The delimitation of evolutionarily significant units (ESUs) frequently results in controversy, but prioritising populations with evolutionary independence is essential for effective in situ conservation management. The American manatee (Trichechus manatus) is distributed along subtropical and tropical coastal waters from Florida (USA) to Alagoas (Brazil), and two subspecies are traditionally recognised, namely, T. m. latirostris, restricted to the Florida peninsula, and T. m. manatus, found in the remaining areas. However, this subspecific classification is not supported by genetic and morphologic evidence, which, rather, recognises two deeply differentiated populations or ESUs called Atlantic (Brazil) and Caribbean (from Venezuela to Florida). In this viewpoint paper, we compare both intraspecific divisions of T. manatus and the conservation implications. First, we used all available mtDNA evidence to test the genealogical clustering of the two American manatee ESUs by using a tree-based coalescent method. Second, we have used different models under a coalescent framework to estimate the historic gene flow among manatee populations. The analysis of the spatial distribution of mtDNA clusters confirmed the existence of the two suggested ESUs, rather than the two claimed subspecies. Furthermore, the best model to explain historic migration indicates that Brazilian manatees belong to an isolated population, whereas Florida and Caribbean populations are connected by more recent gene flow. These results have confirmed that T. manatus of the Caribbean, Gulf of Mexico and Florida belong to the same deme or Caribbean ESU, and the relatively isolated population inhabiting the Atlantic coast of Brazil belongs to the Atlantic ESU. Furthermore, both ESUs are separated by an interspecific hybrid zone (with the Amazonian manatee) located around the mouth of the Amazon River towards the Guianas coastline. The subdivision of two ESUs is also highly supported by karyotypic, morphological and ecological data, and is in clear disagreement with the traditional subspecies designations and the IUCN priorities, which manages Brazilian manatees as part of the Antillean manatee subspecies (T. m. manatus). Rather, Brazilian manatees should be considered as a full priority for conservation and require further taxonomic research; because of their deep history of isolation, they present high genetic and morphologic differentiation from all other American manatees.

对进化重要单位（ESUs）的划定经常引起争议，但是优先考虑具有进化独立性的种群对于有效的原位保护管理至关重要。美国海牛（Trichechus manatus）沿亚热带和热带沿海水域分布，从佛罗里达州（美国）到阿拉戈斯州（巴西），传统上公认有两个亚种，即T. m.。仅限佛罗里达半岛的latirostris和T. m。马纳特斯，在其余区域中找到。但是，这种亚种分类不受遗传和形态学证据的支持，遗传和形态学证据可以识别两个高度分化的种群或ESU，即大西洋（巴西）和加勒比海（从委内瑞拉到佛罗里达）。在这篇观点论文中，我们比较了T. manatus的两种种内分裂及其保护意义。首先，我们使用所有可用的mtDNA证据，通过基于树的合并方法来测试两个美国海牛ESU的族谱聚类。其次，我们在合并框架下使用了不同的模型来估计海牛种群之间的历史基因流。线粒体DNA簇的空间分布分析证实存在两个建议的ESU，而不是两个声称的亚种。此外，解释历史性迁徙的最佳模型表明，巴西海牛属于一个孤立的种群，而佛罗里达和加勒比海的种群则由最近的基因流动所联系。这些结果证实了T. manatus在加勒比海地区，墨西哥湾和佛罗里达州属于同一个deme或加勒比海ESU，居住在巴西大西洋沿岸的相对孤立的人口属于大西洋ESU。此外，两个ESU被位于亚马逊河河口附近通往圭亚那海岸线的种间混合带（与亚马逊海牛）分隔开。两个ESU的细分也得到了核型，形态学和生态学数据的高度支持，并且与传统的亚种名称和IUCN优先事项明显不同，IUCN优先管理作为安的列斯海牛亚种一部分的巴西海牛（T. m。manatus）。）。相反，巴西海牛应被视为保护工作的重中之重，需要进一步的分类学研究；由于其深厚的隔离史，它们与其他所有美国海牛相比具有高度的遗传和形态学差异。