The vertical migration of the first stage of five diadromous atyids (Caridina multidentata, C. typus, C. leucosticta, C. serratirostris, Paratya compressa) and two diadromous palaemonids (Macrobrachium japonicum, M. formosense) and a non-diadromous palaemonid shrimp (M. nipponense) and second zoeae of C. multidentata and M. formosense were observed under different light and salinity conditions. The first and second zoeae spread over the whole of the water column during darkness, and sank to the bottom under the light independent of the various salinity and light conditions, though there were slight differences among species. The swimming ability of the zoea is limited, thus this vertical migration is considered to be not useful for homing migration. This vertical migration at night is considered to be useful in order to feed on organic material and avoiding predators on the bottom.

To clarify the effects of intertidal groundwater discharge on macrofaunal community structures of tidal flats, the species richness, abundance and biomass of macrobenthos at the groundwater-seepage area were assessed quantitatively, measuring environmental conditions including sediment temperature, median diameter, ORP and ignition loss, and salinity, water temperature and nutrient concentration of pore water at four sites (Nakatsu River mouth, Sanbyakuken, Higashihama, Oshinden) on the Nakatsu tidal flat, Oita prefecture, Japan, at low tides in June and July, 2019. A total of 1,233 individuals, belonging to 51 macrobenthic species (37 families) were collected. Mean species richness and biomass were significantly greatest at Higashihama, whereas mean abundance was greatest at the Nakatsu River mouth. Lower species richness, abundance and biomass were recorded at Sanbyakuken. At Higashihama, infaunal species such as the bivalve Glauconome angulata and polychaete Ceratonereis erythraeensis were dominant in the lower salinity and higher nutrient environments. A canonical correspondence analysis showed a positive correlation between nutrients and dominant bivalve abundance. The overall results suggested that intertidal groundwater influences the macrofaunal community structure, providing low salinity and nutrient-rich conditions, which contribute to the high productivity and diversity of marine organisms on tidal flats.