Hiatella is one of the most widespread marine bivalve genera, occurring in diverse habitats from the temperate to polar latitudes in both hemispheres, and in fossil strata since almost 150 Myr ago. Despite variation in some biological and morphological traits, characters to resolve the current systematic structure consistently across the range of the genus are not known: all samples are often referred to a single species, Hiatella arctica (L.). Exploring the systematics of Hiatella using partial sequences of three genes (mitochondrial COI, and the nuclear ANT and 28S rRNA), we find high diversity of deep lineages (11–22% p-distance in COI), and identify at least 13 distinct taxa both by heuristic criteria (congruence of the nuclear and mtDNA data) and by coalescence-based analyses. At several localities, two or three of these cryptic species were found in sympatry. In the framework of previous fossil evidence and of hypotheses of paleoceanographical connections, scenarios of the phylogeny and biogeographical history of the identified species at a range of different time scales are outlined. The distinction between the main North Pacific and North Atlantic Hiatella clades and systematic diversification within each of them seem to have followed a Miocene trans-Panamanian invasion. Apart from such earlier intra-basin diversification, the data suggest that three successive counter-invasions from the Pacific to the Atlantic via the Arctic Ocean route have later contributed to the current North Atlantic Hiatella diversity. These invasions probably took place in connection with (i) the Great Trans-Arctic Biotic Interchange in the Pliocene, (ii) the last interglacial period c. 120 kya and (iii) the Holocene, postdating the last glaciation. This sequence of trans-Arctic invasions is largely analogous to that hypothesized for some other boreal-arctic bivalves.
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