Ubiquity of microscopic animals? Evidence from the traditional morphological approach in species identification

Zoologists always hope to find weird and interesting new animals in exotic places. Over the last few centuries, scientific expeditions in remote places outside Europe have indeed discovered new species and even higher taxa of vertebrates, insects and other macroscopic animals, completely different from the ones previously known in the home country. In contrast, scientists working on microscopic animals, looking at samples from remote areas, have mostly found organisms that could be ascribed to species and genera already known in their home country. Microscopic animals have thus been considered not interesting in biogeography, as their distribution may not be limited by geography.Are microscopic animals really widely distributed? Is their cosmopolitism an actual biological property or only a common misconception based on false assumptions and evidence? Is the scenario more complex than the claimed clear-cut difference between micro- and macro-organisms? This chapter will review all the faunistic knowledge gathered so far on the global distribution of microscopic animals like gastrotrichs, rotifers and tardigrades, which are all smaller than 2 mm. Moreover, we will deal with microscopic free living species in other groups of animals like nematodes and flatworms, which have both micro- and macroscopic species. The focus will be on species identification from traditional taxonomy based on morphology, whereas the following chapter will deal with more recent evidence gathered from analyses on molecular phylogeny and phylogeography from the same groups.