294 



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



While plankton in the straits leading to the Baltic Sea does not differ much 

 from that of the North Sea, in the upper parts of the Gulfs of Bothnia and 

 Finland the plankton has a purely fresh-water character. Instead of the 

 numerous marine species Chaetoceros, Rhizosolenia, Ceratium tripos and С 

 fusus, a considerable number of hydro-medusae, Copepoda (Oithona nana, 

 Eurytemora hirundo, Paracalanus parvus, Acartia longiremis), the marine 

 species of Rotifera (species of the genus Synchaeta mastigocera), numerous 



Fig. 138. Penetration of certain Copepoda into the depths of the 

 North Sea and into the Baltic Sea (Pesta). 1 Eurytemora 

 hirundoides typicus; 2 E. hirundo; 3 Oithona; 4 Southern 

 boundary of Oithona similis (northeastern in the Baltic Sea) ; 

 5 Northern boundary of Oithona rnana ; 6 Eastern boundary of 

 Centropages; 7 Metridia longa; 8 Northern boundary of 

 Paracalanus parvus ; 9 Northern boundary of Pseudocalanus 

 elongatus; 10 Northern boundary of Acartia bifilosa; J J 

 Southern boundary of Limnocalanus grimaldi. 



Tintinnoidea (Parafavella, Tintinnopsis), a series of the species Sagita, the 

 pteropod mollusc Limacina retroversa and others, we have east of the Darss 

 ridge throughout the Baltic the blue-green algae Aphanizomenon flos-aquae , 

 Nodularia spumigena and Anabaena baltica; the diatoms Chaetoceros wig- 

 hami, Thalassiosira baltica and Ch. danicum, which sometimes bring about a 

 summer and autumn flowering of the peridineans P. depressum and P. pellu- 

 cidum, Prorocentrum micans, Dinophysis baltica, Goniaulax catenata and 

 others, some Infusoria, for instance Tintinnopsis campanula, Helicostomella 

 subulata, among the Rotifera a preponderance of Brachionidae, and the 

 species Collotheca pelagica. Most of the Rotifera belong to euryhaline fresh- 

 water forms (Brachionus angularis, B. pala, B. bakeri, Anuraea aculeata, 



