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Alien species and economic impacts


Lateral furrow Mnemiopsis leidyi Ctenophores are delicate, transparent, and gelatinous animals; their body is made up of 98% water and they look like jellyfish. They have a quasi-spherical or flattened shape. The most striking aspect of these animals is the delicate polychromatic iridescence, which are due to a series of longitudinal cilia on the entire body that allow the animal to move about. Ctenophores are hermaphrodites. They release the eggs and sperm into the external environment where fertilisation takes place, so that one individual is capable of establishing a new population.

In the early 1980s, the ctenophore Mnemiopsis leidyi (a jellyfish-like organism), native of the eastern coast of America, was introduced to the North Sea through ballast water carried by merchant ships. Here this ctenophore found a large quantity of food and an absence of predators and, in the summer of 1989, this voracious zooplankton predator multiplied its numbers astronomically. This provoked a sudden decrease in zooplankton and as a consequence the fish that feed on this zooplankton (anchovies, sardines, mackerel, etc) died and the fisheries collapsed.

Areas of origin

Areas of di diffusion

Food chain In the sea, the food chain is a complex process where the plants (phytoplankton) utilise chemical substances dissolved in the water. The phytoplankton becomes food for the herbivorous animals (zooplankton) , which in turn, serve as food for the larger carnivorous animals and so on along the chain up to the great predators. The process is in a dynamic and natural equilibrium. Among the various elements that may disrupt this equilibrium there are: - pollution; - intensive fishing; - the introduction of alien species; - climate change.

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Sequence of cilia, the “combs�

The decline of fisheries in the North Sea

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An alien in the North Sea

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Ministero dell'UniversitĂ e della Ricerca


Ministry of the University and Scientific and Technological Research

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With the grant of

Effects on the food chain

Economic repercussions

The effects produced through the introduction of the ctenophore Mnemiopsis leidyi were: - the drastic reduction in zooplankton; - the drastic reduction in the small pelagics that fed on the zooplankton; - the abnormal growth of phytoplankton


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The Black Sea has a fundamental role for all the fisheries activity in Turkey. Prior to the introduction of the ctenophore, the fisheries were mostly based on the European anchovy (Engraulis encrasicolus) and the Mediterranean horse mackerel (Trachurus mediterraneus), of which a few hundred thousands tonnes were regularly fished. Between 1988 and 1991, at the same time of the introduction of the ctenophore Mnemiopsis leidyi, the quantity of these two species landed fell to around one-fourth the numbers quoted above. Since the fishing for these small pelagic species constitutes an important food source and income for the Turkish population, the negative effects on the economy were particularly serious. The economic impact was estimated at several hundred million dollars for Turkey alone.

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Realized by

Maritime Admiral Michelagnoli Foundation NGO non-profit Fondazione Marittima Ammiraglio Michelagnoli Onlus - Taranto - Italy

Beroe ovata

Photo archive "Mare Nostrum" , property of Fondazione Michelagnoli & Giuseppe Pignataro (international champion) English version translated by Dr Adrian Mallia - Adi Associates Environmental Consultants Ltd - Malta



Mnemiopsis leidyi

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The appearance in 1997 of another ctenophore, Beroe ovata, allowed the ecosystem to recover. In fact, Beroe ovata feeds primarily on Mnemiopsis; hence, since its appearance, the numbers of Mnemiopsis in many regions have been significantly reduced and, at the same time, an increase was observed both in the zooplankton and in the amount of fish caught. Currently, a growth in the numbers of Mnemiopsis is only recorded during short periods in late summer, due to the natural effects controlling its predator.

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Biological control


In agreement with

National Interuniversity Consortium For Marine Sciences - Italy Co.N.I.S.Ma. - Consorzio Nazionale Interuniversitario di Scienze del Mare

7.Alien species and economic impacts copia  
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