We conducted field experiments in Illinois Caverns which indicated that Gammarus troglophilus Hubricht and Mackin (Amphipoda: Gammaridae) and Gammarus acherondytes Hubricht and Mackin, the federally endangered Illinois Cave Amphipod, both preferred larger (12.7 to <50.8 mm) gravels over smaller (2.36 to <12.7 mm) gravels in individual trials, and this preference did not vary significantly with size of the amphipods.

We also investigated microhabitat usage of the amphipods as determined by substrate size distributions and densities of other invertebrates in monthly sampling during a year-long study. Stream gravel size distributions varied among the four cave study sites, as did the composition and abundance of the community of crustaceans and other invertebrates. Variations in abundance and size of the amphipods Crangonyx forbesi (Hubricht and Mackin) (Amphipoda: Crangonyctidae), G. acherondytes and G. troglophilus were explained in part by time of year, indicating that the reproductive activity of these amphipods is influenced by seasonal factors. Gravel substrate characteristics and the densities of other taxa, especially the troglophilic isopod Caecidotea brevicauda (Forbes) (Asellidae), also were important in explaining the distribution of the amphipods.

Gammarus acherondytes, G. troglophilus, and C. forbesi species pairs often co-occurred in samples, suggesting commensal and/or predator-prey relationships, or concentration of animals around limited resources (e.g., food, shelter).

Various characteristics of the four study caves were correlated with the relative success of the Illinois Cave Amphipod, but causal relationships could not be demonstrated from the available data. Organic enrichment, pH, and dissolved oxygen were among the factors implicated as being potentially important in explaining the relative abundance of G. acherondytes at the four study sites.