Charles Darwin's Observations on the Behaviour of Earthworms and the Evolutionary History of a Giant Endemic Species from Germany, Lumbricus badensis (Oligochaeta: Lumbricidae)

The British naturalist Charles Darwin (1809–1882) began and ended his almost 45-year-long career with observations, experiments, and theories related to earthworms. About six months before his death, Darwin published his book on The Formation of Vegetable Mould, through the Actions of Worms, With Observations on their Habits (1881). Here we describe the origin, content, and impact of Darwin's last publication on earthworms (subclass Oligochaeta, family Lumbricidae) and the role of these annelids as global “ecosystem reworkers” (concept of bioturbation). In addition, we summarize our current knowledge on the reproductive behaviour of the common European species Lumbricus terrestris. In the second part of our account we describe the biology and evolution of the giant endemic species L. badensis from south western Germany with reference to the principle of niche construction. Biogeographic studies have shown that the last common ancestor of L. badensis, and the much smaller sister-taxon, the Atlantic-Mediterranean L. friendi, lived less than 10？000 years ago. Allopatric speciation occurred via geographically isolated founder populations that were separated by the river Rhine so that today two earthworm species exist in different areas. 1. Introduction In his Autobiography, Charles Darwin (1809–1882) briefly commented on his last major publication in the following words: “I have now (May 1, 1881) sent to the printers the MS. of a little book on The Formation of Vegetable Mould through the Actions of Worms. This is a subject of but small importance; and I know not whether it will interest any readers, but it has interested me. It is the completion of a short paper read before the Geological Society more than forty years ago, and has revived old geological thoughts” [1, page 136]. In a foot-note on the same page, Darwin's son Francis (1848–1925), who edited the letters as well as other documents after his father's death, remarked that “between November 1881 and February 1884, 8,500 copies were sold.” Charles Darwin's “little book” [2] later gave rise to the scientific concept of “bioturbation”, which can be defined as “the biological reworking of soils and sediments by all kinds of organisms, including microbes, rooting plants and burrowing animals” [3]. These ongoing activities of different soil (or sediment) organisms, which leads to a modification of geochemical gradients and the redistribution of organic substances, can be viewed as a kind of “ecosystem engineering.” Moreover, it is obvious that soil texture is modified and different soil particles are