Effects of Macrophytic Cover on the Distribution of Grass Shrimps, Palaemonetes pugio and P. vulgaris

Abstract

Experiments were conducted to evaluate the effect of different macrophytes on the distribution of grass shrimps. Populations of Palaemonetes pugio and P. vulgaris were placed separately, then together, in an experimental (aquarium) environment comprising equal areas of four microhabitats: an area of unvegetated gravel, an area containing plastic plants resembling and areas with live thalli of the flat green alga Ulva or the branched green alga Codium. The distribution of individuals of P. vulgaris among the microhabitats (3% on bare gravel, 10% on Ambulia, 15% on Ulva, 63% on Codium) did not change significantly when this species was mixed with P. pugio. The distribution of P. pugio alone was much like that of P. vulgaris, but it did change significantly when P. vulgaris was present (from 3 to 7% on gravel, 10 to 16% on Ambulia, 19 to 29% on Ulva, 63 to 44% on Codium). Characteristics of the macrophytes (physical complexity, availability of food items) and the shrimps (residual predator conditioning, interspecific agonistic contacts) are suggested as factors in the distributions observed. Additional key words: algae, Decapoda, coexistence Palaemonetes pugio HOLTHUIS 1949 and Palaemonetes vulgaris (SAY 1818), commonly known as grass shrimps, are closely related sympatric species inhabiting shallow water, often in great numbers, from Cape Cod Bay to the Gulf of Mexico (Anderson 1985). This abundance combines with their rather similar morphology to stimulate interest in the nature of coexistence between these two species. Circumstances that may contribute to coexistence of closely related species are superabundance of resources (Slobodkin 1961) and resource or habitat partitioning (MacArthur 1972; Schoener 1974). The latter may be expressed in the field as a spatial separation of grass shrimps into different microhabitats, as evidenced by different species composition of samples from sites separated by only a few meters (Chambers 1982; Knowlton et al. 1994). Two potentially important microhabitat differences that might serve as the basis of spatial separation among these demersal animals are variation in substrate (Knowlton et al. 1994; Khan et al. 1995) and presence and character of different macrophytes. Macrophytic algae and flowering plants (e.g., Zostera marina) are important to many marine invertebrates by providing foraging sites, protection from predators, a Present address: Dept. of Biology, Armstrong Atlantic State University, Savannah, GA 31419, USA. and nursery grounds (Weinstein 1979; Heck & Thomas 1981; Nelson 1981; Rozas & Odum 1987). Differences in macrophytic cover may contribute to habitat partitioning, but few published accounts (Coen et al. 1981; Ferrell & Bell 1991) demonstrate this with controlled experiments. Physical separation of different species may also result from behavioral dominance of one species by another. Aggressive behavior by one species may force another species out of (or prevent entry into) a particular microhabitat. In studies of agonistic behavior, Chambers (1981) showed that P. vulgaris generally dominated P. pugio, using antennae and second chelae to displace the latter species from an area. Thorp (1976) found that P. vulgaris displaced P. pugio from a preferred oyster-shell substrate that offered greater protection from predation. To determine if available macrophytic cover affects distribution of P. pugio and P. vulgaris, we offered experimental laboratory populations of each species a choice of three different macrophytic microhabitats and an uncovered one, in the absence or presence of the other shrimp species.