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Meiofauna (small interstitial animals and protists living in aquatic sediments) are ubiquitous. Owing to their high abundance and diversity, widespread distribution, rapid generation times and fast metabolic rates, meiofaunal organisms are important contributors to ecosystem processes and functions, including nutrient cycling and provision of food to higher t rophic leve ls , among others (Woodward 2010; Schratzberger and Ingels 2017). Meiofauna, however, have typically received less attention than the larger and more easily studied macroand megafauna, and more needs to be done to train the next new generation of meiobenthologists. In order to bridge this gap, the second meiofauna summer school, BMeioScool2016 a dive in a microscopic world^, was held in Brest, France, 27 June −1 July 2016, and attracted 131 scientists and students from 26 countries (Fig. 1; https://meioscool2016.sciencesconf.org/). Like the first summer school held in 2013, the objectives of MeioScool2016 were to bring together meiofaunal experts in several meiofauna-related sub-disciplines from all over the world to: (1) increase awareness of researchers, students and general public to the fundamental role of meiofauna in marine ecosystems from the coastal zone to the deep sea, and (2) train students and researchers in the identification and description of meiofaunal communities through several complementary disciplines (taxonomy, ecology, molecular biology). The first 2 days were devoted to oral and poster presentations by invited speakers, researchers, and students, while the three other days consisted of practical workshops and field and laboratory work. Two sessions of this summer school were devoted to the meiofauna of extreme and changing environments: BSession 4: Deep-sea and extreme meiofauna^ and BSession 5: Meiofauna response to anthropogenic impacts^. Extreme environments (i.e., environments characterised by one or more environmental parameters permanently close to the lower or upper limits for life; CAREX 2011) cover more than 50% of the Earth’s surface (Zeppilli et al. 2017), and offer many opportunities for investigating the biological responses and adaptations of organisms to stressful life conditions (Rothschild and Mancinelli 2001). The fauna adapted to extreme environments may be particularly sensitive to environmental changes (Catalan et al. 2006; Bellard et al. 2012) and can be used as biological indicators of pollution and global change (Zeppilli et al. 2015). Among the communities present in extreme natural environments, we constantly find meiofauna organisms. This Special Issue of Marine Biodiversity comprises studies and reviews related to extreme environments including taxonomy of extreme meiofauna, and biodiversity and ecology of meiofauna in extreme or changing environments (Table 1). The first contribution to this Special Issue provides a description of a new marine arthrotardigrade species (GomesJúnior et al. 2017). Ligiarctus alatus sp. nov. was discovered in sediments of the Brazilian continental shelf in the Southwestern Atlantic Ocean, from sites located in the major oil extraction basins in Brazil (Campos and Potiguar basins). The review of Rosli et al. (2017) describes trends in the ecology of deep-sea meiofauna with focus on patterns and processes at small to regional spatial scales described in studies published since the last review of deep-sea meiofauna of Soltwedel (2000), and highlights areas needing further research. Zeppilli et al. (2017) present an integrated review of the biodiversity, ecology and physiological responses of marine meiofauna inhabiting extreme marine environments, including mangroves, submarine caves, polar ecosystems, hypersaline areas, hypoxic/anoxic environments, hydrothermal vents, cold seeps, carcasses/sunken woods, deep-sea canyons, * Daniela Zeppilli [email protected]