By volume as well as surface area, the Caspian Sea is the largest lake of the world. It is lacking an outlet. Because of the origin from the Parathetys and repeated reconnections to the Mediterranean and Black Sea, the water of the Caspian Sea is brackish. But salinity is not uniformly distributed, ranging from almost fresh near the river deltas at the northern Caspian up to hyperhaline concentration at shallow gulfs at the eastern shore. Due the almost anoxic conditions below 100 m, the Caspian Sea reveals relatively low degree of endemism, if taking surface area into account.
Despite the fact that most taxa are historically immigrated into the Caspian Sea either by the Thetys, as glacious relicts or from the Black and Mediterranean Seas, the number of accidentally introduced species is raising since the beginning of the 20th century. Also, due enhanced lake level changes, pollution and over fishing, the Caspian Sea suffers increasing anthropogenic pressure. The Caspian Sea region is also well known for being a source region for important continental freshwater invasions.

 

Turricaspia sp. (photograph: T. Hauffe)

South-eastern shore of the Caspian Sea (photograph: N. Nahavandi)