Evolution and biogeography of <i>Litorogammarus</i> Marin, Palatov &amp; Copilaș-Ciocianu, 2023 (Amphipoda: Gammaridae) with description of two new Caspian endemic species

The extensive radiation of gammaroidean amphipods in the brackish Ponto-Caspian region gave rise to almost 100 species occupying a wide range of ecological niches and exhibiting striking morphological diversity. However, much of this fauna remains poorly known. Here, we examine the origin and evolutionary timescale of the genus <i>Litorogammarus</i> and describe two new endemic species from the Caspian Sea by integrating multilocus phylogenetics, fossil-based molecular clock calibration, and morphological observations. We show that the genus originated in the Paratethys during the Late Miocene and that its major lineages arose during the Pliocene when the Paratethys split into the Black and Caspian seas. Both newly described Caspian species (<i>L. glareophilus</i> sp. nov. and <i>L. samuricus</i> sp. nov.) split from their Black Sea endemic sister species during the last two million years, coinciding with two chronologically distinct periods of isolation following major transgressive episodes (Apsheronian and Bakunian/Chaudian). Our results further confirm the evolutionary importance of the recurrent isolation-reconnection episodes between the Black and Caspian seas throughout the Pleistocene, which we herein interpret as functioning like a “species pump”. http://zoobank.org/urn:lsid:zoobank.org:pub:41A6311F-09A8-47A3-91D0-01D409F8F7AB