The lack of gelatinous blooms data reveals scant attention to such phenomena due to their irregular occurrence (Boero et al. 2008), to modern publication trends that give no importance to observations (Boero 2013b), to current monitoring programs that usually do not cover jellyfish (Riisgård et al. 2012) and to the lack of remote sensing tools to detect jellyfish. From 2009 the Italian project Occhio alla Medusa started involving citizens to record sightings of gelatinous organisms. In 2010 till 2015, the Italian popular science magazine FOCUS supported the campaign dedicating a webpage and releasing a smartphone app to upload records.

A lot for information has been derived from three critical sources: the first is the Action plan for the conservation of the coralligenous and other calcareous bio-concretions in the Mediterranean Sea by the UNEP-MAP-RAC/SPA (2008) followed by the report in the State of knowledge of the geographical distribution of the coralligenous and other calcareous bio-concretions in the Mediterranean by UNEP (2009). Together with these two initiatives, at present, there are other projects carried out at national and international scale focusing on bioconstructions, their distribution and the driving forces affecting their structure and function.

This dataset shows modelled spatial distributions of coralligenous outcrops, maërl beds across the Mediterranean Sea. These bioconstructions are typical Mediterranean underwater seascapes, comprising coralline algal frameworks that grow in dim light conditions. They are the result of the building activities of algal and animal constructors, counterbalanced by physical, as well as biological, eroding processes. Because of their extent, biodiversity and production, coralligenous and maërl habitats rank among the most important ecosystems in the Mediterranean Sea, and they are considered of great significance both for fisheries and carbon regulation. <br/> The dataset also shows modelled spatial distributions for <i>Posidonia oceanica</i>. <i>Posidonia oceanica</i> is endemic to the Mediterranean Sea, where it is the dominant seagrass.

The majority of Hydrozoa is represented by not readily noticeable, small species. In recent decades, however, taxonomic knowledge of the group has increased worldwide, with a significant number of investigations focused on the Mediterranean Sea. Over more than two decades, 115 species of hydrozoans were recorded from coastal waters along nearly 300 km of the Salento Peninsula (Apulia, Italy). For each species, records from different collections were merged into single sheets of a general database.

The dataset of seagrasses distribution across the Mediterranean produced in the framework of MEDISEH derived from the compilation of published and unpublished information. For example literature search was made using mainly the ISI Web of Knowledge engine. At the same time a new search started from the “Bibliographic References” at the end of each paper. We amended the data set with our own unpublished data (grey literature). Moreover, we included data from reports of national and EU research projects and data networks. A major contribution to the improvement of our knowledge on seagrasses in the Mediterranean and especially <i>Posidonia</i> derived from the establishment of Natura 2000 network.

The Mass Mortality Events database (hereafter MME-T-MEDNet) is a collaborative initiative involving more than 30 research institutions from 10 Mediterranean countries including EU and non-EU countries. This initiative aims to facilitate the access to information (published in scientific journals and gray literature or still unpublished) related to Mediterranean Mass Mortality Events (MMEs).