Extreme marine environments cover more than 50% of the Earth’s surface and offer many opportunities for investigating the biological responses and adaptations of organisms to stressful life conditions. Extreme marine environments are sometimes associated with ephemeral and unstable ecosystems, but can host abundant, often endemic and well-adapted meiofaunal species. In this review, we present an integrated view of the biodiversity, ecology and physiological responses of marine meiofauna inhabiting several extreme marine environments (mangroves, submarine caves, Polar ecosystems, hypersaline areas, hypoxic/anoxic environments, hydrothermal vents, cold seeps, carcasses/sunken woods, deep-sea canyons, deep hypersaline anoxic basins [DHABs] and hadal zones). Foraminiferans, nematodes and copepods are abundant in almost all of these habitats and are dominant in deep-sea ecosystems. The presence and dominance of some other taxa that are normally less common may be typical of certain extreme conditions. Kinorhynchs are particularly well adapted to cold seeps and other environments that experience drastic changes in salinity, rotifers are well represented in polar ecosystems and loriciferans seem to be the only metazoan able to survive multiple stressors in DHABs. As well as natural processes, human activities may generate stressful conditions, including deoxygenation, acidification and rises in temperature. The behaviour and physiology of different meiofaunal taxa, such as some foraminiferans, nematode and copepod species, can provide vital information on how organisms may respond to these challenges and can provide a warning signal of anthropogenic impacts. From an evolutionary perspective, the discovery of new meiofauna taxa from extreme environments very often sheds light on phylogenetic relationships, while understanding how meiofaunal organisms are able to survive or even flourish in these conditions can explain evolutionary pathways. Finally, there are multiple potential economic benefits to be gained from ecological, biological, physiological and evolutionary studies of meiofauna in extreme environments. Despite all the advantages offered by meiofauna studies from extreme environments, there is still an urgent need to foster meiofauna research in terms of composition, ecology, biology and physiology focusing on extreme environments.

Owing to technical problems and difficult taxonomic identification, meiofauna have been generally less studied than macrofauna. However, the role of meiofauna in marine ecosystem functioning, and their effective and rapid response to anthropogenic alterations and climatic changes have recently been acknowledged, leading to increasing scientific and applied interest. At present, systematic and biogeographic knowledge of the meiofauna of the Adriatic Sea is extremely heterogeneous, because most of the data are limited to a few taxa and the sampled areas are scattered, being located mainly in the coastal areas of the northern basin. Analysis of the composition and distribution of meiobenthic groups in the Adriatic Sea highlights the presence of several endemisms. Meiofauna also include bioindicator taxa, which allow assessment of the quality of marine sediments; this is particularly useful in systems characterised by the synergistic effect of different forms of anthropogenic impact, such as the Adriatic basin. Current knowledge about the ecology of the meiofauna and use of this component in applied ecological studies, along with the availability of a standardised protocol for the analysis of meiofaunal assemblages, allows us to recommend formal acknowledgement of the need to integrate information derived from the analysis of macrofauna with information derived from the study of meiofauna. Future research based on the simultaneous use of both of these benthic components will allow faster and more accurate evaluation of the response of coastal marine ecosystems to anthropogenic disturbance.

Coralligenous bioconstructions are among the most important Mediterranean habitats for biodiversity maintenance. However some characteristic and sensitive organisms, such as the fan corals, are considered endangered in the international community; indeed, they may be severely damaged by fishing activities causing mechanical damage and increasing sedimentation rate. ROV (Remotely Operated Vehicle) investigations were carried out in order to characterize different morphological types of coralligenous habitat (rim, bank and shoal) located in the southern Bay of Naples (Italy), and to assess the presence of lost fishing gears and their impact on these benthic communities. A rapid classification of different fishing pressures and impacts was obtained through the development of new, representative and synthetic categories. Image analysis revealed the presence of rich and diversified communities, characterized by several fan coral colonies. However, fishing activity dramatically affects these coralligenous habitats, entangling and covering benthic assemblages and leading to necrosis and to parasitic epibionts growth especially on branched organisms. Monitoring programmes may provide a detailed assessment of coralligenous habitats characterization, distribution and health status. An accurate evaluation of fishing pressure and impact may be considered a useful tool to improve sustainable management of these valuable habitats.

The existence of deep red coral (Corallium rubrum) banks in the gulfs of Naples and Salerno (South Tyrrhenian Sea, Mediterranean Sea) is well known in historical records due to the heavy coral harvesting that occurred during the 19th and 20th centuries, by both trawling gears and scuba diving. In 2010 and 2012, during two Remotely Operated Vehicle (ROV) surveys on board of the Research Vessel (R/V) Astrea, red coral banks were detected in 16 of the 25 visited localities, between 45 and 150 m depth. Seven of these banks, located in the inner part of the Gulf of Naples, were already explored in 1918 by a scientific survey reporting the occurrence of red coral. Healthy populations (densities > 90 colonies m⁻²) were present only around the coasts of the Phlegrean Islands (Ischia and Procida Islands). Very low densities (< 5 colonies m⁻²) or the absence of coral were recorded in all other sites of the Gulf of Naples (including all historical re-visited banks), and a variable percentage of dead colonies was observed. This evidence suggests a huge state of stress likely favoured by the hydrodynamic conditions in the Gulf, enhancing water pollution and sedimentation rate. Finally, the documented high fishing pressure plays a major role in the hard-bottom communities’ degradation. A recent mass mortality episode was also recorded along the Amalfi coast, around Li Galli Islands (Gulf of Salerno), at a depth range between 80 and 100 m, where the mortality affected 80% of the largest colonies, estimated to be around 70 years old. Several possible reasons for this mortality have been hypothesised, such as the formation of local down-welling currents inducing an unusual drop of the thermocline, or sudden warm water emissions (sulphur springs) in an area characterised by important volcanic activities, or local landslides generating turbidity currents along the steep slopes.