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Remotely operated vehicle (ROV) investigations of the Mediterranean deep continental shelf and bathyal zone have provided, in the last decade, a large amount of data concerning an enigmatic group of anthozoans: the black corals. Taxonomic investigations as well as ecological and biological studies contributed to unveil the diversity, population structure, environmental preferences, bathymetric and geographical distribution of these corals. Remotely operated vehicle footage and accidental bycatch data have provided evidences of large impact by fishing activities on the Mediterranean black coral forests such that today, due also to their important ecological role as structuring species, they are considered part of vulnerable marine ecosystems and conservation strategies need to be developed targeting their protection.

The term cold-water coral sensu lato groups taxa with a more or less pronounced frame-building ability (e.g. Lophelia pertusa and Madrepora oculata) with forest-forming organisms both on hard (e.g. Leiopathes glaberrima, Parantipathes larix, Callogorgia verticillata and Viminella flagellum) and soft bottoms (e.g. Isidella elongata, Funiculina quadrangularis and Kophobelemnon stelliferum). Cold-water coral species and their occurrence in the Mediterranean Sea are here reviewed and discussed from a biogeographic point of view, considering geographical areas of occurrence and bathymetric distribution. The present-day occurrence of living cold-water corals is then compared to the main deep currents of the Mediterranean Sea. Due to the proper interaction between topography and a combination of cold, oxygenated and trophic-carrying water masses (i.e. Levantine Intermediate Water, deep waters and cascading effects), cold-water coral communities develop in a mosaic-like situation along the main paths that such water masses follow within the basin. Finally, knowledge gaps and future perspectives in the study of cold-water coral occurrence, distribution and biogeography are highlighted. The currently still scarce knowledge on the Mediterranean deep-sea and on the basin-scale distribution of the most important cold-water corals species represents crucial biogeographical information. This gives fundamental indications on the location of the Mediterranean vulnerable deep marine ecosystems for future management strategies.

About 63% of the known antipatharian genera occur at mesophotic depths (30–150 m), with the majority extending into the deep sea. Along the continental shelf and offshore sites, antipatharians tend to increase in diversity and abundance with depth, reaching a peak at mesophotic depths due to favorable environmental factors enhancing their settlement and growth and biotic factors associated with lower levels of competition for space. A review of taxonomic and ecological studies for shallow and mesophotic antipatharians is presented for four regionally based case studies, three in the tropics (1) Central Indo-Pacific, plus adjacent sections of the Western Indo-Pacific, (2) Eastern Indo-Pacific (primarily Hawaiʻi), and (3) the Caribbean Sea) and one at temperate latitudes in the Mediterranean Sea and adjacent sections of the Northeast Atlantic. The mesophotic fauna is mainly represented by the families Antipathidae, Aphanipathidae, and Myriopathidae. The most diverse community is found in the Central/Western Indo-Pacific, followed by the Caribbean Sea. The tropical antipatharians are represented by shallow species that extend their distribution into the upper mesophotic zone (30–60 m), while the temperate antipatharians consist of deepwater (> 150 m) species that extend upward into the lower part of the mesophotic zone. Black corals in mesophotic coral ecosystems can be habitat-forming components of benthic assemblages on hard substratum. They have an enormous potential for hidden biodiversity and play an important ecological role for the broader marine ecosystem. The threats to antipatharians consist of demersal fishing activities and coral harvesting, which may be highly destructive to these poorly understood systems.

Coral gardens are increasingly being reported at 40–200 m depth in the Mediterranean Sea. These coral assemblages are composed of gorgonians from shallow coastal rocky bottoms extending their distribution to deeper areas, as well as by gorgonians and black corals with distribution restricted to the continental shelf and upper slope. Gorgonians and black corals are among the main structural species in deep benthic ecosystems, providing shelter and reproductive grounds for a highly diverse associated fauna. Due to their branching morphology and erect position, these deep coral gardens are extremely vulnerable to several fishing activities (bottom trawling, long-lines and trammel net fishing) and should consequently be protected through an ecosystem-based fishery management.

Knowledge on basic biological functions of organisms is essential to understand not only the role they play in the ecosystems but also to manage and protect their populations. The study of biological processes, such as growth, reproduction and physiology, which can be approached in situ or by collecting specimens and rearing them in aquaria, is particularly challenging for deep-sea organisms like cold-water corals. Field experimental work and monitoring of deep-sea populations is still a chimera. Only a handful of research institutes or companies has been able to install in situ marine observatories in the Mediterranean Sea or elsewhere, which facilitate a continuous monitoring of deep-sea ecosystems. Hence, today’s best way to obtain basic biological information on these organisms is (1) working with collected samples and analysing them post-mortem and / or (2) cultivating corals in aquaria in order to monitor biological processes and investigate coral behaviour and physiological responses under different experimental treatments. The first challenging aspect is the collection process, which implies the use of oceanographic research vessels in most occasions since these organisms inhabit areas between ca. 150 m to more than 1000 m depth, and specific sampling gears. The next challenge is the maintenance of the animals on board (in situations where cruises may take weeks) and their transport to home laboratories. Maintenance in the home laboratories is also extremely challenging since special conditions and set-ups are needed to conduct experimental studies to obtain information on the biological processes of these animals. The complexity of the natural environment from which the corals were collected cannot be exactly replicated within the laboratory setting; a fact which has led some researchers to question the validity of work and conclusions drawn from such undertakings. It is evident that aquaria experiments cannot perfectly reflect the real environmental and trophic conditions where these organisms occur, but: (1) in most cases we do not have the possibility to obtain equivalent in situ information and (2) even with limitations, they produce relevant information about the biological limits of the species, which is especially valuable when considering potential future climate change scenarios. This chapter includes many contributions from different authors and is envisioned as both to be a practical “handbook” for conducting cold-water coral aquaria work, whilst at the same time offering an overview on the cold-water coral research conducted in Mediterranean laboratories equipped with aquaria infrastructure. Experiences from Atlantic and Pacific laboratories with extensive experience with cold-water coral work have also contributed to this chapter, as their procedures are valuable to any researcher interested in conducting experimental work with cold-water corals in aquaria. It was impossible to include contributions from all laboratories in the world currently working experimentally with cold-water corals in the laboratory, but at the conclusion of the chapter we attempt, to our best of our knowledge, to supply a list of several laboratories with operational cold-water coral aquaria facilities.