Deep-Sea Phylogeographic Structure Shaped by Paleoenvironmental Changes and Ongoing Ocean Currents Around the Sea of Japan in a Crangonid Shrimp, Argis lar

Small vs. large eggs: comparative population connectivity and demographic history along a depth gradient in deep-sea crangonid Argis shrimps

The crangonid shrimps Argis hozawai, A. lar and A. toyamaensis, co-distributed in the Sea of Japan, exhibit intriguing differences in geographical and bathymetric distributions and in reproductive biology. Argis hozawai (150–250 m depth) and A. lar (200–300 m) are broadly distributed in the north-western Pacific Ocean and spawn relatively large numbers of small eggs, whereas A. toyamaensis (250–2000 m) is distributed in the Sea of Japan and spawns a small number of large eggs. We examined the relationship between egg size and dispersal patterns in the deep sea by comparing genetic population structures using mitochondrial DNA sequence variation. We found little or no genetic divergence within the Sea of Japan for A. hozawai and A. lar, whereas there was a slight but significantly higher genetic differentiation in A. toyamaensis. This suggests that A. toyamaensis has lower dispersal ability than A. hozawai and A. lar, and therefore might maximize larval survival through larger size at hatching, with either direct or abbreviated larval development, to adapt to the deep-sea environment in the Sea of Japan. We also detected the effects of drastic environmental changes during the Pleistocene glacial periods on their demographic processes in the Sea of Japan.

Unexpected low genetic differentiation between Japan and Bering Sea populations of a deep-sea benthic crustacean lacking a planktonic larval stage (Peracarida: Tanaidacea)

Information on the extent, diversity and connectivity of populations is lacking for most deep-sea invertebrates. Species of the order Tanaidacea (Crustacea), one of the most diverse and abundant macrofaunal groups in the deep sea, are benthic, lack a planktonic larval stage, and thus would be expected to have narrow distributional ranges. However, with molecular evidence from the COI gene, we show here that the deep-sea tanaidacean Carpoapseudes spinigena has a distributional range spanning at least 3700 km, from off northern Japan to the south-eastern Bering Sea. Living individuals found in a sediment core indicated that the species is a sedentary burrower. COI analyses revealed a low level of genetic diversity overall, and low differentiation (p-distance, 0.2–0.8%) between the Japan and Bering Sea populations. One hypothesis to explain the low genetic diversity over a broad region is that the Japan population was founded by individuals transported by ocean currents from the Bering Sea. However, due to limited data, other explanations cannot be ruled out. Our results indicate that continued sampling is of fundamental importance to understanding how genetic and taxonomic diversity originate and are maintained in the deep sea.

Evaluating the occurrence of cryptic invasions of a rocky shore barnacle, Semibalanus cariosus, between the north-eastern Pacific and Japan

Many coastal barnacles are introduced to non-native regions. However, data are lacking on cryptic invasion, which is defined as an invasion that remains unrecognised because the invader is mistaken for a native or previously introduced species or clade. In this work, cryptic invasions of an intertidal barnacle, Semibalanus cariosus, between Japan and the north-eastern Pacific were evaluated based on population genetic analyses. A significant genetic differentiation was found between the Japanese and north-eastern Pacific populations, suggesting a limited introduction of non-native genotypes between these regions. Haplotype frequencies did not differ significantly between the past (museum samples collected in 1971 from Hokkaido, Japan) and present Japanese populations, implying the rare occurrence of human-mediated migration from the north-eastern Pacific to Japan. Migrate-n analysis revealed a low level of directional gene flow in S. cariosus from the north-eastern Pacific to Japan, possibly by natural stepping-stone dispersal via directional water currents or human-mediated transport.