Vast numbers of grass shrimp (Palaemonetes spp.) occur in shallow water of estuaries and bays of the U.S. Atlantic and Gulf coasts (Anderson, 1985), where they congregate on areas of bare mud, sand and shell flats, and on wooden pilings (Knowlton et al., 1994), but they are particularly abundant in dense stands of underwater macrophytes (Orth et al., 1984). The two most common species of Palaemonetes associated with tidal marshes and creeks of the mid-Atlantic region are P. pugio and P. vulgaris (Williams, 1984). Because these two species are especially common in areas with macrophytic cover, the nature of the vegetation may be presumed to have a major influence on their abundance, distribution, and loss to predation. Indeed, distributional differences in relation to structural characteristics of macrophytic algae have been demonstrated in a laboratory setting (Khan et al., 1997).
Predation has been shown to influence the distribution and abundance of species within marine habitats (Kneib, 1988; Primavera, 1997). When predators are present, prey organisms frequently switch microhabitats, choosing a site where they are less vulnerable (Main, 1987; Everett and Ruiz, 1993). Both laboratory and field experiments have indicated that for many species of invertebrates, including Palaemonetes spp., macrophytic vegetation can provide protection from predators (Rozas and Odum, 1988; Tayasu et al., 1996). With the decline of submerged aquatic vegetation in deeper waters of Chesapeake Bay, many animals have been reported to shift from deeper water (>1 meter) to shallower conditions (<35 cm) along the shoreline where important structural features created by woody debris provide refuge from predation (Ruiz et al., 1993).
Interactions between prey species, especially as they influence the use of cover, may alter the impact of a predator upon those prey species. For example, Thorp (1976) felt that oyster shell provides protection from predation and that P. vulgaris is more resistant to predation because it displaces P. pugio from a shelly substrate.
The killifish Fundulus heteroclitus is a permanent marsh resident and is abundant in areas inhabited by Palaemonetes. Studies of stomach contents have indicated that grass shrimp are an important component of the killifish diet (Heck and Thomas, 1981; Overstreet and Heard, 1982).
The role of macrophytic covers in predation served as the focus of the present study whose primary purpose was to determine how the presence of F. heteroclitus quantitatively affects survival of P. pugio and P. vulgaris in the various experimental covers. The questions asked were: 1) Is predation pressure the same for both species of grass shrimp? 2) Does macrophytic cover reduce the amount of predatory loss for one or the other prey species of grass shrimp? 3) If macrophytic cover reduces loss to predators, are all kinds of macrophytic cover equally effective in reducing the loss? 4) Does the presence of the second prey species of grass shrimp enhance or lessen the predatory loss of the first prey species?