More than 60 marine non-indigenous species (NIS) have been removed from previous lists and 84 species have been added, bringing the total to 986 alien species in the Mediterranean [775 in the eastern Mediterranean (EMED), 249 in the central Mediterranean (CMED), 190 in the Adriatic Sea (ADRIA) and 308 in the western Mediterranean (WMED)]. There were 48 new entries since 2011 which can be interpreted as approximately one new entry every two weeks. The number of alien species continues to increase, by 2-3 species per year for macrophytes, molluscs and polychaetes, 3-4 species per year for crustaceans, and 6 species per year for fish. The dominant group among alien species is molluscs (with 215 species), followed by crustaceans (159) and polychaetes (132). Macrophytes are the leading group of NIS in the ADRIA and the WMED, reaching 26-30% of all aliens, whereas in the EMED they barely constitute 10% of the introductions. In the EMED, molluscs are the most species-rich group, followed by crustaceans, fish and polychaetes. More than half (54%) of the marine alien species in the Mediterranean were probably introduced by corridors (mainly Suez). Shipping is blamed directly for the introduction of only 12 species, whereas it is assumed to be the most likely pathway of introduction (via ballasts or fouling) of another 300 species. For approximately 100 species shipping is a probable pathway along with the Suez Canal and/or aquaculture. Approximately 20 species have been introduced with certainty via aquaculture, while >50 species (mostly macroalgae), occurring in the vicinity of oyster farms, are assumed to be introduced accidentally as contaminants of imported species. A total of 18 species are assumed to have been introduced by the aquarium trade. Lessepsian species decline westwards, while the reverse pattern is evident for ship-mediated species and for those introduced with aquaculture. There is an increasing trend in new introductions via the Suez Canal and via shipping.

The hydromedusan subumbrellar muscle tissues originate from a mass of endo‐ and ectoderm derived cells proliferating inwardly. This mass of cells, called entocodon, is separated by the ecto‐and endoderm through a layer of extracellular matrix, thus forming a locally triploblastic arrangement of tissues. By cavitation and differentiation, the entocodon gives rise to the striated and smooth muscle layers of the subumbrella. The structure of the striated muscle is histologically identical to that described for triploblasts, where striated muscle is mainly mesodermic. Together with the mode of development, this suggests that not all Cnidaria are diploblastic and that the subumbrellar cavity is a coelom‐like structure. The subumbreHar cavity is formed late in ontogeny, whereas the coelom in higher animals is normally formed during embryonic development. Instead of remaining closed, the subumbrella becomes open, with the muscular mesothelium in contact with the environment. This view of cnidarian structures can be pushed further while considering the regulation of modularity and metamery. The occurrence of homeotic genes in cnidarians and other ‘lower’ animals shows that the specification of body architecture has a common molecular ground throughout the animal kingdom. It is suggested that the expression of homeotic genes while specifying a given architecture might be regulated by iteration of their activity, leading to ‘low’ modular organisms and to ‘higher’ metameric ones. Replicated parts as modules and metameres might be analogous, but the affinity in their molecular specification (and possibly regulation) might indicate homology. The number of resemblances between cnidarians and ‘higher’ metazoa suggests that this phylum might be a centre of metazoan radiation, having structures which represent the living premises of the main steps of animal evolution.