Data from: Low inter-basin connectivity in a facultatively diadromous fish: evidence from genetics and otolith chemistry

Southern smelts (Retropinna spp.) in coastal rivers of Australia are facultatively diadromous, with populations potentially containing individuals with diadromous or wholly freshwater life-histories. The presence of diadromous individuals is expected to reduce genetic structuring between river basins due to larval dispersal via the sea. We use otolith chemistry to distinguish between diadromous and non-diadromous life-histories and population genetics to examine inter-basin connectivity resulting from diadromy. Otolith strontium isotope (87Sr:86Sr) transects identified three main life history patterns: amphidromy, freshwater residency and estuarine/marine residency. Despite the potential for inter-basin connectivity via larval mixing in the marine environment, we found unprecedented levels of genetic structure for an amphidromous species. Strong hierarchical structure along putative taxonomic boundaries was detected, along with highly structured populations within groups using microsatellites (FST = 0.046 – 0.181), and mtDNA (ΦST = 0.498 – 0.816). The presence of strong genetic subdivision, despite the fact that many individuals reside in saline water during their early life-history, appears incongruous. However, analysis of multi-elemental signatures in the otolith cores of diadromous fish revealed strong discrimination between river basins, suggesting that diadromous fish spend their early lives within chemically distinct estuaries rather than the more homogenous marine environment, thus avoiding dispersal and maintaining genetic structure.

澳大利亚沿岸河流中的南银汉鱼属（Retropinna spp.）为兼性洄游物种，其种群可同时包含洄游型与终生淡水型的个体。由于幼体可通过海洋完成扩散，洄游型个体的存在本应降低不同河流流域间的遗传结构分化程度。本研究通过耳石（otolith）化学特征区分洄游型与非洄游型生活史类型，并利用种群遗传学方法探究洄游行为介导的流域间连通性。耳石锶同位素（87Sr:86Sr）剖面分析识别出三种主要生活史模式：广盐洄游（amphidromy）、淡水居留型以及河口/海洋居留型。尽管海洋环境中存在幼体混合导致的流域间连通潜力，但本研究发现该广盐洄游物种的遗传结构分化程度远超以往认知。基于微卫星标记（microsatellites）（FST=0.046~0.181）与线粒体DNA（mtDNA）（ΦST=0.498~0.816）的分析结果显示，种群沿推测的分类学边界呈现显著的层级遗传结构，且类群内部种群亦存在高度分化。尽管多数个体在早期生活史阶段栖息于咸水环境，但种群却存在强烈的遗传分化，这一现象看似矛盾。不过，对洄游型鱼类耳石核心的多元素特征分析显示，不同流域间的特征差异显著，这表明洄游型鱼类的早期生命阶段并非栖息于相对均质的海洋环境，而是处于化学特征迥异的河口区域，从而避免了跨流域扩散，维持了种群的遗传结构。