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



form evolved in an environment different from that in which the relict form 

 exists today. Ekman calls a relict form pseudorelict if it has penetrated by a 

 second stage into its present environment from some other body of water. As 

 regards the Baltic Sea fauna it is often difficult to decide whether some form is 

 a relict or a pseudorelict, especially if one takes account of the fact that during 

 the colder phases of the post-glacial period some of those forms may have had 

 a continuous habitat across the North Sea. Thus the mark of a relict is its isola- 

 tion from its main habitat either in space or in time. Many forms which are 

 abundant in the central and northern parts of the Baltic Sea (Figs. 162 and 

 163) are either entirely absent from, or rare off, the western and northern 

 coasts of Scandinavia (Figs. 164, 165 and 166), their main habitat being the 

 Arctic Ocean. 



All these forms in the Baltic Sea may be considered as marine ice relicts of 

 the Yoldian stage, which in an earlier, colder period had a continuous habitat 

 including the Arctic basin. The following are such relicts : among Hydrozoa : 

 Halitholus cirratus', among molluscs: Astarte borealis (Fig. 167); among 

 worms: Halicryptus spinulosus (Fig. 166); among crustaceans: Mesidothea 

 entomon, Pontoporeia affinis and P.femorata (Fig. 163) ; Pallasea quadrispinosa, 

 My sis oculata, M. mixta, Limnocalanus grimaldi (Fig. 162); among fish: 

 Myoxocephalus quadricomis (Fig. 164) ; and among mammals : Phoca hispida 

 {Ph. foetida). 



S. Ekman (1935) has subdivided all these Ice Age marine relicts into three 

 groups. In the first group he includes the forms which at the present time also 

 live in their main habitat, the Arctic basin, only in greatly diluted or fresh 

 water. They are usually called true brackish-water forms or, strictly speaking, 

 Arctic brackish-water relict fauna. This group includes : Mesidothea entomon, 

 Pontoporeia affinis, Limnocalanus grimaldi, Pallasea quadrispinosa* The 

 second group consists of the extremely euryhaline forms Phoca hispida, 

 Myoxocephalus quadricomis and Mysis oculata, which can thrive equally well 

 in sea and fresh water. Euryhaline marine and brackish forms, of less eury- 

 halinity than the previous group, belong to the third group, namely : Ponto- 

 poreia femorata, Mysis mixta, Halicryptus spinulosus, Astarte borealis and 

 the hydroid Halitholus cirratus. 



It is clear from the charts that the representatives of this last group avoid 

 the least saline parts of the Baltic Sea. Ekman suggests that during the Ancylus 

 stage these forms must have disappeared from the Baltic Sea ; he admits that 

 they may have found a refuge in the western part of the Sea within the region 

 of the present-day straits. These forms populated the Baltic Sea again during 

 the Littorina period. 



Some of these relicts are found in the Baltic Sea in greater numbers than 

 anywhere in the Arctic region. Moreover the fauna of the Baltic Sea contains 

 a number of forms which are, as it were, intermediate between relicts and the 

 forms with a continuous distribution. These latter are abundantly represented 

 in the Baltic and the Arctic basin but are not found in large amounts in the 

 intervening areas. Such forms include the polychaetes Artacama proboscidea 

 and Harmothoe sarsi, and the molluscs Macoma (Tellina) baltica amd others. 



* S. Sagerstrale explains the genesis of P. quadrispinosa in a different manner (see later) 



