In short- or long-term relationships, size does matter: body size patterns in the Mesostigmata (Acari: Parasitiformes)

We examined a dataset of body sizes comprising 3209 species of Mesostigmata to test two hypotheses regarding body size patterns: that species living in symbiotic relationships differ in size compared with those that are free-living, and that mite species show geographic body size gradients. We found that Mesostigmata in symbiotic relationships are considerably larger than those that are free-living, but the type of symbiosis (commensal versus parasitic) had little influence on body size. We also found that free-living mite species fit a Bergmann size cline (larger species in cool climates), but mite species in commensal relationships show a converse Bergmann size cline (larger species in warm climates) while no pattern was found for parasitic species. These results fit a resource-based explanation for both patterns. We hypothesize that animals that use more easily obtainable resources grow larger, through resource pulses in temperate zones for free-living mites and use of concentrated resources for commensal and parasitic species. Available microhabitats could also promote smaller sizes in free-living Mesostigmata: i.e. soil and plants offer more and smaller microhabitats to exploit and are hence occupied by smaller species.