Its scientific name is Mnemiopsis leidyi, but it is commonly known as “sea nut”, and is one of the 100 most harmful invasive species in the world. It has been present in the Adriatic Sea for almost a decade, causing serious problems for fishing operators, especially in lagoons. A real calamity, very similar to a jellyfish, which in addition to plankton, devours the larval stage of fish, molluscs and crustaceans. A “diet” which has its harmful effects on Italian fishing, as underlined by the League MEP, Anna Maria Cisint: In the upper Adriatic alone, in recent months, clam fishing has recorded a collapse in turnover from 120 to 13 million, with the termination of 700 VAT numbers. I have therefore asked the European Commission for urgent actions, with dedicated compensation measures.”
Sea walnut in the Venice lagoon
Now the Venice lagoon is in serious danger. A research team from the University of Padua and the National Institute of Oceanography and Experimental Geophysics – OGS recently published the study ‘An invader chronicles: local ecological niche of Mnemiopsis leidyi in the Venice Lagoon’ in the journal ‘Estuarine, Coastal and Shelf Science’, which highlights how the species Mnemiopsis leidyi constitutes a potential ecological danger for the Venice lagoon, even worse than the blue crab, thanks to the its adaptability and to ongoing climate changes, which can favor its proliferation to the detriment of other species in the ecosystem.
The research focused on the behavior of the sea walnut in the Adriatic, as explained by Filippo Piccardi, first author of the study and researcher at the University of Padua: “Although the dynamics of this species has been studied in other areas of the world, information relating to Mediterranean lagoons, characterized by a strong spatial and seasonal variability of environmental conditions, is still limited. Our study constitutes the first integrated field and laboratory investigation of the ecological niche of Mnemiopsis leidyi in the lagoon of Venice. We adopted an interdisciplinary approach and monitored the spatial distribution of the species for two years with controlled experiments to define the main environmental survival thresholds. The results show that Mnemiopsis leidyi follows a seasonal trend, with bloom (mass reproduction events) in late spring and between late summer and early autumn, probably linked to higher temperatures and optimal salinity conditions.”
Temperature and salinity
The abundance of the species is therefore positively correlated to both water temperature and salinity. Laboratory experiments, integrated with in situ observations, indicate that Mnemiopsis leidyi is able to survive in a wide range of temperatures (10-32 °C) and salinity (10-34). However, the study highlights how the extreme conditions of these ranges, very high temperatures (32 °C) or low salinity (10) can significantly reduce the survival of the species.
“By integrating field observations and experimental results on the tolerance of Mnemiopsis leidyi to changes in temperature and salinity, our study provides new information on the ecological niche of this species in the Venice lagoon – explains Valentina Tirelli, co-author of the study and researcher at the National Institute of Oceanography and Experimental Geophysics – Our results suggest that ongoing climate changes could favor increasingly suitable environmental conditions for this ctenophore, increasing its presence in large aggregates and, consequently, increasing the risk of severe repercussions on the functioning of the entire lagoon ecosystem”. The study therefore highlights the need for targeted monitoring and adaptive management strategies to mitigate the ecological and socio-economic consequences of the expansion of Mnemiopsis leidyi.