Mnemiopsis leidyi (A. Agassiz, 1965)Mnemiopsis leidyi

Taxonomy

Phylum: Ctenophora Esch 
Class: Tentaculata Chun
Order: Lobata Esch 
Family: Mnemiidae Ech. 
Genus: Mnemiopsis L.Agassiz; Mnemiopsis leidyi (A.Agassiz) 1965
Synonyms: Mnemiopsis gardeni L. Agassiz,1980; M.mccradyi Mayer,1900.

Common names: Russian: Mnemiopsis; English: comb-jelly Mnemiopsis

General view of the organism (with references to other INTERNET sources)

Illustrated Database at the GAAS web site:
http://www.zin.ru/projects/invasions/gaas/mnelei_i.htm

Photo Mnemiopsis leidyi from the Caspian Sea.

Taxonomic description of speciesGeneral illustration of the organism

Mnemiopsis leidyi - is the lobate ctenophore. Two oral lobes are derivatives of the ctenophore body (spherosome). Four smaller lobes -auricules are situated under the principal two oral lobes. During their movements the lobes in fold completely its buccal orifice. The oral lappets carry tentacular rings. Its central part is situated above the lips of the mouth crevice. Both "lips" are extremely contractible (Agassiz,1860; Seravin, 1994). They have a pronounced lappet in its middle part, which prolongs into the distal end of pharynx canal. Around the outer end of both "lips" four labial (orifice) metalabile canals are passing being formed by bifurcated distal ends of the pharynx canal (Fewkes,1882). The tentacular apparatus is situated above the mouth lip. The tentacular bulb composed on two lobes is protected by tentacular bulb with a kind of two-lapped hood. The hood is contractible. The bunches of filamentous tentacles pass through the special canals to the mouth lip and between the ctenophore body (sphaerosoma) and the lobes. The plane passing via the central part of the sphaerosome and aboral apex divides the animal per two symmetrical parts. Along its both sides the meridianal canals pass. Under these subtentacular canals the subtentacular comb flappers are situated. The plane perpendicular to the latter is named as sagital. The subsagittal canals or tubes, under which rows of the subsagital comb flappers are situated, frame them. The meridianal canals end with the auricule canals, over which the auricule comb flappers are situated. Between the subtentacular and auricular flapper rows always is an interval. From the double lobed tentacular apparatus along the sides of animal's body the bunches of the fine tentacles (tentilles) are protruding via special furrows. Their ends came out stretching to the mouth lip. Their ends form an extremely efficient catching apparatus between the internal surface of the side lobes and the sphaerosoma surface (Agassiz, 1865, Seravin, 1994). The tentacles are armed with the colloblasts-special catching cells having inside the spical felaments and special gluing substance to immobilize the potential preys. 
It moves due to beating of cilia (tentillia) covering the surface of its locomotive comb flappers. The animal swims its mouth ahead mainly up or sometimes down. Its locomotion is controlled by its nervous system and by its apical statocyte. The subepidermal net of nervous system is located mainly under the rows of comb flappers. 
Morfology of gonadsMnemiopsis leidyi- is self-fertilizing hermaphrodite. It possesses gonads containing both the ovary and the spermatophore bunches in their gastrodermis. The gonads are situated along eight meridianal canals of its gastrovascular system. They are fixed between the ctenes. The rows of spermatophores are spreading along the meridianal canals. The rows of ovaries are fixed reciprocally in the neighboring canals. Gonads are forming in the central their parts beginning from the statocyst's level to the oral side (Zaika and Revkov, 1994). In the auricule canals only ovaries were observed. They are forming their 12 rows stretched along the main ax of the body. In the gonads fixed along the meridional canals in the intervals between ctenes usually present one spermatophore and between 1 to 4 eggs, while in the auricular gonads one egg presents. The specimens 5-7 cm long have 100-140 ctenes in their subsaggital rows spreading along the lobe surfaces, while in the shorter subtentacular rows their numbers are less -between 55-90. Such a specimen carries some 150 eggs along each meridianal canal and over 100 of them - in each auricular canal. Total numbers of simultaneously forming eggs depends of food availability and on temperature. 
Luminescence. Mnemiopsis is remarkably phosphorescent. The seat of the phosphorescence is confined to the rows of locomotive flappers. They are extremely sensitive a slightest shock to the jar in which the ctenophores are kept is sufficient to make them plainly visible by the light emitted from the eight phosphorescent ambulacums. The light producing cells are fixed in the meridional canal, being asymmetrically located with respect to the axis of the canal, as viewed perpendicular to the plane of the comb plates. A group of cells can be identified in stained a section that is located below testicular tissue on the sidewall of the meridional canal being separated from the lumen of the canal by a layer of gastrointestinal cells (Moore,1924).
Intraspecific forms. Some specimens of this ctenophore collected in the Black Sea have small papillae on their body surface, same as had been described by Fewkes (1881) during the primary description of M.leidyi. The morphology of this ctenophore in the Black Sea appears to be rather variable, they can be more or less transparent, mainly in dependence on ctenophore's size. With increase of body size transparence decreases, the largest individuals attain 150-180 mm in the Black Sea (Shiganova et al,1998). We did not find papillae on their body surface of Caspian Mnemiopsis and they are more transparent due to their smaller size.
Local forms. Morphology of Mnemiosis In the Northern Caspian Sea Mnemiopsis has some differences in morphology.
It was found differences in morphology of oral part of sphaerosoma - metalabial canal elongate downward and as a result metalabial and labial canals form loop. Probably it is effect of unfavorable conditions of the Northern Caspian. Some individuals from other areas of the Caspian had the same structure of these canals.

Distribution of species within the Caspian Sea

According to results of population Mnemiopsis leidyi study in 2001 the main area of its distribution is the Southern Caspian, where Mnemiopsis spends winter and than in spring it begins to distribute to the northward. First, it appears in the Middle Caspian, than it spreads to the Northern Caspian in the areas where salinity it not less than 4 in late July-early August.

Distribution of Mnemiopsis leidyi in the Caspian Sea in 2001
Distribution of Mnemiopsis leidyi in the Caspian Sea in 2001 ( a- June; b- July; c-August; d- in October (Data of Tamara Shiganova, SIO RAS, Andrey Kamakin, CaspNIRKH)

Status as per International Red Data Book. N/A.
Status as per National Red Data Books. N/A.
First record for the Caspian Sea. In Russian area Mnemiopsis leidyi was first recorded in early November 1999 at two stations above the depths 29-32 m ( 4054 N. 5250 E. 3950 N 5150 E. ) at the temperature 16,5-19,5 and salinity 11.76-13.07 by scientists of CaspNIRKH . . Kamakin and V.B. Ushivtzev (Shiganova et al., 2001). 
Redescription of species. The exact estimation of a new ctenophore thus appeared in the Black Sea was significantly aggravated by inconsistency in description of morphological features of the genus Mnemiopsis by its first researchers (L.Agassis, 1960; A.Agassis, 1965; Fewkes, 1881) and latest description of this genus by Mayer (1912), Main (1928) and Pratt (1935).
There were three species in genus Mnemiopsis M.gardeni L. Agassiz, M.leidyi A. Agassiz M. mccradyi Mayer ( Moser, 1908; 1909; Mayer, 1912; Main, 1928, Pratt, 1935) before its arrival in the Black Sea. After invasion Mnemiopsis in the Black Sea L.N.Seravin,1994) revised genus with conclusion that it includes only one polymorphic species of lobate ctenophore Mnemiopsis leidyi, M. mccradyi M.gardeni as valid species don't exist and should be synonyms of Mnemiopsis leidyi. Dr. Harbison has the same point of view (person. com., 1993)

General characteristics of species

Ecological group. Macrozooplankton.
Origin. North American species might be brought into the Black Sea with ballast water by Russian tankers driving oil to the ports at eastern coast of USA. From the Black Sea Mnemiopsis might be transferred into the Caspian Sea also by tankers driving oil though the Volga-Don Canal.
World distribution. The native habitat of the ctenophore, Mnemiopsis, is in temperate to subtropical estuaries along the Atlantic coast of North and South America (Harbison, et al, 1978). In the early 1980s, it was accidentally introduced to the Black Sea (Vinogradov et al, 1989), where it flourished and expanded into the Azov, Marmara, eastern Mediterranean, and Caspian Seas (Studenikina et al, 1991, Shiganova et al, 2001a, Shiganova et al, 2001b) 
Habitat. Mnemiopsis leidyi inhabits coastal areas and surface layers (above termocline) open sea. Some large ctenophores can spread deeper and even can be found near the bottom in the coastal areas of the Caspian Sea. 
Migrations. Transferred with the currents. Dial vertical migrations were not recorded, although it is more abundant near the surface at night where they feed and reproduce.

Relation to abiotic environmental factors

Relation to salinity. Euryhalinic species. Salinities range from 2 to 38 (Kremer, 1994). In the seas of Mediterranean basin M.leidyi occurs in waters with salinities ranging from 3 in the Sea of Azov to 39 in the eastern MediterraneanIn the Caspian Sea its distribution is limited isohalines of 4 . 
Relation to temperature. Eurythermic species. Temperatures range from 0C in northern native locations in the winter, to 32C in the southern estuaries during the summer
In the seas of Mediterranean basin Mnemiopsis occurs in waters with temperatures ranging from 4C in winter to 31C in summer.
In the Black Sea can not live at the temperature below 2 (experimental data by Shiganova), in the Sea of Azov entire population dyes in autumn when temperature droppes below 4 (Mirsoyan, 2001).
Vertical distribution. No remarkable vertical migrations were recorded, although it can be more abundant near the surface at night where it feeds and reproduces. 
In the coastal areas could be found in the whole column, although it inhabit mainly in the surface layer above termocline in the open sea. 
Relation to oxygen conditions. Mnemiopsis can live in conditions with very low oxygen content up to 0.2-0.3 mg .l-1 (Purcel et al., 2001).
Relation to fluctuations of the sea level. Information is not available

Feeding

Feeding type. Heterotrophic, carnivorous 
Feeding behavior. The larvae of Mnemiopsis can retract entirely their two tentacles into the tentacular sheaths on either side of the body, between the oral and aboral poles. In order to "set" their tentacles for food collection animals swim vigorously, oral pole toward, in a curved or helical pathway relaxing and trailing their tentacles behind them, while the lateral branches also relax and expand up to two body widths from the main tentacle. As the tentacles reach an extension over 10 times the body diameter, the ctenophore body comes to rest. The tentacles can be some 25 times longer than the body diameter of the animal. The animals drift in this manner. In the absence of contact, the animals periodically contact and then reset their tentacles. When having contact with the ctenophore manipulates its tentacle into the mouth, deposits the prey, and withdraws its tentacle. The animal immediately resets its tentacle. Examining the prey after ingestion, small sections of the tentacle can be seen wrapped around it (Reeve and Walter, 1976; Walter, 1976). The tentacles must presumably be continuously growing from the base to compensate for this loss.
The adult lobate ctenophores searching for food, swim mouth forward with their lobes spread open like wings, upon the surface of which prospective food organisms impinge. The prey appears to become enmeshed on the mucus lined inner surface of lobes, which also contain colloblast cells. The currents induced by the auricular cilia move the prey towards the small tentacles and along the pharyngeal grooves into the mouth. Newly ingested copepods can be seen still active as they move up the pharynx into the stomach area. Digestion time takes between from 1 to3 h depending on size of prey. Mnemiopsis prey is captured by both lobes and secondary tentacles (Harbison et al, 1978; Reeve & Walter, 1978). Mostly the lobes trap active and relatively large preys, such as copepods. They collide with lobes to become entangled in mucus and, or cause lobe closure or curling. Relatively inactive prey like nauplii and eggs, bypass the lobes and are swept into the tentacles where they are trapped. The feeding strategy enables Mnemiopsis to feed on a greater variety of preys (Larson, 1988). 
Mnemiopsis larvae exhibit different feeding behavior patterns depending upon the level of starvation (Reeve and Walter 1978). The starving larvae fully extend their tentacles fully, frequently changing position and re-setting as described above. If no food is encountered, these search periods are followed by periods of inactivity. When food becomes abundant they reduce the tentacle length making frequent contacts with food direct as they swim actively until their guts are full. At this point, active swimming and feeding cease. Their tentacles become extended to 2-3 x their body diameter as they drift. Lobate adults appear bell-shaped under starved conditions. They often swim vertically up and down in an aquarium with their lobes extended at right angles to their direction of travel. The lobe width in this condition can be more than the animal length. Upon the addition of food to the aquarium, the lobe width decreased while the animal feeds. When a number of copepods are in the oral area, the ctenophore folds its muscular lobes inward. It prevents escaping the copepods thus ensuring their ingestion. In high food concentrations after having been starved for 24 h, the lobate ctenophores exhibit the superfluous feeding. Once their guts are full they still continue to "feed" by entangling preys in mucus, this excess food forms a bolus at the oral region. Quite frequently, they will either "spit out" these enmeshed ball of copepods or completely evacuate their guts, and continue to feed. Thus at high food concentration the ctenophore captures and kills much more planktonic animals than can ingest and digest.
This behavior pattern can continue for several hours until the concentration of food is reduced to a point where all the copepods that are captured can be ingested. By this time, the bottom of the experimental container is covered with a layer of partially digested, and undigested but dying copepods. Examination of the guts of these animals after one hour of exposure to low and high concentrations of food indicates a definite difference in the amount of digestion that has taken place. In high concentrations the food organisms are still whole and recognizable, whereas in low concentrations the copepods are partially digested and in some cases, indistinguishable.
The number of ingested food organisms is a function of animal length (Walter,1976; Reeve et al,1978). A large lobate ctenophore can hold up to 100 food items in their gut at any one time. 
Thus the ctenophore larvae and adults alternates their feeding habits depending on degree of starvation of food concentration. The food collection in them is discontinuous, because their tentacles must be withdrawn from fishing to permit ingestion. As food concentration increases, the proportion of non-fishing time might also increase at very high food density. Mnemiopsis feeds mostly passively entrapping the food partiles due their accidental collections with the tentacular filaments ( Reeve et al, 1978). In addition, they might feed actively, when moving up and down vertically with its mouth lobes wide open. The swimming velocity of ctenophores is variable depending upon their size and physiological state. Maximal cruising velocity of foraging Mnemiopsis ranging between 0.2-1.2 cm -1 ( Larson,1988). 
The primary sorting of food particles occurs at the interval surfaces of oral lobes, where attached captured plankters often present being wind over with tentacular filaments and mucus. Most often they are dead. The selected food objects arrear then dragged by the ciliar apparatus to the mouth. Sometimes the food material is directly moved to the mouth by contracting the lobe (Sergeeva, et al, 1990). The food thus swallowed is moved to the aboral end of gasrtrovascular cavity. Where it is digested. Undigested chitin covers of crustaceans are regurgitated by the ctenophore back via the mouth (Sergeeva and Zaika, 1990). 
The lobate ctenophores are recognized as the carnivores, however the suggestions have been made, that they must also rely at times on phytoplankton or detritus (Nelson,1925; Miller,1970; Miller and Williams, 1972). Anyway Baker and Reeve (1974) have established experimentally that Mnemiopsis starved when exposed to phytoplankton or detrital suspensions on. 
Occasionally found in the guts phytoplankton most probably original from the digested copepods (Hirota 1974). Mnemiopsis ingests any organism that it is able to capture with its oral lobes, including holoplanktonic organisms, the planktonic larvae of bentic organisms, fish eggs and larvae (Nelson, 1925; Main, 1928; Tsikhon-Lukanina et al,1993, own observations). M.leidyi feeds superfluously if the food is abundant (Harbison, et al, 1978). The ctenophore continues to capture prey, even if its stomadeum is full, regurgitating back an excess of undigested plankton wrapped in a bolus of mucus.
The food spectrum of Mnemiopsis varies depending on season, place and time of the day. In summer it feeds mainly with the planktonic crustaceans: small specimens prefere cladocerans, while large the copepods together with fish eggs and larvae (Tsikhon-Lukanina et al, 1991a, 1991b,1992). The mean size of preys varies between 0.75 and 1 mm. The ctenophore population inhabiting coastal waters have more variable food spectrum on comparison with that in the open sea (Sergeeva et al, 1990; Tzikhon-lukanina et al, 1991a). It includes more copepods and fish eggs, fish and invertebrate larvae. Among the latter mussel veligers are usually predominate.
Food spectrum. The lobate ctenophores are recognized as the carnivores, however the suggestions have been made, that they must also rely at times on phytoplankton or detritus (Nelson,1925; Miller,1970; Miller and Williams, 1972). Anyway Baker and Reeve (1974) have established experimentally that Mnemiopsis starved when exposed to phytoplankton or detrital suspensions on. 
Occasionally found in the guts phytoplankton most probably original from the digested copepods (Hirota 1974). Mnemiopsis ingests any organism that it is able to capture with its oral lobes, including holoplanktonic organisms, the planktonic larvae of bentic organisms, fish eggs and larvae (Nelson, 1925; Main, 1928; Tsikhon-Lukanina et al,1993, own observations). M.leidyi feeds superfluously if the food is abundant (Harbison, et al, 1978). The ctenophore continues to capture prey, even if its stomadeum is full, regurgitating back an excess of undigested plankton wrapped in a bolus of mucus.
The food spectrum of Mnemiopsis varies depending on season, place and time of the day. In summer it feeds mainly with the planktonic crustaceans: small specimens prefere cladocerans, while large the copepods together with fish eggs and larvae (Tsikhon-Lukanina et al, 1991a, 1991b,1992). The mean size of preys varies between 0.75 and 1 mm. The ctenophore population inhabiting coastal waters have more variable food spectrum on comparison with that in the open sea (Sergeeva et al, 1990; Tzikhon-lukanina et al, 1991a). It includes more copepods and fish eggs, fish and invertebrate larvae. Among the latter mussel veligers are usually predominate.
Quantitative characteristics of feeding. Because of the ability of adult ctenophores for the superfluous feeding, its rate in them depends on the food concentration in water within an extremely wide range between 1 up to 3000 copepods per liter (Bishop, 1920; Miller, 1970; Reeve et al,1978; Kremer,1970). However, the larvae animals display the optimum nutrition at about 200 copepods per liter. Because of such an dependence of adults feeding rate upon the food concentration, the food ration in them can become very large. By extreme copepod density at 3000 ind.l-1. The food ration can reach in them 1000% of body weight being expressed in carbon units (Kremer,1979). At the food concentrations (Acartia) 20 to 200 ind.l-1, the food ration varied between 120 and 1500 % (Finenko et al, 1995). In the natural habitats it was estimated close to 70%.
The digestion time in adult ctenophores takes usually 2 to 3 hours by temperature 20 to 23C ( Sergeeva et al .1990; Tzikhon-lukanina et al., 1992; 1993). The digestion time expands with increasing of food swallowed.
The threshold food concentration in experiments was established at about 1 to 3 copepods per liter. Thus at experimental conditions in aquaria the ctenophores begin to starve at food content rather high on its comparison with the open sea waters. During such experiments starving the size of animals decreased for 2-3 times during only 2 weeks (Sergeeva et al,1990). The ctenophore populations often encounter with deficiency of available food in the sea. In such areas with poor zooplankton concentration, most of them have their gastrovascular cavity (stomadueum) does not contain any kind of food (Tzikhon-lukanina et al., 1991a). Its mean concentration in surface waters before the ctenophore invasion was about0.15 mg.l-1 (dry weight), which could provide food ration not more than 0.2% of body carbon per day (Vinogradov et al,1989). Even during the invasion period the mean fullness of gastrovascular cavity in a natural population of Mnemiopsis was estimated at only 10% (Tzikhon-lukanina et al., 1991a). The daily rhythm of ctenophore feeding in the sea is characterized by its maximal rate at midnight (Sergeeva et al,1990).
The digestion efficiency in ctenophores was estimated at about 70% on a copepod dry weight basis by food concentration at 100 ind.l-1 (Cosper and Reeve,1975). Increasing this concentration up to 1000 ind.l-1 the efficiency estimations becomes much more erratic, yielding negative figures in some cases because in this case animals ingest food without digestion while losing a lot of mucus produced. Average digestive affiance at the highest food concentration was 20% although values widely ranged.
Unless food was rapidly ingested, gut residence time beyond the initial hour during which feeding was permitted was fairly constant and not dependent on food quantity, and ranged between 2,5-3,5 hours. Fecal material always appeared to be evacuated through the mouth. The fecal material of ctenophores does not form a distinct pellet (Reeve, Cosper and Walter, 1975), but is loosely bound by mucus and sinks to the bottom of the experimental container.

Reproduction

Reproduction type. Mnemiopsis leidyi- is a self-fertilizing hermaphrodite 
As most planktonic ctenophores Mnemiopsis leidyi is a simultaneous hermaphrodite and capable of self fertilization, and thus viable offspring can be produced from a single adult (Planka,1974; Hirota,1972; Reeve & Walter;1976). They can produce offspring long before they reach their upper size limit. They are also capable for reviewed cases of paedogenesis (sexual maturity of larvae and juveniles) and dissoggony (sexual maturity of larvae followed by regression of gonads and subsequent rematuring of adults) in Mnemiopsis reproduction (Planka, 1974). 
It possesses gonads containing both the ovary and the spermatophore bunches in their gastrodermis. The gonads are situated along eight meridianal canals of its gastrovascular system. They are fixed between the ctenes. The rows of spermatophores are spreading along the meridianal canals. The rows of ovaries are fixed reciprocally in the neighbouring canals. Gonads are forming in the central their parts beginning from the statocyst's level to the oral side (Zaika and Revkov, 1994). In the auricule canals only ovaries were observed. They are forming their 12 rows stretched along the main ax of the body. In the gonads fixed along the meridional canals in the intervals between ctenes usually present one spermatophore and between 1 to 4 eggs, while in the auricular gonads one egg presents. The specimens 5-7 cm long have 100-140 ctenes in their subsaggital rows spreading along the lobe surfaces, while in the shorter subtentacular rows their numbers are less -between 55-90. Such a specimen carries some 150 eggs along each meridianal canal and over 100 of them - in each auricular canal. Total numbers of simultaneously forming eggs depends of food availability and on temperature. By the optimal food supply the ctenophores began to propagate attaining oral-aboral length 10mm and having 13.5 mg dry weight (Finenko et al, 1995).
Terms of reproduction. The development of gonads and their current state displays definite daily rhythm. The rape eggs that appear at late evening before the midnight, while any their rudiments are visible still at 5 p.m. The spawning begins at late evening with peak at midnight or late at one or two o'clock a.m. (Freeman and Reynolds,1973, Zaika and Revkov, 1994).
Reproduction areas. Reproduction starts in the Southern Caspian in June when temperature reaches 21. In July Mnemiopsis reproduces also in the whole Middle Caspian, and in August with penetration to the Northern Caspian it reproduces there as well. Peak of reproduction occurres in August and continues until October similar the Black Sea pattern. 
Fecundity. The eggs spawned acquire a thick cover within 1 minute after contact with the seawater. 
The fecundity of ctenophores depends upon the body size. Large specimens produce between from 2 to 8x 1000 eggs during the spawning. The largest animals in native habitat can produce from 10 to 14 x 1000 eggs ( Baker and Reeve,1974; Kremer ,1975). The ctenophores may begin to produce eggs 13 days after their hatching having 26 mm size. During 10 following days they produce up to 12000 eggs. The fecundity apparently depends also upon the food concentration and habitat .
Experimental studies on Mnemiopsis have indicated that Mnemiopsis begins to produce eggs in the Caspian Sea when it reaches length about 15 mm, although eggs were obtained even from specimen 12 mm and weight 0.5 g. The most abundant size of reproducing Mnemiopsis was 20-30 mm in the Caspian Sea. This is much smaller than the largest size of 64 mm, which the ctenophore can attain in the Caspian Sea.
Average fecundity of Mnemiopsis in the Caspian Sea was 1174 eggs/day with maximal 2824 eggs/day for specimens 30-39 mm length and about weight 2.0-2.7 g Dependence egg production of Mnemiopsis on wet weight.
Dependence egg production of Mnemiopsis on wet weight in fed Mnemiosis specimens.
Dependence egg production of Mnemiopsis on wet weight in fed Mnemiosis specimens.

Fed Mnemiopsis had higher egg production per day. Mean fecundity of Mnemiopsis relatively to the wet weight in the Caspian Sea is approximately the same, even less that we found for the Black Sea (Shiganova,unpublished)
Limiting factors. The main factors which limit reproduction are low temperature and low food concentration.

Life history and development

Life-history stages. The exact duration of the embriogenesis depends upon the water temperature.
Percentage of hatched eggs was not high in our experiments with Caspian Mnemiopsis the range from 9 to 92% after 24 hours (according to our data in the Black Sea percentage of hatched eggs were 94-96%). Size of spawning eggs was 120-140mm. The eggs are spherical, at the beginning can have a little oval shape. After development they encased in a thin membranous capsule. After 21-24 hours eggs hatch from the capsules. Larvae resemble members of the order Cydippida. The eggs and larvae are very sensitive and survival can be even less than 30% in aquarium.
The size of hatched eggs is the same as it was found for the Black Sea (Shiganova, unpublished) and for American coastal waters. But size of developmental stages in the Caspian Sea is less and the size of e of reproductive maturity in Caspian Mnemiopsis is less than ones in the Black Sea. It was defined by the about 16 mm. Although Mnemiopsis continues its growth and can reach 64.5 mm.

Approximate size and timing of some developmental stages in the life history of Mnemiopsis for the Caspian Sea.

Developmental stage
(t=23-250C)
Size with lobe length Approximate time
Spawning egg 120-140 m 0 h
4-cell stage   1.0 h
Epiboly (begins)   3.0 h
Second round of micrometeres form at oral pole    
Gastrulation (begins)   6.0 h
First appearance of ctenes 200m  
Ctene coordination( Apical organ formation) 250m  
Tentacle outgrowth 300m  
Tentaculate stage
Hatching
260-320m 21-24 h
Polar fields grow out from apical organ 1.5-2.0 mm  
Lobate stage    
Meridional canal meets  2.2-2.5 mm  
Auricles begin o grow out 3 mm  
Normal adult (begins)
Reproduction
16 mm + About two weeks

So, we can conclude that the size of developmental stage of reproductive maturity in ctenophores cannot be as readily specified as it can be in the case of copepods. A few eggs may even be produced by larvae (the phenomen of dissogeny). The onset of adult maturity in Mnemiopsis as defined by the beginning of continuous egg production. 
The embryo is forming completely within the original egg cover. It has size of about 0.12-0.14 mm and acquires its specific form and tentacular structures (Shiganova, unpublished). When the larva attains mobility the egg cover softens and became flexible. 
The embryo acquires a double rows of cilia, a well-developed pair of lateral tentacies, 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 is 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 5 mm 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 10 mm 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 23C. The size of hutched larvae 0.3-0.4 mm.
Relation to environmental factors.The main factors, which are important for reproduction, are temperature and food concentration.
Age of maturity. The size of reproductive maturity is much less in the Caspian Sea (about 15-16 mm) than in the Black Sea (about 30 mm). The maximal size of Mnemiopsis in the Caspian Sea is also less (6.4 mm) that it was recorded for the Black Sea, where Mnemiopsis reaches 10-12 cm with maximal 18 cm (Shiganova, 1997).
Quantitative characteristics of growth. The growth rate of ctenophores can be characterized by the coefficient of exponential growth (k) (Winberg, 1971). In experiments Mnemiopsis at 26C reached 30 mm length in about 10 days (coefficient 0.78) by which time they had become completely lobate (Reeve and Baker,1975). Growth slowed over the next 10 days (k=0.23, up to 40mm) and again from 20 to 40 days (k=0.07 up to 68 mm).
The newly hatched larvae of Mnemiopsis having 0.3-0.4 mm size can increase their weight seven times during first week. The individual growth rate of larvae is very variable and their mortality is high. 
The variability of growth rate and survival of Mnemiopsis larvae under identical conditions, even when they are produced hermaphroditically from a single parent, may have important ecological implications. When food is plentiful, rapidly growing larvae can get through the phase of vulnerability to destruction by copepods, and instead take advantage of the copepods by feeding on them to maintain a high growth rate. The fastest growing larva increased its weight, 2.3 times each day (growth coefficient 0.83), which may be the fastest growth rate recorded for any metazoan (Stanlaw, et al, 1981).
Regeneration. Adult ctenophores M.leidyi regenerates damaged body parts ( Coonfield, 1937). Only eight parts of the body loss this ability while halves, thirds and fourths of Mnemiopsis readily regenerate the lost parts.
Some of the halves and fourths failed to regenerate rows of plates, however these pieces continued to live as normal animals. The sequence of regeneration of organs was the apical organ, rows of plates, and plates on these rows. The regeneration plates proceeds within three days after formation of the rows. There are certain evidences, that the apical organ of this animal is the regulating center of regeneration.

Structural and functional population characteristics

Quantitative characteristics. In 2001 Mnemiopsis population already reached value of biomass, which were recorded in the Black Sea in 1988 (first year of colonization), density were even much higher (9103 ind.m-2 in the entire sea and 15587 ind.m-2 in the Southern Caspian) the maximal value, estimated for the Black Sea (7600 ind.m-2 in 1989 .).
Population trends. According to our data for the Black Sea population study significant interannual variation in population size in the Black Sea, probably resulting on its environmental interactions and food availability (Shiganova, 1998; Shiganova et al, 2001)
Zooplankton population size can determine the size of the Mnemiopsis population, and Mnemiopsis populations also can greatly affect zooplankton.

Interspecific relations

Food competitors of Mnemiopsis in the Caspian Sea are fishes, which consume zooplankton. First of all they are all species of kilkas. There are no known predators of Mnemiopsis in the Caspian Sea now.

Impact on the ecosystem

In the Black Sea, dramatic reductions in zooplankton, ichthyoplankton, and zooplanktivorous fish populations have been attributed to Mnemiopsis (Shiganova, 1997,1998).
There was remarkable effect on the Caspian ecosystem already in 2000, decrease of zooplankton biomass and density and their species diversity (Shiganova et al, 2001). 
Effect on the ecosystem was very considerable in 2001. Density and biomass of zooplankton were decreasing from month to month with increasing Mnemiopsis population size. Average daily catch of kilka dropped in tree times in 2001 comparing with 2000. Since the decreasing of kilka stocks, rations and share of kilka in diet composition of beluga reduced. Average daily catch of kilka dropped in tree times in 2001 comparing with 2000. (Data of CaspNIRKH).

Importance of species to bioresources production of the Caspian Sea

It is necessary to develop and implement measurements to control Mnemiopsis population, in another case the Caspian ecosystem damage and decline of its fish stocks will not be able to recover. 
Economic significance of species. Great economic lost has already recorded in all Caspian countries, due to drop kilka stocks.
Conservation measures. Not required

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Compiled by: 

T. Shiganova, P.P.Shirshov Institute of Oceanology RAS

Acknowledgment:

Author greatly appreciate CaspNIIRKh assistance in author's participance in the cruises of CaspNIIRHK and opportunity to use some data for estimations.