Aliens Associating with Caspian Sea Endemic Bivalves

Abstract

Endemic species of the Caspian Sea fauna and flora are potentially and actually threatened by invasive species, particularly since the opening of the Volga-Don Shipping Canal in 1952. While the invaders are more or less known, their impact on the Caspian ecosystem is poorly understood, maybe with few exceptions such as the impact by the immigrated ctenophore Mnomiopsis leidyl. Here we describe the association of the invasive cirripedian crustacean Balanus improvisus and the cheilostomate bryozoan Conopeum seurati with the limnocardiid bivalve Didacna sp. endemic to the Caspian Sea. The description represents only the first step towards an understanding of the interactions between the invasive and endemic species. The Caspian Sea is the largest lake on our planet spanning an area of about 400,000 km 2 and with almost 79,000 km 3 water volume (Dumont, 1998). The extension of the Caspian Sea has varied quite significantly during the last several million years, due to tectonic events and climatically induced lake level changes, which have particularly strong impact on the shallow areas of the Caspian depression. In historical times lake level fluctuated in a range between )29 and )19 m below sea level. Since the last couple of years it varied around )27 m, having risen from )29 m since the late seventies of the 20th century (Kosarev and Yablonskaya, 1994; Dumont, 1998; De Mora and Turner, 2004). The Caspian Sea is the only extant lake which was formed from an ancient ocean. The Caspian depression with its two deep basins was once part of the Paratethys, a sea which disintegrated into several more or less isolated basins during the late Neogene and the Pleistocene (Nevesskaja et al. 2001). The Caspian Sea is considered to constitute a lake since the Messinian (late Miocene), but there were connections to the Black Sea basin several times during the Pliocene and the Quaternary (Grigorovich et al. 2003). During certain phases of Pleistocene glaciation cycles, the Caspian Sea probably had also temporal connections with northern periglacial lakes and with the Aral Sea. These connections facilitated the faunal exchange between the water basins with the consequence that the Caspian Sea fauna has different geographical origins. The Holocene has been a period of relative isolation until the beginning of the 20th century when man started to introduce alien species to the Caspian Sea deliberately, such as the bivalve Abra ovata, to serve as fish food (Grigorovich et al. 2002). With the opening of the Volga–Don Shipping Canal in 1952, a permanent connection between the Caspian Sea and the Black Sea was established. Since then quite many new invaders arrived or were accidentally