Classification of the Intertidal and Estuarine Upogebiid Shrimps (Crustacea: Thalassinidea), and Their Settlement in the Ryukyu Islands, Japan

Classification of the Intertidal and Estuarine Upogebiid Shrimps (Crustacea: Thalassinidea), and Their Settlement in the Ryukyu Islands, Japan

Upogebiid shrimps (Crustacea: Thalassinidea) inhabit sandstones exposed on the intertidal zone of two of the islands of the Ryukyu Arc, Japan (islands located between 24°N, 123°E and 28°N, 129°E). The upogebiids here are morphologically similar; so, their classification is open to debate. In addition, when the upogebiids settled on the Ryukyu Arc has not been studied till date. Based on morphological features, we suspect that six stone upogebiid species are distributed in the Ryukyu Arc. This hypothesis was tested by molecular phylogenetic analysis of mitochondrial DNA markers. Furthermore, Bayesian evolutionary analysis, which was used to estimate TMRCA (time to most recent common ancestor) of each upogebiid, showed that stone upogebiids diverged 5-15 Ma (second half of the Miocene). Stone upogebiids may have occurred in the islands near the equator 5-15 Ma. On the contrary, Bayesian analysis indicated that the time of ‘intraspecific’ divergence varied from 1.15 Ma to 0 Ma, suggesting that stone upogebiids may have settled in the Ryukyu Arc from the Pleistocene to the present. In Iriomote Island, sedimentary layers suitable for burrowing occurred on the shores from the west to the northern half of the island, but not on the east. The Ryukyu Arc was a passive continental margin until the beginning of the Pleistocene, but the warm Kuroshio current completely separated the Ryukyu Arc from the Eurasian continent about 1.55 Ma. With subduction of the Ryukyu Arc northwestward, sedimentary layers for burrowing may have appeared on the ‘front’ of the island (i.e. on the shores from the west to the north), thereby providing suitable habitats for the stone upogebiids to settle, since 1.15 Ma.