This study aims model the distribution of meiofauna indicators in relation to environmental variables from the Santos Basin continental margin, SE Brazil, using machine learning techniques, to provide baseline information and foster future monitoring programs. A total of 100 sampling stations were distributed in eight transects and 11 isobaths (25 to 2,400 m) perpendicular to the coast. In each station, three replicates were sampled for meiofauna and 38 environmental parameters. A total of 28 meiofauna taxa were found, with a mean richness varying from 3 to 15 taxa per station. Meiofauna mean density varied between 55 and 2,001 ind. 10 cm-2. Density of meiofauna and its most frequent taxa (Nematoda, Copepoda, Kinorhyncha, and Polychaeta), and taxa richness were used as descriptors for the models. Meiofauna and nematode density showed the highest training and testing accuracies, with R² values above 0.74. Based on the distribution of meiofauna descriptors and their responses to environmental conditions, we suggest a mosaic of six benthic zones. The La Plata Plume zone and the Cabo Frio Upwelling zone are two of the most diverse and productive zones in the continental shelf, wich are separated by the less productive Central Continental Shelf zone. A fourth zone, with very low meiofauna densities, corresponds to the carbonated sediments of the shelf-break. The Upper and Mid-Slope is a narrow zone along the entire basin, with intermediate densities and small amounts of high-quality organic carbon. The largest, impoverished zone, the Lower Slope and Plateau comprises the deepest areas and the São Paulo Plateau. The study showed that, although some zones can be recognized by most meiofauna descriptors, others are better characterized by specific ones, implying that meiofauna indicators should be monitored concomitantly. We recommend the optimization of sampling design based on our model to reduce costs and increase our understanding of the system.

In the current study we provide a reviewed list of valid genera and species of Xyalidae, a widespread family of mostly marine free-living nematodes. Comments are added about the historic background and taxonomic situation of the family, all valid genera and, when necessary, diagnostic characters are given. Additionally, information about distribution and geographical location of species recorded along the Brazilian coast is provided. Our review recognized 46 valid genera, 450 valid species and 73 descriptions without enough morphological information for identification (species inquerendae). Nearly 80 % of the species inquerendae belong to Daptonema and Theristus. To avoid homonymies, two Daptonema species were renamed, Daptonema biwaensis (Tsalolikhin, 2002) new name (former Mongolotheristus timoshkini) and Daptonema vietnamensis (Gagarin and Thu, 2008) new name (former D. curvatum sensu Gagarin and Thu, 2008). Cenolaimus sapeloensis is transferred to Xyala sapeloensis comb. nov. Along the Brazilian coast 28 genera and 41 species have been recorded. The species Elzalia floresi, Metadesmolaimus tersus, Paramonohystera stricta, Pseudosteineria scopae, Rhynchonema cemae, R. veronicae, Steineria ericia, S. marcorum, S. pavo, S. tripartita, Theristus acribus, T. flevensis, T. macroflevensis, T. pertenuis, T. stranus, Trichotheristus heterus, T. setosus and Zygonemella striata have the Brazilian coast as the type locality. Among all species, three occurred across three geographic regions, while the large majority was restricted to one. Xyalidae is typically encountered in oceanic sandy beaches, with only the species belonging to Daptonema, Theristus, Trichotheristus and Zygonemella being recorded in estuarine sediments. This observation suggests that the colonization of inland waters occurred multiple times along the evolutionary history.

Species descriptions of the family Xyalidae, as well as of most marine nematodes, were in general largely made in the past century (e.g. Allgén, 1927; Cobb, 1920; Gerlach, 1957; Lorenzen, 1977). Many of these descriptions were based on one or two specimens or even on juveniles with relatively few features of taxonomic value. Lack of types and inaccuracy in sampling localities are other problems associated with taxonomy of marine nematodes. These issues, together with the fact that in the past researchers had slower exchange of information and reduced access to some journals, led to the multiplication of synonyms. To propose a new species of nematode, particularly within a genus with a convoluted historical background, requires caution and critical taxonomical review prior to the description (Adams, 2001; Fonseca & Decraemer, 2008).