THE BALTIC SEA 



323 



comprising mainly eurythermic species. The first group is qualitatively poor 

 and uniform, consisting almost exclusively of such Arctic species as Mesido- 

 thea entomon, Polynoe cirrata, Mysis mixta, Halicryptus spinulosus, Ponto- 

 poreiafemorata. Demel likewise includes the relict forms Terebellides stromii, 

 Macoma baltica and Diastylis rathkei. The shallower zone is inhabited by a 

 fairly varied fauna ; its most characteristic forms are : Cardium edule, Mya 

 arenaria, Nereis diversicolor, Macoma baltica, Mytilus edulis, Gammarus 

 locusta and Balanus improvisus. The boundary of the two groups of fauna (the 

 surface and the bathypelagic) lies at a depth of 25 to 40 m. 



As regards the fate of the Macoma baltica biocoenosis farther north, it should 

 be pointed out that the area of Gotland Island serves as a kind of boundary 

 dividing this biocoenosis into two parts. South of Gotland the benthos has not 

 yet the distinct oligomixed nature characteristic of the more northerly 

 regions. Mytilus, Mya and Cardium do not yet lose their importance com- 

 pletely ; on the other hand such forms as Pontoporeia affinis and Mesidothea 

 entomon are still not yet developed to a significant extent. Pontoporeia femo- 

 rata still supplants its kindred species P. affinis. These two characteristic 

 biocoenoses of the Baltic fauna are adapted to two different biotopes. P. 

 affinis inhabits the less saline, shallower parts of the Sea and is often pre- 

 ponderant on sand bottoms. P.femorata keeps to more saline, deeper layers 

 and is frequently found in large numbers on mud bottoms. 



The benthos composition in the area of Gotland and the Aland Islands is 

 set out in Table 130. 



Table 130 



Encircling Gotland Island at depths below 80 m lives an impoverished 

 benthic community. As the depth increases, firstly the molluscs disappear, 

 then the worms and finally the crustaceans. Only the polychaete Scoloplos 

 armiger can five on mud bottoms infected with hydrogen sulphide. The 

 last representatives of the remaining animal population — Pontoporeia femorata 

 and Terebellides stromii — disappear a little earlier. 



Another very characteristic Baltic Sea form, Mesidothea entomon, which 

 is abundant in the western and northern parts of the Sea, gradually disappears 

 south of the latitude of Aland Island. 



As one moves farther to the north into the Aland Islands area the selection 

 of saline-loving forms {Cardium edule, Nereis diversicolor, Terebellides stromii, 



