Data from: One species for one island? Unexpected diversity and weak connectivity in a widely distributed tropical hydrozoan

Isolation by distance (IBD) is one of the main modes of differentiation in marine species, above all in species presenting low dispersal capacities. This study reports the genetic structuring in the tropical hydrozoan Lytocarpia brevirostris α (sensu Postaire et al, 2016b), a brooding species, from 13 populations in the Western Indian Ocean (WIO) and one from New Caledonia (Tropical Southwestern Pacific). At the local scale, populations rely on asexual propagation at short distance, which was not found at larger scales; identical genotypes were restricted to single populations. After the removal of repeated genotypes, all populations presented significant positive FIS values (between 0.094*** and 0.335***). Gene flow was extremely low at all spatial scales, between sites within islands (<10 km distance) and among islands (100 to>11 000 km distance), with significant pairwise FST values (between 0.012*** and 0.560***). A general pattern of IBD was found at the Indo-Pacific scale, but also within sampled ecoregions of the WIO province. Clustering analyses identified each sampled island as an independent population, whereas analysis of molecular variance indicated that population genetic differentiation was significant at small (within island) and intermediate (among islands within province) spatial scales. The high population differentiation might reflect the life cycle of this brooding hydrozoan, possibly preventing regular dispersal at distances more than a few kilometres and probably leading to high cryptic diversity, each island housing an independent evolutionary lineage.距离隔离（Isolation by distance, IBD）是海洋物种分化的核心模式之一，对于扩散能力较低的类群尤为突出。本研究报道了热带水螅虫Lytocarpia brevirostris α（遵照Postaire等（2016b）的定义）的遗传结构特征，该育幼型物种的采样种群涵盖西印度洋（Western Indian Ocean, WIO）的13个种群，以及新喀里多尼亚（热带西南太平洋）的1个种群。在局域尺度上，种群依赖短距离无性繁殖，该繁殖模式未在大尺度样本中被检测到；相同基因型仅局限于单个种群内部。去除重复基因型后，所有种群均呈现显著为正的FIS值（区间为0.094***至0.335***）。各空间尺度下的基因流均处于极低水平，无论是岛屿内部（距离<10公里）还是岛屿之间（100公里至>11000公里），两两FST值均具有统计学显著性（区间为0.012***至0.560***）。研究在印太尺度以及西印度洋海域的采样生态区内均发现了典型的距离隔离模式。聚类分析将每个采样岛屿判定为独立种群，而分子方差分析结果显示，种群遗传分化在小尺度（岛屿内部）和中尺度（海域内岛屿之间）均显著。高种群分化或与该育幼型水螅虫的生活史特征相关：其可能阻碍了数公里以上的常规扩散，并可能促成了极高的隐存多样性，每个岛屿均拥有独立的演化支系。