Мнемiопсис лейди: Шығыс Атлант мұхитынан бастау алған инвазивті түр

Genus of comb jellies

Mnemiopsis leidyi, the warty comb jelly or sea walnut, is a species of tentaculate ctenophore (comb jelly). It is native to western Atlantic coastal waters, but has become established as an invasive species in European and western Asian regions. Three species have been named in the genus Mnemiopsis, but they are now believed to be different ecological forms of a single species M. leidyi by most zoologists.

Mnemiopsis leidyi was introduced in the Black Sea in the 1980s, where only one species of comb jelly, the small sea gooseberry Pleurobrachia pileus occurred until then. The most likely cause of its introduction is accidentally by merchant ships' ballast water. The first Black Sea record was in 1982. By 1989, the Black Sea population had reached the highest level, with some 400 specimens per m3 of water (>10 animals/cubic foot) in optimal conditions. Afterwards, due to depletion of foodstocks resulting in lower carrying capacity, the population dropped somewhat. In the Black Sea, M. leidyi eats eggs and larvae of pelagic fish. It caused a dramatic drop in fish populations, notably the commercially important anchovy Engraulis encrasicholus (known locally as hamsi, hamsiya, hamsa, etc. ), by competing for the same food sources and eating the young and eggs.

In 1999 the species was introduced in the Caspian Sea via the Unified Deep Water System of European Russia. The establishment of this population led to a 60% reduction in the number of sprat, which in turn led to a reduction in the population of sturgeon and seals.

Since then, the species has apparently spread throughout the Mediterranean basin and the northwestern Atlantic. In 2006, it was first recorded in the North Sea, and since October 17, 2006 in the western Baltic Sea, namely the Kiel Fjord and The Belts. Up to 100 animals per cubic metre were counted in the Baltic, whereas the population density in the North Sea was at a much lower 4 animals/m3 at most. The impact of the species on the already heavily stressed Baltic ecosystem is unknown. The species overwinters in the deep waters where the temperature does not drop below 4 °C (39 °F); the fact that the Baltic is heavily stratified, with the waters above and below the halocline mixing little, is believed to aid its survival. At least technically possible given the species' euryhaline habits is an alternative route of dispersal through continental Europe, being carried with ballast water in ships travelling from the Black Sea to the Rhine Estuary via the Rhine Main Danube Canal. The latter route is known to be the point of entry into continental Europe for numerous invasive freshwater neozoons from the Ponto Caspian region, such as the zebra mussel, the quagga mussel, the amphipods Dikerogammarus villosus and Chelicorophium curvispinum, and the polychaete Hypania invalida.

Both the nuclear and mitochondrial genomes of Mnemiopsis leidyi have been sequenced, providing insight into the evolutionary position of Ctenophora (comb jellies). In the original 2013 paper reporting the nuclear genome sequence, phylogenetic analysis of the presence and absence of genes, introns, and amino acid alignments suggested that the comb jelly is the sister lineage to the rest of all animals. However, a 2015 study applied different methodologies and found support for Porifera as the sister group to all other animals, and confirmed findings from the original study that amino acid alignments gave mixed support for this hypothesis. The position of Ctenophora and Porifera is currently being actively debated. Its mitochondrion shows several interesting features. It is 10 kilobases in length making it the smallest animal mitochondrial DNA sequence known to date. It has lost at least 25 genes, including MT ATP6 and all the tRNA genes. The atp6 gene has been relocated to the nuclear genome and has acquired introns and a mitochondrial targeting presequence. All tRNA genes have been genuinely lost along with nuclear encoded mitochondrial aminoacyl tRNA synthetases. The mitochondrial rRNA molecules possess little similarity with their homologs in other organisms and have highly reduced secondary structures. The genome of Mnemiopsis leidyi appears to lack recognizable microRNAs, as well as the nuclear proteins Drosha and Pasha, which are critical to canonical microRNA biogenesis. It is the only animal thus far reported to be missing Drosha. MicroRNAs play a vital role in the regulation of gene expression in all non ctenophore animals investigated thus far except for Trichoplax adhaerens, one of three known members of the phylum Placozoa. In Mnemiopsis leidyi, NOS is present both in adult tissues and differentially expressed in later embryonic stages suggesting the involvement of NO in developmental mechanisms. Ctenophores also possess soluble guanylyl cyclases as potential NO receptors with weak but differential expression across tissues. Combined, these data indicate that the canonical NO cGMP signaling pathways existed in the common ancestor of animals and could be involved in the control of morphogenesis, cilia activities, feeding and different behaviors.