Does the Dispersal of Fairy Shrimps (branchiopoda, Anostraca) Reflect the Shifting Geographical Distribution of Freshwaters Since the Late Mesozoic?

Order Anostraca (fairy shrimp of large branchiopods) is a primitive crustacean group, retaining ancient forms and ecology. The Holarctic family Chirocephalidae originated over 100 million years ago; it is a very long-lived freshwater taxon that has survived from the Mesozoic to the present. Thus, using this taxon as an indicator, we verified how the geographical distribution of freshwaters shifted during the ancient era. We used newly collected samples of Drepanosurus uchidai from Aomori (northern Japan) and Galaziella baikalensis and Branchinecta orientalis from Olkhon Island (the largest lake-bound island of Lake Baikal, Russia) to sequence 658 bp of COI gene. These sequences, plus those of 16S rRNA gene (~ 550-bp mt16S rDNA fragment), were compared with those retrieved from GenBank. To re-evaluate and clarify phylogenetic relationships among the Chirocephalidae that remains confused till now, six genera of the family, including Polyartemiella, Drepanosurus, Eubranchipus, Chirocephalus, Artemiopsis, and Galaziella species were used for molecular analyses. Small water bodies usually have comparatively short lives and fade away sooner or later due to growth of aquatic plants and accumulation of bottom sediments. In such environments, refugia formed on the shores of large-scale lakes were necessary for large branchiopods to survive several Ice Ages. The lake shorelines have moved with the growth or decline of the lakes, but the habitats of large branchiopods sporadically left behind can now be confirmed as a history of the shifting geographical distribution of global freshwaters. For example, two types of large branchiopod populations from island-bound water bodies on Olkhon Island are recognized: (1) populations on the northwestern coast that are closely related to the group in the Mongolian Gobi steppe region, and (2) populations in other areas distant from the coast that are highly endemic to the island. Based on fossil records and genetic distances, an absolute differentiation time of world chirocephalids can be estimated as 140 million years ago in the Mesozoic era. On the other hand, the age of Lake Baikal is only 25‒30 million years at the most. Therefore, extant large branchiopods of Olkhon Island must have first appeared near the present lake catchment after separation from their ancestral populations that had originated in Europe, and before the formation of Lake Baikal.