Size-dependent interactions in salt marsh fish (Fundulus heteroclitus Linnaeus) and shrimp (Palaemonetes pugio Holthuis)

Competition and predation are major biotic interactions that influence the structure of aquatic communities. The relative importance of each type of interaction is usually a function of the relative size of the organisms involved. This study used field experiments to assess potential size-dependent interactions among small juvenile killifish (Fundulus heteroclitus) and grass shrimp (Palaemonetes pugio) and larger adult killifish. Density, species combinations, and predation risk were manipulated within field enclosures on an intertidal salt marsh on Sapelo Island, Georgia. Intraspecific and interspecific competitive effects between small juveniles (larval killifish and postlarval grass shrimp) were examined in the absence and presence of predators (larger adult killifish) to determine interactions and modifications of interactions due to predators. In the absence of predators both negative intraspecific and interspecific effects occurred in the form of reduced growth of juvenile grass shrimp. Survival of juvenile shrimp was the same in the presence or absence of juvenile killifish. However, when larger adult killifish were present, the interspecific competitive effect of reduced growth in juvenile grass shrimp was not significantly different from controls. Growth and survival of juvenile fish were unaffected by increased density of conspecific juveniles and the presence of juvenile grass shrimp and adult fish predators. These results provide field evidence for interspecific competitive interactions between juveniles of two of the most abundant species of nekton utilizing salt marsh habitat along the eastern coast of North America. However, when larger adult killifish predators were present these interspecific competitive effects among similar sized juvenile killifish and grass shrimp disappeared.  1999 Elsevier Science B.V. All rights reserved.