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''Mnemiopsis leidyi'', the warty comb jelly or sea walnut, is a
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
of tentaculate ctenophore (comb jelly). It is native to western
Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe an ...
coastal waters, but has become established as an
invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...
in European and western Asian regions. Three species have been named in the
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...
''Mnemiopsis'', but they are now believed to be different
ecological Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overlaps wi ...
form Form is the shape, visual appearance, or configuration of an object. In a wider sense, the form is the way something happens. Form also refers to: *Form (document), a document (printed or electronic) with spaces in which to write or enter data ...
s of a single species ''M. leidyi'' by most
zoologist Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not uncommon. is the branch of biology that studies the Animal, animal kingdom, including the anatomy, structure, embryology, evolution, Biological clas ...
s.


Description and ecology

''Mnemiopsis'' have an oval-shaped and transparent lobed body, with four rows of
cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
ted combs that run along the body vertically and glow blue-green when disturbed. They have several feeding
tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...
s. Unlike cnidarians, ''Mnemiopsis'' does not sting. Their body comprises 97% water. They have a maximum body length of roughly and a diameter of . It is euryoecious, tolerating a wide range of
salinity Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...
(2 to 38
psu PSU may refer to: Organizations Military *Police Support Unit, a paramilitary wing of the Zimbabwe Republic Police *Port Security Unit, a U.S. Coast Guard expeditionary force protection unit Political parties * Parti Socialiste Unifié (disambig ...
), temperature (), and water quality. ''Mnemiopsis'' is a
carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other sof ...
that consumes
zooplankton Zooplankton are the animal component of the planktonic community ("zoo" comes from the Greek word for ''animal''). Plankton are aquatic organisms that are unable to swim effectively against currents, and consequently drift or are carried along by ...
including
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...
s, other comb jellies, and eggs and larvae of
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
. Many of its
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s are
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, ...
s, including
bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
s and
fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
. Others are members of
gelatinous zooplankton Gelatinous zooplankton are fragile animals that live in the water column in the ocean. Their delicate bodies have no hard parts and are easily damaged or destroyed. Gelatinous zooplankton are often transparent. All jellyfish are gelatinous zoopla ...
such as '' Beroe'' ctenophores and various Scyphozoa (jellyfish). The comb jelly has the capacity for self-fertilization, as they are
hermaphroditic In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have separ ...
. They have
gonad A gonad, sex gland, or reproductive gland is a mixed gland that produces the gametes and sex hormones of an organism. Female reproductive cells are egg cells, and male reproductive cells are sperm. The male gonad, the testicle, produces sper ...
s that contain the
ovary The ovary is an organ in the female reproductive system that produces an ovum. When released, this travels down the fallopian tube into the uterus, where it may become fertilized by a sperm. There is an ovary () found on each side of the body. ...
and
spermatophore A spermatophore or sperm ampulla is a capsule or mass containing spermatozoa created by males of various animal species, especially salamanders and arthropods, and transferred in entirety to the female's ovipore during reproduction. Spermatophores ...
bunches in their
gastrodermis The gastrodermis is the inner layer of cells that serves as a lining membrane of the gastrovascular cavity of Cnidarians. The term is also used for the analogous inner epithelial layer of Ctenophore Ctenophora (; ctenophore ; ) comprise a ph ...
. It carries 150 eggs along each meridional canal. Eggs and
sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...
are released into the water column where fertilization takes place. The spawning commences at late evening or at 1:00 or 2:00 a.m. The spawning eggs develop a thick outer layer within a minute of encountering seawater. As many as 10,000 eggs are produced from large specimens in areas with abundant prey. Egg production can start when the animals reach about 15 mm in length. Egg production increases with ctenophore size, and it is unclear when
senescence Senescence () or biological aging is the gradual deterioration of functional characteristics in living organisms. The word ''senescence'' can refer to either cellular senescence or to senescence of the whole organism. Organismal senescence inv ...
occurs. It has a transient anus, which means that it appears only during defecation. There is no permanent connection between the gut and the rear of the body. Instead, as waste accumulates, part of the gut starts to balloon out until it touches the outer layer, or epidermis. The gut then fuses with the epidermis, forming an anal opening. Once excretion is complete, the process is reversed and the anus vanishes. The animals defecate at regular intervals: once an hour in the 5-centimetre-long adults, and once every 10 minutes or so in the larvae. The species moves so slowly that it is referred to as "sea walnut".


As an invasive species


1980s – Black Sea

''Mnemiopsis leidyi'' was introduced in the
Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Roma ...
in the 1980s, where only one species of comb jelly, the small sea gooseberry ''
Pleurobrachia pileus ''Pleurobrachia pileus'' is a species of comb jelly, commonly known as a sea gooseberry. It is found in open water in the northern Atlantic Ocean, the North Sea, the Baltic Sea and the Black Sea, and was first described by the Danish zoologist ...
'' 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 carrying capacity of an environment is the maximum population size of a biological species that can be sustained by that specific environment, given the food, habitat, water, and other resources available. The carrying capacity is defined as t ...
, the population dropped somewhat. In the Black Sea, ''M. leidyi'' eats eggs and larvae of
pelagic fish Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that do live on or near the bottom, and reef fish that are associated with coral reef ...
. It caused a dramatic drop in fish populations, notably the commercially important
anchovy An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...
'' Engraulis encrasicholus'' (known locally as ''hamsi, hamsiya, hamsa'', etc.), by competing for the same food sources and eating the young and eggs.
Biological control Biological control or biocontrol is a method of controlling pests, such as insects, mites, weeds, and plant diseases, using other organisms. It relies on predation, parasitism, herbivory, or other natural mechanisms, but typically also invo ...
was tried with ''
Beroe ovata ''Beroe ovata'' is a comb jelly in the family Beroidae. It is found in the South Atlantic Ocean and the Mediterranean Sea and has been introduced into the Black Sea, the Aegean Sea, the Sea of Azov and the Caspian Sea. It was first described ...
'', another comb jelly, with some degree of success; it appears as if a fairly stable predator-prey dynamic has been reached.


1999 – Caspian Sea

In 1999 the species was introduced in the
Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central Asia ...
via the
Unified Deep Water System of European Russia The Unified Deep Water System of European Russia (russian: Единая глубоководная система Европейской части Российской Федерации, translit=Yedinaya glubokovodnaya sistema Yevropeyskoy chast ...
. The establishment of this population led to a 60% reduction in the number of
sprat Sprat is the common name applied to a group of forage fish belonging to the genus ''Sprattus'' in the family Clupeidae. The term also is applied to a number of other small sprat-like forage fish (''Clupeoides'', ''Clupeonella'', '' Corica'', ' ...
, which in turn led to a reduction in the population of sturgeon and seals.


2006 – North and Baltic Seas

Since then, the species has apparently spread throughout the
Mediterranean basin In biogeography, the Mediterranean Basin (; also known as the Mediterranean Region or sometimes Mediterranea) is the region of lands around the Mediterranean Sea that have mostly a Mediterranean climate, with mild to cool, rainy winters and w ...
and the northwestern Atlantic. In 2006, it was first recorded in the
North Sea The North Sea lies between Great Britain, Norway, Denmark, Germany, the Netherlands and Belgium. An epeiric sea on the European continental shelf, it connects to the Atlantic Ocean through the English Channel in the south and the Norwegian S ...
, and since October 17, 2006 in the western
Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...
, namely the
Kiel Fjord Kiel () is the capital and most populous city in the northern German state of Schleswig-Holstein, with a population of 246,243 (2021). Kiel lies approximately north of Hamburg. Due to its geographic location in the southeast of the Jutland pe ...
and
The Belts Belt may refer to: Apparel * Belt (clothing), a leather or fabric band worn around the waist * Championship belt, a type of trophy used primarily in combat sports * Colored belts, such as a black belt or red belt, worn by martial arts practit ...
. 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. One year later, the Baltic population of ''M. leidyi'' was found to have spread east to the
Gotland Basin The Gotland Basin is the large central basin in the Baltic Sea between Sweden and the Baltic countries. It is subdivided into the Gdansk Deep (or Gdansk Basin), the Western Gotland Basin, and the Eastern Gotland Basin. Within the Eastern Gotland ...
and the
Bay of Puck The Bay of Puck or Puck Bay (; ; german: Putziger Wiek), is a shallow western branch of the Bay of Gdańsk in the southern Baltic Sea, off the shores of Gdańsk Pomerania, Poland. It is separated from the open sea by the Hel Peninsula. The bay ha ...
. The impact of the species on the already heavily stressed Baltic
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syste ...
is unknown. The species overwinters in the deep waters where the temperature does not drop below ; the fact that the Baltic is heavily stratified, with the waters above and below the
halocline In oceanography, a halocline (from Greek ''hals'', ''halos'' 'salt' and ''klinein'' 'to slope') is a cline, a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity (in concert with temp ...
mixing little, is believed to aid its survival. Apart from the widespread ''P. pileus'', three comb jelly species occasionally drift into the Baltic from the North Sea but do not seem to be present as a stable population of significant size: '' Bolinopsis infundibulum'', '' Beroe cucumis'' and ''
Beroe gracilis ''Beroe gracilis'' is a species of comb jelly in the family Beroidae. It is a free-swimming species found in the North Sea, the Atlantic Ocean and the Mediterranean Sea. Description ''Beroe gracilis'' is a translucent, elongated, hollow, cylind ...
''. The second species might potentially be used for biological control. The route of dispersal of ''M. leidyi'' to the North Sea/Baltic region is unknown. It might have occurred naturally by drifting individuals, or with ballast water of ships, either from its natural range or from the Black Sea, via the Mediterranean and eastern Atlantic. At least technically possible given the species'
euryhaline Euryhaline organisms are able to adapt to a wide range of salinities. An example of a euryhaline fish is the molly (''Poecilia sphenops'') which can live in fresh water, brackish water, or salt water. The green crab (''Carcinus maenas'') is an e ...
habits is an alternative route of dispersal through continental
Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...
, being carried with ballast water in ships travelling from the Black Sea to the
Rhine Estuary ), Surselva, Graubünden, Switzerland , source1_coordinates= , source1_elevation = , source2 = Rein Posteriur/Hinterrhein , source2_location = Paradies Glacier, Graubünden, Switzerland , source2_coordinates= , sou ...
via the Rhine-Main-Danube Canal. The latter route is known to be the point of entry into continental Europe for numerous invasive
freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. Although the term specifically excludes seawater and brackish water, it does include ...
neozoon An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there ...
s from the Ponto-Caspian region, such as the
zebra mussel The zebra mussel (''Dreissena polymorpha'') is a small freshwater mussel. The species originates from the lakes of southern Russia and Ukraine, but has been accidentally introduced to numerous other areas and has become an invasive species in ma ...
, the
quagga mussel The quagga mussel (''Dreissena rostriformis'', also known as ''Dreissena bugensis'' or ''Dreissena rostriformis bugensis'') is a species (or subspecies) of freshwater mussel, an aquatic bivalve mollusk in the family Dreissenidae. It has an aver ...
, the
amphipod Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far descr ...
s ''
Dikerogammarus villosus ''Dikerogammarus villosus'', also known as the killer shrimp, is a species of amphipod crustacean native to the Ponto-Caspian region of eastern Europe, but which has become invasive across the western part of the continent. In the areas it has ...
'' and '' Chelicorophium curvispinum'', and the
polychaete Polychaeta () is a paraphyletic class (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...
'' Hypania invalida''.


Genomics

Both the nuclear and mitochondrial genomes of ''Mnemiopsis leidyi'' have been sequenced, providing insight into the evolutionary position of
Ctenophora Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), an ...
(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 ''MT-ATP6'' (or ''ATP6'') is a mitochondrial gene with the full name 'mitochondrially encoded ATP synthase membrane subunit 6' that encodes the ATP synthase Fo subunit 6 (or subunit/chain A). This subunit belongs to the Fo complex of the large, ...
and all the
tRNA Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ac ...
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 synthetase An aminoacyl-tRNA synthetase (aaRS or ARS), also called tRNA-ligase, is an enzyme that attaches the appropriate amino acid onto its corresponding tRNA. It does so by catalyzing the transesterification of a specific cognate amino acid or its pre ...
s. 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
microRNA MicroRNA (miRNA) are small, single-stranded, non-coding RNA molecules containing 21 to 23 nucleotides. Found in plants, animals and some viruses, miRNAs are involved in RNA silencing and post-transcriptional regulation of gene expression. miRN ...
s, as well as the nuclear proteins
Drosha Drosha is a Class 2 ribonuclease III enzyme that in humans is encoded by the ''DROSHA'' (formerly ''RNASEN'') gene. It is the primary nuclease that executes the initiation step of miRNA processing in the nucleus. It works closely with DGCR8 and ...
and
Pasha Pasha, Pacha or Paşa ( ota, پاشا; tr, paşa; sq, Pashë; ar, باشا), in older works sometimes anglicized as bashaw, was a higher rank in the Ottoman Empire, Ottoman political and military system, typically granted to governors, gener ...
, 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 ''Trichoplax adhaerens'' is one of the three named species in the phylum Placozoa. The others are ''Hoilungia hongkongensis'' and ''Polyplacotoma mediterranea''. The Placozoa is a basal group of multicellular animals (metazoa). ''Trichoplax'' ar ...
'', the only known member of the phylum
Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', ''Hoilungia hongkongensis'', an ...
.


References


External links


Mnemiopsis Genome Project Portal at the National Human Genome Research Institute, NIHResearch group Limnology
at Ghent University, Belgium

* * {{Taxonbar, from=Q133630, from2=Q3317486 Monotypic ctenophore genera Bolinopsidae Fauna of the Atlantic Ocean Taxa named by Louis Agassiz