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Two field studies involving periodic sampling of the surface waters of the upper reaches of the Fowl River estuary in southwestern Alabama were completed to describe temporal changes in the densities of selected species of crustaceans larger than 505 flm. Regardless of tidal phase, triplicate 5·min surface tows collected very few crustaceans during the day, while nighttime zooplankton samples showed much higher densities of the amphi· pods Gammarus tigrinus, Corophium lacustre, Grandidierella bonnieroides, the isopod Munna reynoldsi, the cumacean Almyracuma proximoculi and the mysids Taphromysis spp. These results strongly indicate nocturnal vertical migration by crustaceans that are traditionally considered benthic. In addition, these species showed significantly higher densities near the water surface during nocturnal flood tides than during nocturnal ebb tides, indicating tidal vertical migration. These crustaceans are reported to inhabit low-salinity areas, and a transect along the length of this estuary showed relatively higher densities of these crusta· ceans in the lower-salinity waters upstream than in the higher-salinity waters downstream. While the adaptive value of vertical migration for an otherwise benthic organism is not clear, the nocturnal and tidal timing of such a migration appears to provide these oligohaline· mesohaline crustaceans with a behavioral mechanism that generally retains them in the upper reaches of the estuary with minimal exposure to visual predation in the water column. [

Nocturnal and Tidal Vertical Migrations of "Benthic" Crustaceans in an Estuarine System With Diurnal Tides