Ahmet E. KIDEYS
Galina A. FINENKO, Boris E. ANNINSKY
Tamara A. SHIGANOVA
Abolghasem ROOHI, Mojgan ROUSHAN TABARI, Mehdi YOUSEFFYAN, Mohamad T. ROSTAMIAN, Hosseinali ROSTAMI
In addition, M. leidyi has an ability to regenerate from fragments larger than one quarter of an individual (Coonfield, 1936). It is also a generalist carnivorous feeder (Tzikhon-Lukanina & Reznichenko, 1991) and occurs over a broad range of salinity conditions (Harbison & Volovik, 1994).
The native habitat of ctenophores in the genus Mnemiopsis is in estuaries along the eastern coastline of north and south America. 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 (Vinogradov et al, 1989). It has spread from the Black Sea to the Seas of Azov, Marmara and eastern Mediterranean (Studenikina et al., 1991; Shiganova, 1993; Kideys & Niermann 1994; Shiganova et al., 2001). Its impact is a serious problem for the ecosystems of most of these basins.
A steep decline in ichthyo- and mesozooplankton abundance and change in species composition follow its bloom (Vinogradov et al., 1992; Shiganova, 1997; Kovalev et al., 1998, Konsulov & Kamburska, 1998). The catches of zooplanktivorous fish sharply dropped (Volovik et al, 1993; Kideys, 1994; Prodanov et al, 1997; Shiganova, 1997, 1998; Kideys et al., 1999). By the late 1980s and early 1990s the pelagic ecosystem of the Black Sea had become a dead-end gelatinous food-web.
But in 1999 first bloom of a new invader Beroe ovata was recorded in the Black Sea (Finenko et al., 2000, Shiganova et al., 2000). It is not clear whether this species is also introduced with ballast waters or naturally transferred from the Mediterranean Sea. However, there are signs that the ecosystem of the Black Sea began to recover (Kideys & Romanova, 2001; Shiganova et al, 2001; Yunev et al. 2001) due to sharp decreases in Mnemiopsis population. Investigations in the Black Sea have shown that Beroe almost exclusively feeds on Mnemiopsis and very effective in controlling its levels (Kideys et al., 2000; Finenko et al., 2000; 2001, Shiganova et al, 2000, 2001).
At the end of 1990s, however, we are faced with a new problem – the invasion of Mnemiopsis in the Caspian Sea (Ivanov et al., 2000). The most probably that this ctenophore was transported with ballast water taken aboard in the Black Sea or the Sea of Azov (where Mnemiopsis occurs in warm months) and released after ballast-loaded ships passed through the Volga Don Canal either into the Caspian Sea.
The impact of Mnemiopsis on the Caspian Sea ecosystem may be much worse than in the Black Sea due to greater sensitivity of this closed basin. Due to damage observed in the Black Sea, there has been a fast response over the presence of Mnemiopsis in the Caspian Sea. Since Mnemiopsis is a voracious predator on zooplankton, both abundant small pelagic fish feeding on zooplankton as well as large predators feeding on these fish such as white sturgeon (Huso huso) and endemic Caspian seal (Phoca caspica) would be under significant threat in the Caspian Sea.
Investigations of a new invader Mnemiopsis leidyi in the Caspian Sea in 2000 showed that Mnemiopsis was distributed mainly in the middle and southern Caspian Sea. By July and in September it was found everywhere including the northern Caspian where the salinity could be as low as 4 ppt. Density of Mnemiopsis ranged between 3 and 100 ind. m-2 in July. In October density of the ctenophore increased up to 170 ind. m-2 (Shiganova et al. 2001). Preliminary studies suggested a decrease in the quantity of mesozooplankton in the northern Caspian. In August 2001 density and biomass of Mnemiopsis increased signiificantly in all areas of Caspian comparing with the same season 2000 (Kideys et al., in preparation; Shiganova et al. in preparation), but the most considerable increase was recorded in the southern Caspian Sea.
Following the invasion of Mnemiopsis, sharp decreases were observed in the pelagic (mainly kilka, Clupeonella spp.) fishery of Iran (Kideys et al., 2001a) and Azerbaijan (Kideys et al., 2001b) as well as of Russia, showing the scale of the problem.
Positive effect of appearance Beroe ovata on the Black Sea ecosystem is a great example for the control of an invader, which caused such damage to an ecosystem. According to the decision of the First International Workshop, organized by Caspian Ecological Program (CEP) in April 2001, it was concluded that Beroe ovata is the best candidate to control Mnemiopsis population in the Caspian Sea. However, scarcity of systematic data on Mnemiopsis levels and ecology from the Caspian Sea, was strongly felt, and therefore CEP stressed the need for good, comparable data on this subjects. As a result:
Within the framework of the latter two programs, specially designed laboratory experiments were performed in Mazandaran Fisheries Research Center in Iran from 13 of September to 11 of October. The main purposes from this activity were:
1). Studying survival and tolerance of Beroe ovata in the Caspian Sea
2). Studying some aspects of ecological physiology of Beroe ovata in the Caspian Sea water to determine possibility its introduction into the Caspian Sea
3). Studing some aspects of ecological physiology of Mnemiopsis leidyi in the Caspian Sea to identify its impact on the Caspian zooplankton community:
MATERIAL AND METHODS
A. EXPERIMENTS WITH BEROE OVATA IN THE CAPIAN SEA WATER
Acclimation of Beroe ovata to the Caspian Sea water salinity
Beroe generally made up of small individuals (10-40 mm) was transported into Caspian coast of Iran in two batches. For the first batch, 30 Beroe sampled from Sinop, Turkey (southern Black Sea; salinity is about 18 ppt) on 11 September 2001 were transported to the laboratory in Khazerabad (Mazandaran, Iran) in a 10 l jar. The total journey time was about 48 hours for this batch of Beroe. In the second batch about 60 Beroe were collected from Bosporus (salinity around 22 ppt) on 13 September 2001. Journey time was about 24 hours for the second batch.
Acclimation of Black Sea Beroe ovata to the Caspian Sea water salinity was done by changing salinity step by step from 22 ppt to 12.6 ppt with three intermediate salinities of 17.4, 15.0, 13.5 and 12.6 ppt.
Upon arriving to the laboratory on 14th September, healthy looking specimens of Mnemiopsis were put into a large tank (15 liters seawater) in the room with controlled temperature of 21 0C. After 4 hours the salinity was decreased down to 17.4 ppt with filtered Caspian Sea water (12.6 ppt off Khazerabad) (1:1). During next days salinity was decreased every 9-17 hours and the behavior of animals in new salinity conditions was observed.
Table 1. Acclimation of Bosporus B. ovata to Caspian water salinity
7 individuals from Sinop were kept individually at 26 oC in conditions of original salinity 18 ppt in 5-1 containers during 1 day; next day they were removed to the container with lower salinity water about 16 ppt and a day later transferred into the container filled totally the Caspian water (12-13 ppt).
All containers were provided with aeration. After Beroe ovata acclimation to the salinity of the Caspian Sea, the experiments on its feeding, respiration, growth and reproduction rates were started.
Estimation of ingestion rate and digestion time of Beroe
Two independent methods were used to determine the feeding rate of B. ovata at 21 oC. The first method included two series of experiments at 21 oC and another series of experiment at 26 oC. In the first series of experiments in each of 15 containers (3.5 1iter capacity each) 12 Mnemiopsis of four size groups (<5, 5-10, 11-15 and >15 mm, being 3 Mnemiopsis from each size group) as a food for individual Beroe were placed. The 16th container contained only 12 Mnemiopsis without Beroe to keep as control. The length of B.ovata in these experiments ranged from 13 to 35 mm. All Beroe starved at least 24 hours before the feeding experiment.
This experiment was designed so that in addition to the feeding rate we could determine prey-size preference, digestion time as well as interval between two following engulfs in B. ovata with monitoring all bottles every 30 min during 24 hours.
In these conditions B. ovata had many available preys and could feed ad lib over the whole period. Total biomass of preys was about 1.66 ± 0.31 g/l., duration of experiment was 24 hours. The numbers and length of M.leidyi in the containers were counted and measured at the start and the end of experiment. The daily ingestion rate (number or biomass of preys that one Beroe could consume per day) was calculated from the measured differences in total numbers of the preys at the start and the end of observations. To estimate the ration in weight units the relationship between length and weight of Mnemiopsis was used: W=0.0011*L2.34, where W-wet weight of M.ledyi, mg, 1-length, mm (Kideys et al., 2001a). Beroe weight was computed from length- weight relation: W= 0.00071*L2.467 (Finenko et al, unpublished data).
The second series of feeding experiments at 21 oC as conducted to determine ration value at different prey sizes (3 size group: I -5-6 mm, II-10 mm and III - 30-40 mm Mnemiopsis,) in the same biomass concentration (about 1 g/l). The temperature conditions of these experiments were similar to those ones in the first series.
The third series of feeding experiments were designed for determining the digestion time with respect to the size ratio of prey and predator at 26 0C. Once ingestion occurred the B. ovata specimens were monitored every 15 min until defecation was complete and the gut was empty. Then, knowing the size distribution of ctenophores from field data, this relationship could be used to calculate maximum potential ingestion rate in field.
In the second method, a long-term experiment (14 days) to study ration and growth was performed at 24-260 C. Five specimens Beroe with wet weight of 3.17 –4.64 g or length of 30-35 mm were placed individually in each container (4.2-17.0 l volume). 5 to 12 specimens Mnemiopsis (size 10 –30 mm) were placed into each container depending on the volume of container. Initial concentration of preys was about 1 ind/ l. Number of preys refreshed every day after calculation of remained Mnemiopsis after Beroe feeding .
Daily ration was estimated from the number and wet weight of ingested preys.
Growth rate of Beroe
Growth rate was examined simultaneously with ration study in above long-term experiment (10-14 days) at 24-26 0 C. Growth rate was estimated from regular measurements of Beroe length in each container every day at the same time.
Respiration rate of Beroe
Fourteen replicates were conducted to study respiration rate with 21 specimens of Beroe at temperature 21 (7 specimens) and 23o C (14 specimens). The individual ctenophores were kept in the dark in respiration chambers (0.250 l volume) filled with 120 micron filtered sea water for 7-18 h. During the experiments the concentration of oxygen was not allowed to decrease more than 10% . Oxygen content was determined by Winkler method ( Omori & Ikeda, 1984).
Reproduction of Beroe
Aquariums with Beroe were observed every after night with the aim to obtain eggs of Beroe.
Eggs and early embryos were obtained from the fed specimens of Beroe in aquariums with Caspian seawater and replaced in incubator and 100 ml dishes for development and hatching . They were examined every several hours to observe hatching and development Beroe in the Caspian Sea water.
B. EXPERIMENTS WITH MNEMIOPSIS OF THE CASPIAN SEA
Feeding experiments with Mnemiopsis
Feeding experiments with M. leidyi were conducted from 26 September to 7 October 2001. Animals were collected in region of Khazerebad with a METU net (designed by A. E. Kideys) of 500 micron mesh size and put in large aquarium (20 l volume). Undamaged Mnemiopsis were taken out gently from aquarium into experimental containers within 1-2 hours before experiments.
The zooplankton preys were collected daily, with horizontal tows (3 times during 10 min. each of them) using 100 micron mesh size, then they were filtered through 50 micron mesh to get samples without Mnemiopsis and large zooplankton species. Prey consisted mostly of nauplii, copepodites and adult Acartia clausi. One day (30th September) there were plenty Nereis larvae in the sample. Sea water filtered through a 30 micron mesh was added into the sample to have the total volume 1000 ml. Before experiment the number of Copepoda was counted 3 times in 10 ml of this sample. To have initial concentration 100 Acartia /l in each from 4 experimental jars (5 l volume) we put there certain volume (85-140 ml in different experiments) of this sample, that was calculated from average concentration of Acartia in the previous counted sample. 2 jars with the same volume of zooplankton without Mnemiopsis were kept as controls. Four experiments were conducted at 210 C, one was at 24 0C. All experiments were carried out in the dark, duration of experiments was 6 h. Three size groups of Mnemiopsis were studied: I- 5 mm, II- 10 mm, III- 15-20 mm. Into each jar 10 animals of I group and 5 Mnemiopsis of II and III size groups were added.
After experiment Mnemiopsis were removed from experimental jars and water with zooplankton was filtered through the 30 micron mesh to have total volume 200-250 ml. The number of different stages of Acartia (copepodits and adult) was counted in every jar. The average concentration of prey in the jar (mg/l) was calculated from numbers and individual weight of each stage in control jars. The clearance rate was computed on difference between control and experimental jars using the equation
Cl = V* (lg Cc- lg Ce)/ 0.4343*n t,
After the experiment the lengths of Mnemiopsis specimens were measured and weighed in the each experimental jar.
Respiration rate of Mnemiopsis
45 replications for study of respiration rate of M. leidyi were performed with numbers of Mnemiopsis from 1 to 240 specimens per container depending on Mnemiopsis size. Ctenophores were kept 10-24 h for acclimation to new temperature conditions. Experiments were carried out at the temperature 18 oC (2 replications); 24-25 oC (2 replications); 28-29 oC (1 experiment). Duration of experiments was from 7 (28-29 oC) h to 24 h (18 oC). During the experiments the concentration of oxygen decreased not more than 10% of initial concentration.
Calculations of the metabolic rates were made from the measured difference in oxygen concentration between bottles with and without ctenophores. At the end of the incubation period, oxygen concentrations were measured in subsamples of seawater transferred into 30 ml biochemical-oxygen demand bottles. Oxygen concentrations were determined by titration using Winkler method (Omori & Ikeda,1984).
Reproduction of Mnemiopsis
Specimens of Mnemiopsis were collected from the boat with the net of a 500 m m net, having a diameter of 0.5 m (METU Net).
Adult specimens of different size were picked up from the sample and put into the two-liter containers filled with filtered seawater at 22-25 oC and placed in the dark for spawning. Some of them were specially fed before experiment.
Fertilized eggs and early embryos were obtained during night and after calculating total numbers, put into another container or incubator with filtered seawater for development and hatching. After 24 hours percent of hatched eggs was estimated.
Will be added in due time….
Our experiments in Fisheries Research Center of Mazandaran show that Beroe ovata can live and grow intensively in the Caspian Sea water. Beroe should be acclimated gradually to lower salinity of the Caspian water (from 18-22 ppt of the Black Sea) in a few days. Salinity should be decreased not more than 2-3 ppt a day. We managed to keep Beroe in aquarium at salinity 12.8 ppt and temperature 21 oC during about 1 month (13 September - 9 October). The high growth rate of Beroe was observed in high food concentrations.
Beroe feeding rate at salinity 12.8 ppt is quite high (it can be as high as > 100% of body weight a day) for different size Beroe. Daily rations at 12.8 ppt are close to these at 18 ppt (the Black Sea) (Finenko et al., 2001, Anninsky et al., 1998).
So based on physiological data one could suggest that in the Caspian Sea Beroe is able to grow and ingest Mnemiopsis intensively and decrease its abundance sharply as it happened in the Black Sea.
Although we were able to reproduce Beroe into eggs and on a few occasions into larvae, larval development experiments were not successful in the laboratory. However it is still possible that successful development may take place in field conditions. We also quantified different physiological characteristics of Mnemiopsis leidyi successfully. These data will be evaluated with respect to ongoing monitoring studies to evaluate the impact of Mnemiopsis on the pelagic ecosystem of the Caspian Sea in near future.
Our data on feeding, respiration, reproduction rates and effect of temperature on metabolism are ecological-physiological background to estimate predatory impact of Mnemiopsis on the Caspian Sea zooplankton community. Mazandaran Fisheries Research Center and another Iranian Institute (Guilan) on the Caspian coast have ongoing monitoring programs for estimating abundance and biomass of Mnemiopsis population since July 2001 (Kideys et al., 2001a). Joint study of population dynamics and ecological-physiological characteristics of Mnemiopsis in this region will be conducted to estimate critical abundance that will be harmful for ecosystem of the Caspian Sea.
We greatly appreciate hospitality and kind help of all staff of laboratory of Mazandaran Fisheries Research Center in sampling and preparing laboratory equipment as well as great attention to our experiments. Special thanks to Drs. Rezvani (Head of IRFO) , A. Salmani, S. Ghasemi, F. Parafkandeh, M. Najafpour (IFRO, Tehran), M. Nazaran, A. R. Kihansani, J. Sharifi, T. M. Pormand, A. Nasrollatabar, M. Ebrahimzadeh, A. Jafarei, R. Ahmedinejad. This study was made possible by organizational efforts of Dr. Vladymyr Vladymyrov (Caspian Environment Program) under financial support of UNOPS, project RER98G32 and IFRO (Iranian Fisheries Research Organisation, Shilat) Tehran.
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