662 



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



Fish-parasites. Seventy-one species of different fish-parasites should be added 

 to this list. According to V. Dogjel and B. Bykhovsky (1934) they are distri- 

 buted among the following groups : 



Protozoa 



Coelenterata 



Trematoda 



Cestoda 



Nematoda 



Hirudinea 



Crustacea 



14 species 



1 species 

 30 species 



9 species 

 10 species 



2 species 

 5 species 



Fig. 313. Zonal distribution of Aral fauna (Zenkevitch, 1951). 



For the Sea itself only 33 species of parasites have been confirmed. All 

 the others are inhabitants of low-salinity areas. Among these parasites the 

 Coelenterata Poly podium hydriforme and the Trematoda Nitschia sturionis, 

 brought into the Aral Sea with starred sturgeon during its acclimatization, are 

 of great interest.* About a hundred specimens of starred sturgeon were 

 introduced into the Aral Sea from the Caspian Sea, without being dis- 

 infected against parasites. 



The parasite of the gills of the starred sturgeon is the trematode Nitschia, 

 which is widely dispersed in the Caspian Sea and does little harm there. No 

 more than 40 specimens per fish have been recorded in that Sea. Once in the 

 Aral Sea, however, the trematode transferred to the local sturgeon Acipenser 

 and caused a serious epizootic epidemic which led to high mortality among 

 fish. Up to 600 trematodes have been counted on one sturgeon Acipenser. 



Over a period of some years the number of Trematoda dropped sharply ; 



* There is, however, an opinion that Nitschia sturionis existed in the Aral Sea before the 

 starred sturgeon was introduced. 



