Two types of earthworms, Eisenia fetida and Eisenia andrei, were studied to see if they are different species. The researchers found that these two earthworms are indeed separate species with different characteristics. Eisenia andrei is better for vermicomposting and grows and reproduces faster. Contaminants may affect the two species differently, so they should be kept apart in studies. The earthworms have a way to prevent mating between the two species, which affects their population dynamics.