔大洋进行传播。在更精细的尺度上，3个设有多个采样点位的岛屿中，有2个岛屿内检测到了遗传分化；其中空间遗传结构最精细的点位为珍珠与赫米斯环礁（Pearl & Hermes Atoll），其泻湖与礁前生境之间的距离不足10公里。尽管本次研究使用了能够检测微弱遗传分化的分子标记，但并未发现任何证据表明暴发种群是大种群中的异常遗传亚群。总体而言，在不同种群分区中同期发生的暴发种群彼此间遗传独立，其成因大概率为营养盐输入，以及协同促进浮游生物水华形成的相似气候与生态条件。