The application of molecular tools has led to the discovery of substantial and unexpected genetic and species diversity in natural communities. Cryptic species, which are reproductively isolated from each other but phenotypically indistinguishable, have been identified in a wide range of taxa and biogeographic regions. The frequency of cryptic species is still unknown, but they may represent an important component of the world’s biodiversity. However, we still know little about how these species interact. For example, the ecological similarity of cryptic species may reduce the strength of stabilizing interspecific interactions. Alternatively, subtle niche differences may be sufficient to produce stabilizing density dependence. Hyalella amphipods were once thought to comprise a single species, but are now known to encompass at least 30. In the state of Michigan there are at least 4 different species, 3 of which are morphologically indistinguishable (species “A”, “B”, and “C”). We surveyed ten lakes in southwest Michigan to determine how commonly Hyalella species coexist, and if they differ in their patterns of habitat use within lakes. We used species-specific PCR primers to genotype all individuals collected.

Results/Conclusions

Coexistence of cryptic Hyalella amphipods is widespread throughout southwest Michigan. We found that all three species were present in 9 out of 10 lakes examined. Additionally, we sampled separate habitats within each lake: shallow nearshore waters, surface waters above macrophyte beds, and deeper waters within macrophyte beds. All three species were found in each of these habitats, however, not in all lakes, and all three taxa had different distribution patterns across the three habitat types. Overall, the most common species in almost all the lakes was found mainly in shallow waters above macrophyte beds; a second species was mainly found in nearshore waters; and the third species was mainly found in deeper waters within the macrophyte beds. Thus, there may be a role for surprisingly fine-scale habitat partitioning within lakes even among these extremely similar cryptic species that widely coexist.