Why Do Earthworms Vary in Size? A Study of Influencing Factors

We investigated the impact of various factors on the size and mass of eleven earthworm taxa, abundant in southern part of West Siberia. Earthworms were collected during in 2022 and 2023 in the West Siberian Plain, Gorny Altai, and Rudny Altai (Kazakhstan). We analyzed 1053 individuals from 256 samples at 24 test sites. To account for the impact of physical-geographical factors on body size, sampling sites were established in mountain, lowland, and intrazonal landscapes, across various natural zones and habitats. To account for edaphic factors, layered measurements of soil density, pH, electrical conductivity, temperature, moisture, and organic matter were performed in all soil samples. Variance analysis showed that body mass is influenced by gut cleansing and fixation type. Smaller taxa demonstrate more pronounced mass decrease after gut cleansing. Worms stretched when fixed in formalin and contracted in alcohol. The coefficients of length and width change after fixation significantly depended on earthworm species and the total length, except for width change during alcohol fixation. European invasive species were observed to be 1.1–3.3 times shorter in southern Western Siberia compared to Belarus. The key physico-geographical factors influencing earthworm size and mass were “habitat depth,” “habitat type,” “natural zone,” and “landscape.” The key edaphic factors influencing earthworm size and mass were “soil organic matter” and “soil temperature.”