Nico Eisenhauer, Elina Eisenhauer

Earthworms are some of the most important and popular soil organisms. Their essential roles in ecosystems have not only been recognized by Aristotle, Charles Darwin, and many active scientists around the globe, but also by land managers, farmers, and gardeners. However, many people do not know how diverse earthworms are in terms of their form and function. Here we summarize the current knowledge of the taxonomic, morphological, physiological, reproductive, and functional diversity of earthworms, their action as so-called “ecosystem engineers”, and their many interactions with other organisms below and above the ground. The ~7,000 described species range from a few cm to a length of up to 3 m and are categorized into three major ecological groups that reflect their life style. Taken together, this overview will help appreciate the surprisingly high diversity of a supposedly well-known soil animal group.