Inland salt marshes are a rare habitat in North America. Little is known about the invertebrates in these habitats and their ability to cope with the brackish conditions of the marsh. We studied the population growth of ostracods found in an inland salt marsh (Maple River salt marsh) and of copepods found in the wetland habitat immediately adjacent to the freshwater Kalamazoo River. By studying these species in water from both habitats, we aimed to find out if they performed differently in the two habitats. We also tested Daphnia pulex in water from the two habitats due to the history of Daphnia spp. as model organisms. We found that copepods performed better in water taken from the Maple River salt marsh, and the ostracods and D. pulex performed equally well in either water. This was unexpected, since ostracods are found in the salt marsh and copepods in the freshwater area. As a second experiment, we tested the invertebrates in pairwise interactions. In water from the Kalamazoo River, ostracods outperformed the other two species, but there was no difference between D. pulex and copepods. No species outperformed the other in salt marsh water. Our results show no local adaptation to salinity, suggesting that ostracods and copepods may be limited in their respective distributions by dispersal limitation or habitat suitability.