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Taxonomic name: (Agassiz
1865)
Synonyms: Mnemiopsis gardeni L.Agassiz 1860,
Mnemiopsis mccradyi, Mayer, 1990
Common names: American comb jelly, comb jelly, comb
jellyfish (English), Rippenqualle (German), sea gooseberry, sea walnut
(English), Venus' girdle, warty comb jelly
Organism type: comb jelly,
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The ctenophore, Mnemiopsis ledyi, is a major carnivorous
predator of edible zooplankton (including meroplankton), pelagic fish eggs and
larvae and is associated with fishery crashes. Commonly called the comb jelly or
sea walnut, it is indigenous to temperate to subtropical estuaries along the
Atlantic coast of North and South America. In the early 1980s, it was
accidentally introduced via the ballast water of ships to the Black Sea where it
had a catastrophic effect on the entire ecosystem. In the last two decades of
the twentieth century, it has invaded the Azov, Marmara, Aegean Seas and
recently it was introduced into the Caspian Sea via the ballast water of oil
tankers.
Description
Mnemiopsis ledyi is a comb jelly with a length up to 100mm. The body is
laterally compressed, with large lobes arising near the stomodeum, generating 4
deep, noticeable furrows that characterize the genus. It has four rows of small,
but numerous, cilated combs which are irridescent by day and may glow green by
night (NIMPIS, 2002). The colour is usually transparent or slightly milky,
translucent (Shiganova 2003).
Occurs in:
estuarine habitats, marine habitats
Habitat description
The native habitat of the ctenophore, Mnemiopsis, is in temperate to
subtropical estuaries along the Atlantic coast of North and South America
(Mayer,1912). M. leidyi is tolerant of a wide range of salinity,
temperature and water quality conditions over a broad range of inshore habitats.
Since its unintentional introduction to the Black Sea, Mnemiopsis has
spread to adjacent bodies of water, inhabiting waters of salinities ranging from
3% in the Sea of Azov to 39‰ in the eastern Mediterranean, and temperatures
ranging from 4oC in winter to 31oC in summer (Dumont and Shiganova).
General impacts
Mnemiopsis ledyi is a major zooplankton predator and is associated with
fishery crashes (Costello, 2001). A carnivorous predator on edible zooplankton
(including meroplankton), pelagic fish eggs and larvae, M. leidyi causes
negative impacts right through the foodchain of the areas it has invaded. In the
Black Sea and the Sea of Azov, the zooplankton, ichthyoplankton and
zooplanktivorous fish stocks all underwent profound changes.
The pelagic ecosystem of the Black Sea was degraded, manifesting as sharply
decreased biodiversity, abundance, and biomass of the main components of the
pelagic ecosystem-zooplankton (Dumont and Shiganova). Fish stocks in the Black
Sea and Sea of Azov have suffered due to predation on eggs and larval stages of
food supplies (Shiganova 2003). Effects on the ecosystem in the Caspian Sea were
faster and stronger than in the Black Sea. In 2001, repercussions were felt at
all trophic levels, including that of the top predator, the Caspian seal (Dumont
and Shiganova).
A cascading effect occurred at the higher trophic levels, from a decrease in
zooplankton stock and collapsing planktivorous fish, to vanishing predatory fish
and dolphins. Similar effects occured at lower trophic levels: from a decrease
in zooplankton stock to an increase in phytoplankton, which was released from
zooplankton grazing pressure. The majority of these effects were top-down, but a
few were also bottom-up. Similar effects, but less pronounced, were recorded in
the Sea of Marmara. Effects on Mediterranean food webs have, so far, remained
insignificant. Salinity is probably supraoptimal there, and several predators
prevent M.leidyi from reaching outbreak levels.
Notes
Mnemiopsis is probably the most-studied ctenophore genus in the world
because of its great abundance in estuaries in heavily populated areas of the
United States, and because of its explosive population growth after accidental
introduction into the Black Sea in the early 1980s. But after the invasion of a
new ctenophore of the genus Mnemiopsis into the Black Sea, a question
regarding which species was invasive arose. L.N. Seravin (1994) made a revision
of the genus Mnemiopsis with the conclusion that it includes only one
polymorphic species of lobate ctenophore-Mnemiopsis leidyi, which this
new ctenophore belongs to. Richard Harbison also supports this point of view
(personal communication in Dumont and Shiganova).
Geographical range
Native range: The native habitat of the ctenophore, Mnemiopsis, is
in temperate to subtropical estuaries along the Atlantic coast of North and
South America between 40 degrees north to 46 degrees south (Mayer, 1912,
Costello, 2001).
Known introduced range: The unintentional introduction of M. leidyi
to the Black Sea in the early 1980s allowed it to secondarily expand its range
to the adjacent seas of Azov, Marmara, the Aegean and perhaps the eastern
Mediterranean (Studenikina et al, 1991, Shiganova et al, 2001). However,
nowhere were conditions as optimal and perennial as in the Black Sea and the
surface waters of the Sea of Marmara. It has to re-invade the Sea of Azov each
year. Low numbers take advantage of the Black Sea current to reach the northern
Aegean Sea where they disperse, according to the dominant circulation patterns.
However, its presence in Saronikos Gulf and Elefsis Bay could be also due to
ballast water release as elsewhere in the eastern Mediterranean Sea (Shiganova
et al., 2001).
Invasion pathways to new locations
Ship ballast water: In the early 1980s, Mnemiopsis leidyi was
accidentally introduced via the ballast water of ships to the Black Sea where it
had a catastrophic effect on the entire ecosystem. It was also introduced into
the Caspian Sea via the ballast water of oil tankers.
Reproduction
Mnemiopsis leidyi is a free-spawning, simultaneous hermaphrodite capable
of self-fertilization (Costello, 2001). It possesses gonads containing both the
ovary and the spermatophore bunches in their gastrodermis. Total numbers of
simultaneously forming eggs depends of food availability and on temperature -
2-3000 eggs per day production by adults at high food concentrations is common.
The embryo is formed completely within the original egg cover. It has size of
about 0.12-0.14mm and acquires its specific form and tentacular structures. When
the larva attains mobility the egg cover softens and became flexible. The life
span of egg producing individuals may be many months (Costello, 2001).
Lifecycle stages
Totally planktonic life history; early tentaculate larvae resembling
Cydippida ctenophores but metamorphoses into the mature lobate form. No
current evidence of resting stages (Costello, 2001).
The embryo acquires a double rows of cilia, a well-developed pair of lateral
tentacles, and a large, apical sense-organ. The entodermal part of the
gastro-vascular system consists of 6 lateral diverticula from a central chamber;
2 of these lateral branches lead into the bases of the tentacles and the other 4
lead outward toward the 4 double rows of cilia. The ectodermal buccal pouch or
stomodeum has become a long, laterally compressed tube, with its broad axis 90*
from the tentacular axis of the animal. Until this time the animal swims about
quite freely within the egg-envelope at this stage its cilia may be observed
beating in a normal manner and its tentacles to elongate or contract in response
to stimuli. Soon after this the larva breaks through the egg-envelope and
escapes into the water. Here it passes the development stages which are very
similar to those of the young Pleurobrachia.
The tentacles acquire numerous lateral filaments and elongate greatly, as in
Pleurobrachia. When the animal is 5mm long, the oral lobes begin to develop as
two simple outgrowths on both sides of the mouth in the sagittal plane of the
animal. At the time when the oral lobes begin to develop, the meridional ventral
canals and the paragastric tubes begin to elongate downward. The former give
rise to the characteristic loops in the oral lobes. Four meridional vessels
extend downward and fuse with the circum-oral vessel. The primary tentacle-bulbs
migrate downward to lie close by the sides of the mouth. The auricles appear
last of all, after the lobes have developed to some extent. When attaining 10mm
long the animal becomes ellipsoidal in outline. The appearance of its lobes and
auricles resembles to that in the adult of Bolinopsis. Afterward the deep,
lateral furrows extend upward to the level of the apical sense-organ and the
animal acquires the characteristic of Mnemiopsis ( Mayer, 1912 ). The
embrional development takes about 20-24 hours in the Black Sea upper water layer
by 23 degrees C. The size of hutched larvae is 0.3-0.4mm.
Reviewed by: Dr. Tamara Shiganova. P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences, Russia.
Compiled by: Dr. John Costello, Biology Dept., Providence College, Providence, RI, USA.
Dr. Hermes Mianzan, National Institute for Fisheries Research and Development (INIDEP), Argentina.
Dr. Tamara Shiganova. P.P.Shirshov Institute of Oceanology, Russian Academy of Sciences, Russia.
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