How temperature affects the body size of terrestrial tardigrades

Many vertebrates, both homeo- and poikilothermic, show a significant relationship between body size and environmental temperature. Whether such an association may exist in microscopic invertebrates has been less explored. Therefore, we decided to analyse terrestrial Tardigrada from various habitats worldwide to examine whether these animals reveal any relationship pattern between body size and environmental temperature. Data on minimum, maximum, and mean body sizes were extracted from original descriptions or sometimes from later re-descriptions of the species. Minimum, maximum, and mean temperature data from the type localities of the species were retrieved from WorldClim 2. In general, accounting for geographic and phylogenetic confounding factors, the body size of terrestrial tardigrades decreased as the environmental temperature increased. The same tendency was observed for most of the genera when additional analyses were carried out separately for each genus. This is the first biogeographical analysis demonstrating that terrestrial tardigrades generally conform to the temperature–size rule.