The colonisation of the Tyrrhenian Islands by Hydraena water beetles, with Hydraena reflexa Rey, 1884 reinstated as a valid species endemic to Corsica and Sardinia (Coleoptera, Hydraenidae)

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.

Automated summary

The biogeographical history of the Tyrrhenian Islands and the evolutionary history of their endemic Hydraena water beetles are explored using a time-calibrated phylogeny derived from mitochondrial and nuclear DNA sequence data. The results show that Tyrrhenian species of Hydraena stem from five colonisation events occurring at different intervals in the last 15 million years.