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Also this summer we are witnessing the invasion of the Mnemiopsis leidyi or Sea walnut. This year we are in early July and the sea water is slowly warming up as always in summer. Punctually after a short and intense sea storm we witness an invasion of jellyfish, plastic, tree trunks and branches, various floating debris and as now every year also the Mnemiopsis leidyi, the now infamous Sea walnut.
Mnemiopsis leidyi Sea walnut Invasion Invasione Noce di Mare – intotheblue.it
This ctenophore is an alien species of the “Mare Nostrum“, the Mediterranean Sea and these invasions are now frequent and regular as summer approaches and the sea warms. So we must now live with it and try to reverse the trend of climate change and tropicalization of the Mediterranean, listening to the scientists and climatologists who have been recommending us for a long time to remain below the average of 1.5° of the annual average.
We remind you that this Ctenophore represents one of the largest invasive species on the planet. IUCN, the World Union for Nature Conservation, classifies this species as highly invasive and has been included in the list of the 100 most harmful invasive species in the world.
Mnemiopsis leidyi Sea walnut Invasion Invasione Noce di Mare – intotheblue.it
Sea walnut (Mnemiopsis leidyi A. Agassiz, 1865) is a ctenophore belonging to the Bolinopsidae family. Two other species belong to the genus Mnemiopsis Agassiz, 1860: M. gardeni and M. mccradyi, with a different distribution area; the most accepted opinion, however, is that the three species are different zoological forms of M. leidyi even if they show a certain polymorphism due to environmental adaptations.
Description
Mnemiopsis is easily mistaken for a jellyfish due to its transparency, but it is far from being so. The transparent and oval body has six lobes, two of which are evident at first sight (each large lobe alternates with two small ones), on which rows of ciliated combs are arranged; two rows for each large lobe and only one for each small lobe. These eyelashes glow a blue-green light when stimulated by light or touch.
It uses its numerous tentacles to feed itself, but unlike cnidarians, they are not stinging for humans: they are adhesive colloblasts that do not emit any toxins and are arranged along two long and thin tentacles that float in the water. The entire body is made up of 97% water and is small in size: it measures approximately 7–12 centimeters long by 2.5 cm wide.
Mnemiopsis leidyi Sea walnut Invasion Invasione Noce di Mare – intotheblue.it
Habitat
It is a species native to the Atlantic introduced with the ballast water of tankers in the Black Sea, the Baltic and the Caspian. Over time it also colonized other seas until it reached the eastern Mediterranean.
It lives in shallow and eutrophicated waters. The species are very tolerant to environmental changes (they are euryhaline and eurythermal) and consequently very resistant: they can develop both in estuaries of the Baltic Sea and in warm, oxygen-poor waters.
Mnemiopsis leidyi Sea walnut Invasion Invasione Noce di Mare – intotheblue.it
Ecology
Mnemiopsis is a carnivorous animal that feeds on zooplankton, including crustaceans and other ctenophores, as well as fish larvae and eggs. Its predators are generally vertebrates, including fish and birds, but also other zooplankton, such as the Beroe species or various Scyphozoa jellyfish.
The ecological success of M. leidyi is due to its ability to use its cilia to generate a current that drags large volumes of water to be filtered, without its prey being able to notice it. This “stealth predation” makes Mnemiopsis a very effective generalist predator, which captures a very vast range of microplantonic prey (i.e. approximately 50 µm), copepods (+/- 1 mm) and fish larvae (up to 3 mm or more). The effectiveness and versatility of this mechanism makes M. leidyi a destructive and very invasive species when not controlled by its predators.
Mnemiopsis leidyi Sea walnut Invasion Invasione Noce di Mare – intotheblue.it
The case of the Black Sea
In the Black Sea it was introduced in the eighties, while only one species of ctenophores was present in those waters, the Pleurobrachia pileus; the first recording of M. leidyi in the Black Sea dates back to 1982. The invasion of M. leidyi probably began thanks to the involuntary transport in the ballast water of merchant ships.
In 1989, the population in the Black Sea will have reached very high levels with around 400 specimens per m³ of water. In the following years, due to the intense consumption of food sources, the population decreased. In the Black Sea, M. leidyi feed on eggs and larvae of pelagic fish, significantly reducing their population, with serious effects on the local trade of Engraulis encrasicholus anchovies which, among other things, compete for the same food resources.
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