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.