The biotas of old islands formed from continental terranes usually have a more complex biogeographical history than those of young oceanic islands, including taxa which have originated by vicariance and/or colonisation, over a variety of timescales. The Tyrrhenian Islands of Corsica, Sardinia and the Tuscan Archipelago in the Mediterranean have a complex geological history, shaped by interactions between the African and Eurasian plates since the Mesozoic. Our understanding of the historical biogeography of Tyrrhenian endemics remains limited for many groups, including freshwater invertebrates. Here we use a time-calibrated phylogeny, derived from mitochondrial and nuclear DNA sequence data, to explore the evolutionary history of Tyrrhenian endemic Hydraena water beetles, an ecologically important group in the islands’ streams. Whilst no endemic Hydraena appear to date from the initial separation of Corsica-Sardinia from the European mainland in the Oligocene, we show that Tyrrhenian species stem from five colonisation events, occurring at different intervals in the last ca. 15 Ma, at least two endemic lineages apparently arising through isolation at the end of the Messinian Salinity Crisis. Hydraena reflexa Rey, 1884, long considered a geographical form or subspecies of the widespread H. pygmaea Waterhouse, 1833, is reinstated as a valid species, endemic to Corsica and Sardinia. H. reflexa can be distinguished from H. pygmaea on external and aedeagal characters, documented here in detail. Specimens of ‘H. reflexa’ from southern continental Italy are shown to differ from those on Corsica and Sardinia, having identical male genitalia to H. pygmaea from elsewhere in its range. Genetically, this Calabrian form also clusters with H. pygmaea, and may have arisen through past introgression between a reflexa-like ancestor and H. pygmaea following the connection of the Calabrian block with the nascent Apennines in the Pliocene. The degree of genetic divergence seen between H. pygmaea and H. reflexa suggests that they diverged approximately 5.5 Ma in the Miocene, following the isolation of Corsico-Sardinian and mainland populations at the end of the Messinian.

由大陆地体形成的古老岛屿的生物群通常比年轻的海洋岛屿具有更复杂的生物地理历史，包括在不同时间尺度上因继承和/或殖民而起源的类群。科西嘉岛、撒丁岛和地中海托斯卡纳群岛的第勒尼安群岛有着复杂的地质历史，自中生代以来，受非洲板块和欧亚板块相互作用的影响。对于包括淡水无脊椎动物在内的许多群体，我们对第勒尼安特有物种的历史生物地理学的了解仍然有限。在这里，我们使用源自线粒体和核 DNA 序列数据的时间校准系统发育学来探索第勒尼安特有的九头蛇水甲虫的进化历史，该甲虫是岛屿溪流中具有重要生态意义的群体。虽然自渐新世科西嘉岛-撒丁岛与欧洲大陆最初分离以来，似乎没有出现地方性九头蛇，但我们表明，第勒尼安物种起源于五次殖民事件，这些事件在过去大约 10 年的不同时间间隔发生。 15 Ma，至少有两个地方性谱系显然是在麦西尼亚盐度危机结束时通过隔离而产生的。Hydraena reflexa Rey (1884) 长期以来被认为是广泛分布的H. pygmaea Waterhouse (1833)的地理形式或亚种，现已恢复为有效物种，为科西嘉岛和撒丁岛特有。H. reflexa可以在外部和阳茎特征上与H. pygmaea 区分开来，这里有详细记录。来自意大利大陆南部的“ H. reflexa ”标本与科西嘉岛和撒丁岛的标本不同，其雄性生殖器与其分布范围内其他地方的H. pygmaea相同。从遗传学上来说，这种卡拉布里亚形式也与H. pygmaea聚集在一起，并且可能是在卡拉布里亚地块与上新世新生的亚平宁山脉连接之后，通过过去反射样祖先和H. pygmaea之间的基因渗入而出现的。 H. pygmaea和H. reflexa之间的遗传分化程度表明，在麦西尼亚末期科西科-撒丁岛种群与大陆种群隔离之后，它们在中新世分化了大约 5.5 Ma。