Habitat preference, predation, and the intertidal distribution of gammaridean amphipods in a north carolina salt marsh

Abstract

Habitat islands designed to simulate small patches of Spartina wrack were used to sample intertidal amphipods in Tar Landing Marsh, North Carolina, U.S.A. Orchestia grillus Bosc, Orchestia sp. (an as yet undescribed saline salt marsh endemic), and Gammarus palustris Bousfield were the common species at the study site. In transect samples along a tidal gradient, Orchestia grillus occurred only at extreme high marsh sites, while Gammarus palustris colonized islands that were more frequently inundated by the tide. Orchestia sp. was intermediate in distribution with a tendency toward the high marsh habitat. Other investigators have suggested that predation by the killifish, Fundulus heteroclitus ( L.), maintains the high marsh distribution of Orchestia grillus. Transplantation experiments with sets of habitat islands placed at different vertical positions within a tidal gradient were used in the present investigation to determine whether the natural distribution of O. grillus could occur in the absence of predation. Results suggested that physical conditions associated with relatively frequent and prolonged tidal inundation make the low marsh an unsuitable habitat for O. grillus. The intertidal distribution of this amphipod appears to be independent of killifish predation in Tar Landing Marsh. Collections from naturally colonized habitat islands at different levels in the tidal gradient are consistent with the hypothesis that intertidal distributions of Orchestia sp. and Gammarus palustris are also determined primarily by preferences for specific physical conditions.