They are ctenophores, transparent organisms with a gelatinous consistency, which arrived with naval traffic and rising temperatures. They are not stinging but they clog fishermen’s nets and put the growth of bony fish in the area at serious risk
In the waters of the Venice lagoon, aexplosion of ctenophores is stirring the concern of citizens and fishermen. The University of Padua explained the phenomenon, with a study in collaboration with the National Institute of Oceanography and Experimental Geophysics of Trieste, published in the international journal Hydrobiology. The recent massive invasion in the Venice Lagoon by the ctenophore Mnemiopsis leidyia species known as sea nut, which began exploding in 2014, is connected to the increase in water temperature.
Ctenophores are transparent organisms, like jellyfish, which however have bands of mobile cilia in different parts of the body, used for the capture of zooplankton, which they feed on, and for movement. An interesting aspect is that unlike jellyfish, ctenophores do not have stinging cells, therefore they do not sting, nor do they pose a danger to people who encounter them in the water or on the shore.
The uncontrolled increase in population, however, can have a negative impact on the size of fish populationsas they feed on zooplankton which the larval stages of bony fish also feed on. Furthermore, due to their gelatinous consistency, they completely clog fishermen’s nets, thus producing a serious problem for the fish trade in the area.
The Venice Lagoon is a rapidly changing environment, subject to intense naval traffic (a typical vector for the introduction of alien species) and numerous other human activities and is therefore particularly affected by invasive species.
As Ispra underlines in an in-depth note, the individuals who crowd the lagoon these days are adults Mnemiopsis leidyi, a species native to the coastal and estuarine marine waters of the western Atlantic which in the last thirty years has spread, thanks to the ballast water of oil tankers and other large ships, in the Black Sea and from there to the Caspian (thanks to the rivers and navigable channels that connect the two seas) and in various points of the Mediterranean (including the Gulf of Trieste) and in more recent times also in the North Sea and the Baltic. In some cases, however, the spread of the species could be due to transport via marine currents.
While M. leidyi of the Black Sea was locally introduced with the ballast water of ships coming from the Atlantic coasts of North America (as demonstrated by genetic analyzes of specimens of the species taken from the various European seas, which highlighted their homogeneity and identified the area of probable origin), where the presence of the species was recognized for the first time in 1982, the local population spread progressively in the following years, also increasing in terms of density, until in the two-year period 1989-1990 a concomitant strong decline was observed in biomass and in the catches of anchovies, which constituted the main fishing resource of the entire basin.
Therefore the presence of notable aggregates of M. leidyi in the Venice lagoon is certainly not good news for the fish populations that spend all or part of their life cycle there.
