Coral reefs are important hot spots of biodiversity, but despite the problems related to their conservation, studies on meiobenthic biodiversity are scarce. This paper provides a preliminary overview of the meiofauna and nematodes inhabiting coral sediments in the outer reef and lagoon habitats of the Maldives, and investigates their relationships with micro-habitat type. The abundance and community structure of the meiofauna and nematodes reveal significant differences between stations. Coral fragments and rubble from the outer reefs seem to act as a trap for the finest sediment, which might contribute to creating a heterogeneous micro-habitat suitable for both epifaunal (Epsilonematidae and Draconematidae genera) and sediment-dwelling (Tricoma, Richtersia, Ptycholaimellus and Molgolaimus) taxa. On the other side, the lagoon’s sediments are mainly colonized by dwelling taxa, probably due to the low-flow regime that enhances the deposition of organic matter and pelite fractions. A high level of diversity was recorded in both the habitats studied. Among the factors that probably determine the high diversity in the lagoon is the presence of small biogenic structures. These are characterized by a high degree of angularity, which might add more complexity to the habitat. The nematodes reveal an overlap in the taxonomic composition between the Maldives and other geographically distant areas, possibly supporting the existence of iso-communities that are typical of the coral degradation zone. Nematode trophic composition reflects differences in availability of food resources; epigrowth and non-selective deposit feeders are dominant in the lagoon, while selective deposit and epistrate feeders are dominant in the outer reefs

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The effects of the human impact on the coastal environments and relative biota can be different and even greater than those derived by natural fluctuations. The major disturbance causes in the coastal systems are rivers that may be important sources of nutrients and pollutants, particularly in a semi-closed basin such as the Adriatic Sea. In this context, we investigated the distribution of both meiobenthic and nematode assemblages in the Central Adriatic Sea to evaluate whether and how they are influenced by riverine discharges and which faunal descriptors are the most effective in detecting this type of stress. On the basis of our results, the disturbance effects in the studied area might be caused by both local streams and Po plume, and even if the latter might be considered of lesser extent, it is worthy to note its real impact at a short distance from the coastline. Meiobenthic assemblage structure appears as a good tool for detecting short-term responses of the benthic domain, whereas nematode assemblages seem more useful for defining long-term responses. Accordingly, the former highlighted from poor to sufficient ecological quality status (EQS) of this area, whereas the latter from moderate to bad EQS. Life strategy traits prove to be the most informative faunal descriptor due to their high correspondence with the environmental variables and particularly with this type of disturbance.

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Huvadhoo Atoll is a little-known and generally uncontaminated atoll of the southern Maldives, although the human pressure is increasing. This study represents the first attempt to characterize the meiofaunal and nematode assemblages of its lagoon both from a taxonomic and functional point of view. The nematode assemblage was made up of a total of 131 genera in 33 families. Desmodoridae, Chromadoridae and Xyalidae represented the richest and most abundant families, followed in terms of abundance by Selachinematidae and Comesomatidae. The nematode richness was overall higher than that reported in the previous studies carried out in the Central part of the archipelago. The diversity patterns revealed higher values than those reported for the back-reefs platforms, so confirming the positive influence of the water depth on the biodiversity of the nematode assemblage. The statistical analysis highlighted a significant taxonomic difference of the assemblages between the stations characterized by fine and medium-coarse sands, respectively, in line with the auto-ecological preferences of the taxa detected. The use of some nematode descriptors for assessing the ecological quality status (EQS) of the lagoon has revealed a slight disturbance in the station close to Viligili, one of the most urbanized islands. However, the summarization of all the descriptors used allows the highlighting of the good EQS of the Huvadhoo lagoon. Thus, the results of this study may be taken as the starting point for the future monitoring of the potential and real impact of the anthropogenic activity on the area over time.