From biggest to smallest mud dragons: size-latitude trends in a group of meiobenthic animals worldwide

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Size-latitude trends in the meiobenthic phylum Kinorhyncha, commonly known as mud dragons, have been explored in oceans worldwide. Generalized least squares regression was used to assess relationships between size and latitude, as well as between size, latitude, and two selected environmental variables that exhibit latitudinal gradation: the sea surface temperature and the net primary productivity. Different structures of spatial autocorrelation and potential confounding factors, such as the species richness and the number of kinorhynch records that could affect latitudinal gradients, were also addressed. In addition, generalized mixed models were used to determine the influence of the phylogeny on body size. Size-latitude relationships of Kinorhyncha were commonly found globally, as well as for particular geographic regions (hemispheres and/or coastlines), with important differences between taxonomic groups. These size-latitude trends were heterogeneous and implied the influence of the latitude itself, environmental variables, and phylogeny. These facts indicate that a single underlying process is not likely to explain the observed relationships but a complex interaction of several macroecological patterns both present and past. Perhaps, the inclusion of future new reports, conducted in undersampled areas, may shed some light on the matter and reveal more generalized size-latitude patterns. Nevertheless, it is also likely that broadly generalizable size-latitude relationships may not exist in meiofaunal communities.