Sandy beach ecosystems are ecologically and culturally important, yet they face a wide range of anthropogenic stressors. Their constituent biodiversity is usually poorly resolved, with numerous instances of cryptic species described. In addition, there are few studies on the spatio-temporal patterns of evolutionary diversity, although these are vital for unravelling historical and contemporary processes acting to shape biodiversity, with measures of molecular diversity and differentiation providing important insights towards the conservation and management of beach ecosystems. Here we utilise a comparative phylogeographic approach to describe the mtDNA COI variation of two co-distributed Excirolana (Crustacea, Isopoda) sister-species in South Africa. We found that both species showed medium to high population genetic structure, with many populations isolated from each other. In addition, we describe two deeply divergent lineages for both species in the eastern parts of their ranges, which may constitute two additional Excirolana species in South Africa. We show that populations on the range edge for both species have lower genetic diversity than in the core. Our results on the divergent patterns of molecular diversity, in addition to the high levels of population divergence, highlight the need for the inclusion of molecular indices into conservation planning. This ensures safeguarding not only patterns of species occurrence, but also genetic diversity, critical to maximising resilience of sandy beach species in a changing world.

沙滩生态系统在生态和文化上都很重要，但它们面临着各种各样的人为压力。它们的组成生物多样性通常很难解决，描述了许多隐性物种。此外，很少有关于进化多样性的时空格局的研究，尽管这些研究对于揭示影响生物多样性的历史和当代进程至关重要，分子多样性和分化的措施为海滩生态系统的保护和管理提供了重要的见识。 。在这里，我们利用比较性的地理学方法来描述两个共同分布的Excirolana的mtDNA COI变化（甲壳纲，等足目）在南非的姊妹物种。我们发现这两个物种都显示出中等至高的种群遗传结构，许多种群彼此隔离。此外，我们在其范围的东部描述了两个物种的两个非常不同的世系，这可能构成另外两个Excirolana种在南非。我们表明，这两个物种在范围边缘的种群的遗传多样性都比核心种群低。我们关于分子多样性差异模式的研究结果，除了高水平的人口差异外，还强调了将分子指数纳入保护规划的必要性。这不仅确保了物种发生方式的保护，还确保了遗传多样性的保护，这对于在不断变化的世界中最大限度地提高沙滩物种的复原力至关重要。