ORIGINAL ARTICLE Impact of a new invasive ctenophore (Mnemiopsis leidyi) on the zooplankton community of the Southern Caspian sea Abolghasem Roohi 1 , Zulfigar Yasin 2 , Ahmet E. Kideys 3,5 , Aileen Tan Shau Hwai 2 , Ali Ganjian Khanari 1 & Elif Eker-Develi 4 1 Caspian Sea Research Institute of Ecology, Khazarabad Boolvar, Sari, Iran 2 School of Biological Science, Universiti Sains Malaysia, Penang, Malaysia 3 Institute of Marine Sciences, Middle East Technical University, Erdemli, Mersin, Turkey 4 European Commission, Joint Research Centre, Institute for Environment and Sustainability, Ispra, Italy 5 Present address: Dolmabahce Sarayi, 2 Hareket Kosku, 34353 Besiktas, Istanbul, Turkey Problem Mnemiopsis leidyi is an ecologically important lobate ctenophore inhabiting estuaries along the Atlantic coast of the USA, such as Narragansett Bay (Kremer & Nixon 1976; Deason & Smayda 1981), Barnegat Bay (Mountford 1980), Chesapeake Bay (Herman et al. 1968; Burrell & Van Engel 1976) and estuaries of North Carolina (Miller 1974). A decline in the standing stock of non-gelatinous zooplankton has often been associated with pulses of M. leidyi and other species of ctenophores in seas and oceans of the world (Bigelow 1915; Nelson 1925; Bigelow & Leslie 1930; Russell 1933, Russell 1935; Barlow 1955; Conover 1961; Fraser 1962, 1970; Cronin et al. 1962; Hopkins 1966; Herman et al. 1968; Sage & Herman 1972; Kideys 2002). Keywords Alien; Caspian Sea; exotic; holoplankton; invasion; Iranian coasts; merozooplankton; Mnemiopsis leidyi; seasonal zooplankton distribution. Correspondence Abolghasem Roohi, Caspian Sea Research Institute in Ecology, PO Box 961, Khazarabad Boolvar, Sari, Iran. E-mail: roohi_ark@yahoo.com Accepted: 27 May 2008 doi:10.1111/j.1439-0485.2008.00254.x Abstract The invasive ctenophore Mnemiopsis leidyi (Agassiz), which was transported from the Black Sea into the Caspian Sea at the end of the 1990s, has negatively affected the ecosystem of the Caspian Sea. Zooplankton abundance, biomass and species composition were evaluated on the Iranian coast of the Caspian Sea during 2001–2006. A total of 18 merozooplankton (13 species composed of larvae of benthic animals) and holozooplankton (four Copepoda and one Cladocera) species were identified. The total number of zooplankton species found here was 50% less than in a previous investigation performed in the same region in 1996 before the introduction of Mnemiopsis leidyi into the Cas- pian Sea. Cladocera species seemed to be highly affected by the invasion of Mnemiopsis leidyi; only one species, Podon polyphemoides, remained in the study area, whereas 24 Cladocera species were found in the study carried out in 1996. Whereas among the Copepoda Eurytemora minor, Eurytemora grimmi, Calanipeda aquae dulcis and Acartia tonsa that were abundant before the Mnemiopsis leidyi invasion, only A. tonsa (copepodites and adults) dominated the inshore and offshore waters after the invasion. The maximum in zooplank- ton abundance (22,088 ± 24,840 indÆm )3 ) and biomass (64.1 ± 56.8 mgÆm )3 ) were recorded in December 2001 and August 2004, respectively. The annual mean zooplankton abundance during 2001–2006 was in the range of 3361– 8940 indÆm )3 ; this was two- to five-fold less than the zooplankton abundance in 1996. During 2001–2006, the highest abundance and biomass of Mnemiopsis leidyi were observed during summer-autumn months coincident with warm temperatures and generally when the abundance of other zooplankton organ- isms was low. Marine Ecology. ISSN 0173-9565 Marine Ecology 29 (2008) 421–434 ª 2008 The Authors. Journal compilation ª 2008 Blackwell Publishing Ltd 421