MNEMIOPSIS LEIDYI, an American native Ctenophore, invaded the Caspian Sea in 1999. At the same time, Iranian Fisheries Company (Shilat) reported a sharp decline in the catch of pelagic fish (Such as Anchovey, sprat) in the coastal water of Gilan and Mazandaran. The rate of catches before and after m. LEIDYI's invasion is evaluated and its socio-economical impacts are studied. The results show invasion of lU. LEIDYI has had numerous impacts in the Caspian Sea ecosystem. In addition, the Iranian fishermen have lost more than 44 billion Rials in 2000 -2001. At the final, in order to control of M. LEIDYI's population and reduce its effects, some recommendations have been proposed.

MNEMIOPSIS LEIDYI was sampled from December 2003 to March 2005 in 11 stations located along four regions AnzaH, Khazarabad, Tourkman and Gorgan Bay using METU net in the Iranian coasts of the Caspian Sea. Study of stomach contents of MNEMIOPSIS LEIDYI showed that Acartia sp. belonging to Copepoda and Lamellibranchia sp. had the highest frequency comprising 66% and 13% of food items taken by the organism respectively and the least food items taken were Balanus nauplii, Rotatoria, Tintinnopsis and Podon polyphemoides. The maximum mean abundance of food item was Acartia sp. which was shown to be present in stomach content of MNEMIOPSIS LEIDYI caught in different depths, seasons and regions in the Iranian coasts of the Caspian Sea. For the M LEIDYI larvae, the highest amount of food item was Acartia sp. and Lamellibranchia sp. with an average of 44% and 32% and the maximum abundance of food item for juvenile and mature MNEMIOPSIS LEIDYI was also Acartia sp. with an average of 67.5% and 66%. The results showed that the main food item for MNEMIOPSIS LEIDYI is Acartia sp. of the Copepoda. Hence, MNEMIOPSIS LEIDYI feeding has an important effect in declining zooplankton populations.

The biochemical composition of the Caspian Sea ctenophore, MNEMIOPSIS LEIDYI. was analysed in terms of total fractions of protein, lipid, ninhydrin-positive substances (NPS), polysaccharides (PS) and low molecular- weight carbohydrates (LMC).The sum of organic substance per garm dry weight was 84.18 mg/g. Protein was the major fraction followed by lipid and NPS. The NPS/ protein and LMC/PS ratios were 0.45 and 0:50 mg/g DM, respectively. Starvation of MNEMIOPSIS LEIDYI for one week resulted in reduction of all biochemical fractions, and for 3 weeks, high loss of body weight occurred. The sum of organic substances per gram dry weight after one week starvation was 56.7 mg which was 30% less than the non-starved condition. In unfavorable conditions all biochemical fraction changed simultaneously.

The alien ctenophore MNEMIOPSIS LEIDYI which was transported from the Black Sea into the Caspian Sea by the end 1990s has since affected this new ecosystem. We studied the spatial and temporal distribution of the ctenophore from July 2001 to July 2002 in three transects located in Lisar, Bandar Anzali and Sefidroud, in Guilan province. Four stations were established in each transect at a 5, 10, 20 and 50 meter bottom depth contours.Maximum observed biomass of the organism was 221.3±91g/m2 in October 2001 and the minimum was 8.3±3.8g/m2 in December 2001. The highest biomass of the organism was detected in the upper 0–20m water layer in areas 50 meters deep achieving 145.36± 67.33g/m2. The lowest biomass was in the 20 – 50 layers in areas 50 meters deep amounting to 1.9±0.95g/m2. Small specimens less than 10 mm comprised about 97% of the sampled population and the maximum length measured at 51–55mm. The largest average biomass was 112.3±48g/m2 detected in Lisar and the smallest biomass in the range 67.17±46.8g/m2 was seen in Anzali region. It was concluded that temperature, depth, salinity and food availability affected the distribution of MNEMIOPSIS LEIDYI in the sampled areas. It is also expected that the high biomass of the organism detected in the region may have significant effects on pelagic ecosystems of the Caspian Sea.

Feeding regime of comb-jelly, MNEMIOPSIS LEIDYI, in the Caspian sea was investigated by analysing the stomach contents of about a thousand specimens collected from cities of Fereidoun-kenar, Noushahr and Noor during spring, summer and autumn. The study of feed particles in the stomachs of the juvenile and adult specimens from all the sampling stations at all seasons revealed the presence of mainly Calanoida and Acartia. Copepods and fish seeds followed next in volume; vegetal particles constituted the least volume. Over 80% of the stomachs of the sampled specimens was found to be full indicating that comb-jelly is voracious.

The alien ctenophore LEIDYI was entered from the Black Sea into the Caspian Sea by late 1990s. It has been reported that the M. LEIDYI caused the dramatic decrease of zooplonkton and pelagic fish stock in the Caspian Sea. In this study, spatial and temporal distribution of M.LEIDYI was studied during year 2004 in four transects Bandar Anzali, Khazarabad, Turkman and Miankaleh in the south coasts of the Caspian Sea. Results showed that M. LEIDYI achieved maximum abundance and biomass (6061±5830 n.m2 and 745±715.9 g.m2) in the upper layer of 50 (0-20) m depth during autumn. The Minimum biomass and abundance (22.78±9.64 g.m2 and 385±226 n.m2) of the ctenophore were measured in the coasts regions (5 and 10 m depth) during spring.The biomass and abundance of M. LEIDYI were more in southeastern of Caspian Sea than the southwestern. Larvae (<5 mm) contributed about 85% total abundance of the population. The maximum size of the measured M. LEIDYI was 64 mm in length. The effects of abiotic factors on distribution of M. LEIDYI was discussed.

This project was carried out with the aim of making a suitable net (mesh size, 1mm; total length of filtration area, 143.3cm including cylindrical and conical parts; and bucket volume, 1.7 liter) for catching M LEIDYI with the least injury, having efficient filtration and the most catch.For comparing it with plankton nets, some samplings were carried out in the two length groups 4-9mm and 12-25mm at depths of6 and 12m at 1m/s speed. Result revealed no significant difference between percentage of intact and injured samples of two nets in 4-9mm length group (P>0.05), whilst in 12-25mm length group, intact sample percentage in the experimental net was more than conventional plankton net (P<0.05). Some samplings were also done at depth of 12m at 1 and 1.5m/s speeds by the experimental net in order to find the suitable speed of towing. Result showed significant difference between the number of injured samples at two mentioned speeds (P<0.05) and the suitable speed of towing net for catching of this animal was obtained at 1m/s. Filtration efficiency at speed of 1m/s was and at speed of 1.5m/s was 0.95.

To identify dominant bacterial genera of invading comb-jelly (MNEMIOPSIS LEIDYI), samples were collected in triplicates from 5, 10 and 20 meter depths of the Caspian Sea. Samples were first homogenized in sterile normal saline and, then, were inoculated on tryptic soya agar at 30˚C for 72 hours. The morphological, physiological and biochemical characteristics of the grown bacteria from the second passages on tryptic soya agar were studied using standard bacteriological examinations. Genera of Corynobacter spp., Enterobacter spp., Vibrio spp., Psedotubercolosis spp., Listeria spp. and Yersinia spp. in samples from 5 meter depth, and Micrococcus spp., Enterobater spp., Listeria spp., Actinobacillus spp., Vibrio spp. and Morganella spp. were identified in samples from 10 meter depth. In addition, genera of Vibrio spp., Enterobacter, Aeromonas spp., Micrococcus spp., and Morgunella spp., were characterized in samples from 20 meter depth.Results showed that Vibrio spp. and Enterobacter spp. were dominant bacterial genera in samples from 5 meter depth, while Listeria spp. Vibrio spp. and Enterobacter spp. were dominant in 10 meter depth; Vibrio spp. and Enterobacter spp. were dominant bacterial genera in samples obtained from 20 meter depth.

The alien ctenophore MNEMIOPSIS LEIDYI which was transported from the Black Sea into the Caspian Sea at the end of 1990s has been negatively affecting ecosystem in this new environment. In this study, spatial and temporal distribution of M.LEIDYI were studied from a total of stations located along three transects (Amirabad, Babolsar and Nowshar) in the Eeastern Iranian coasts of the Caspian Sea (Mazandaran province) during July 2001 to November 2002.M. LEIDYI acheived maximum biomass 1024.5 g/m2 in August-October 2002. Minimum biomass (1.5 g/m2 of ctenophore were measured in December-Jaunuary 2001. The highest biomass was at the stations with 10m bottom depth (570.7 g/m2 in autumn and lowest biomass (75.9 g/m2) was obtained at a station with a 50 m bottom depth in winter. The highest average biomass (641.2 g/m2) were measured in ~irabad region and the lowest biomass (207.5 g/m2) observed in Nowshar region.The young specimens (<5 mm) contributed about 90% to the total abundance of the population. The maximum length was 51-55mm which was measured in August. The factors affecting the distribution of M. LEIDYI in the study area were discussed.