Is the ubiquitous distribution real for marine gastrotrichs? Detection of areas of endemism using Parsimony Analysis of Endemicity (PAE)

Abstract Geographical patterns in microorganisms' distribution is a matter of intense debate. Following the assumption that “everything is everywhere” (EiE hypothesis), it is expected that microscopic animals do not have any scale spatial pattern of distribution due to high abundances, resting stages, and long-distance dispersal. Gastrotrichs are one of the most common components of the marine soft-body meiofauna. To assess the validity of EiE hypotheses on marine gastrotrichs, we applied Parsimony Analysis of Endemism (PAE) to identify areas of endemism. We used provinces of coastal and continental shelves of the world as operational units to define the distribution of 458 marine species of Gastrotricha. We found 10 areas of endemism, most of them were recorded from Europe and North America, and some areas were also consistent with the distribution of other benthic groups. An area of endemism is obtained when at least two species are perfectly congruent in their distribution. Moreover, the state of knowledge of the true species distribution is directly related to the intensity and spatial variation of the sampling effort. Thus, it is not a surprise to note that most areas of endemism are in the North Hemisphere, where gastrotrichs have been especially and longer sampled and their geographic range size is well known. On the other hand, the absence of endemism areas in South Hemisphere is related to sampling insufficiency. However, some level of endemism was found and the ubiquitous distribution of gastrotrich species is strongly questioned.