Diel Variation in Immigration of Fishes and Decapod Crustaceans to Artificial Seagrass Habitat

Fish and decapod entry into small (1.5 m2) artificial seagrass habitats positioned on an open sand area in a New Jersey estuary was examined to determine if immigration varied between day and night. To encounter the structured habitats, colonizers had to cross an expanse of bare sand, with its presumably higher predation risk. Contrasts in abundance in the artificial seagrass plots between dawn and dusk indicated higher nighttime immigration for four species, including the fishes Fundulus heteroclitus and Myoxocephalus aenaeus, and the caridean shrimps Palaemonetes vulgaris and Hippolyte pleuracanthus. Size-frequency distributions of colonizers varied between day and night for two fish species, Menidia menidia and Syngnathus fuscus, with a greater proportion of smaller individuals immigrating to the artificial seagrass at night. Callinectes sapidus also displayed a diel contrast in size distribution but, for this species, proportionately more small individuals colonized the plots during the day. We suggest that diel variability in predation risk and/or diel patterns in motor activity may be responsible for these patterns in immigration.