Study of Mnemiopsis leidyi diet and reproduction in the southern coasts of Caspian Sea

Abstract

Mnemiopsis leidyi is one the species of comb-jelly. It belongs to the Ctenophore phylum and lobate order. Mnemiopsis leidyi first appear in the Black Sea in 1980s and reached to the biomass levels up to 1.5-2 kg m2 in the summer of 1989. This species is food rival of pelagic fish. It has been reported that the ctenophore caused the dramatic decrease of zooplankton and pelagic fish stock in the Black Sea. In this study, sampling on Mnemiopsis leidyi, zooplankton and phytoplankton started from July to December 2002 from a total of 2 stations located Anzali and Khazarabad by METU net in the Iranian Coasts of the Caspian Sea. 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 with 12 mm length and weight of 0.5 g. The most abundant size of reproducing Mnemiopsis was 20-30 mm in the Caspian Sea. Average fecundity of M.leidyi in the Caspian Sea was 1174±741 eggs/day with maximum 2824 eggs/day for specimens with length group of 30-39 mm and the weight about 2.0-2.7 g. Percentage of hatched eggs was not high in experiments with M.leidyi the range from 9 to 92 % after 24 hours. The maximum produce egg of M.leidyi measured with mean 845±733 eggs/day in Salinity of 11 ppt and the minimum egg recorded with mean 2 eggs/day in salinity of 4 ppt. The results showed that the biomass of M.leidyi had some fluctuation in different months and its maximum biomass was recorded in August with a figure of 384±156 g.m2 and the minimum biomass was observed in December with a figure of 87±46 g.m2. The gut contents of Mnemiopsis leidyi showed a wide variety of prey including 84 % of zooplanktons and 16% of Phytoplanktons. The most abundance of intaked foods was Acartia (belong to Copepoda) with 34 % and the least was podon polyphemoides (belongs to Cladocera) with 0.74 %. Also 3.0 % of fish eggs and 4.2 % of Lamllibrachia were found among stomach contents. The young specimen (5< mm) consumed more zooplanktons and phytoplanktons comparing to adult ones. Copepoda was dominant zooplankton in the Iranian coasts of the Caspian Sea. Fecundity of Mnemiopsis showed correlation to the length and wet weight in the Caspian Sea and reproduction of M.leidyi was done small size in comparing with other area in the Caspian Sea. Salinity, water temperature and food availability is the most important factor in producing eggs in M.leidyi. It seems, that Mnemiopsis leidyi feeding has been one of the most important factor in declining zooplankton populations and Clupeonella stocks.