566 BIOLOGY OF THE SEAS OF THE U.S.S.R. 



is occupied, side by side with molluscs and fish, by crustaceans, mainly Pera- 

 carida; 136 species of this last inhabit the Caspian Sea. All Caspian mysids 

 belong to the sub-family Mysini, the family Mysidae and the genera Hemi- 

 mysis (1 species), Mysis (4 species), Schistomysis (1 species), Paramysis (10 

 species), Caspiomysis (1 species), Katamysis (1 species), Diamysis (1 species), 

 and Limnomysis (1 species). Only Caspiomysis is strictly endemic to the 

 Caspian Sea. Katamysis and Limnomysis are endemic forms of the Pontic- 

 Caspian region. The others have a wider distribution in the oceans. The Mysis 

 genus stands apart. Most mysids are representatives of plankton-benthos, 

 but Paramysis loxolepis and the species of the genus Mysis (except M. caspia) 

 belong to plankton ; they make daily vertical migrations of some hundreds 

 of metres (up to 500 m). Representatives of the genus Mysis, evolved from 

 the Arctic immigrant Mysis oculata var. relicta, have retained their Arctic 

 aspect, living in depths of more than 50 m. A number of Caspian mysids 

 penetrate fresh waters and become adapted to them. 



Among the Cumacea order representatives of 8 genera (Pseudocumatidae 

 family, Pseudocuma, Stenocuma, Pterocuma, Volgocuma, Caspiocuma, 

 Schizorhynchus, Chasarocuma) and 19 species live in the Caspian Sea. 

 Derzhavin has pointed out that they have probably all evolved from one 

 ancestral form of the genus Pseudocuma, and that they all converge (mor- 

 phologically and biologically) on the Cumacea community of the ocean fauna. 

 All the 19 species of Cumacea inhabit only the Pontic-Caspian region, and 

 10 species are strictly endemic to the Caspian. Some Caspian Cumacea have 

 also penetrated into fresh waters in the basins of the Caspian Sea (10 species), 

 of the Azov Sea (9 species), and of the Black Sea (9 species plus two doubtful 

 ones). Cumacea live at the bottom ; however they are involved in the diurnal 

 rhythm of vertical migrations. 



The large order Isopoda is represented in the Caspian Sea by 3 species only 

 of varied genesis. laera sarsi is an endemic form of the Pontic-Caspian region. 

 Mesidothea entomon f. caspia is an immigrant from the Arctic. The third form, 

 Nannoniseus caspius, described by O. Grimm in 1875 from one specimen, has 

 not been recorded by any one else in the Caspian Sea. Iaera inhabits the 

 shallow littoral zone, while Mesidotea, in contrast, retaining its Arctic aspect, 

 does not rise into the upper warmed layers. 



Among the higher crustaceans the order Amphipoda is the richest in species 

 and the most characteristic of the Caspian fauna. All Caspian amphipods (72 

 species) belong to the only sub-order — Gammarideae — and mostly to the 

 Gammaridae (60 species) and Corophiidae (8 species) families. Of the other 

 four species two Pseudalibrotus species and one of Pontoporeia are immi- 

 grants from the far north, while Caspicola knipovitschi, an original form 

 described by Derzhavin, forms a separate family — the Caspicolidae. Except 

 for these forms the Caspian Amphipoda belong to two families. The follow- 

 ing genera are particularly rich in species : Gmelina (5 species), Amathillina 

 (5 species), Niphargoides (11 species), Pontogammarus (10 species), Steno- 

 gammarus (6 species), Dikerogammarus (4 species), Chaetogammarus 

 (4 species), and Corophium (8 species). The other genera — Niphargus, 

 Boeckia, Gmelinopsis, Gammaracanthus (Arctic immigrant), Cardiophilus, 



