THE BALTIC SEA 309 



into the zones of lower salinity than the other forms ; Idothea baltica enters 

 both Gulfs, while Idothea granulosa and /. viridis do not go beyond the en- 

 trances of the two Gulfs. 



Decapoda of marine origin are very poorly represented in the central area 

 of the Baltic Sea. There are 64 species of decapod crustaceans in the Swedish 

 waters of the Skaggerak and Kattegat ; in Oresund there are 24, and ir. Kiel 

 Bay 10. Only two species — Crangon crangon and Leander adspersusvar.fabricii 

 — inhabit the central basin. 



Apart from Mysis oculata, which densely populates this Sea, Mysis vulgaris 

 and M.flexuosa among the Mysidacea penetrate far into the Sea. 



Of the echinoderms only the most euryhaline, Asterias rubens and Ophiura 

 albida, are found in the Baltic Sea itself. 



Finally the sea fish most common in the Baltic Sea are : the brackish-water 

 race of herring, which occupies first place in the fishing industry ; Clupea 

 harengus membras (the so-called Baltic herring), and then the following : CI. 

 sprattus, Gadus morrhua, Lumpenus lampetriformis, Cottus scorpius quadri- 

 cornis, Liparis liparis, Cyclopterus lumpus, Pholis gunellis, Zoarces viviparus, 

 Spinachia spinachia, Nerophis ophidion, Siphostoma typhle, Ammodytes lan- 

 ceolatus, A. tobianus, Pleuronectes flesus and Bothus maximus. 



Decrease in size. Like many other groups of organisms with a calcareous 

 skeleton, the molluscs diminish in size with decreasing salinity as one moves 

 eastwards (Fig. 147). Mytilus edulis, which is up to 150 mm long off the shores 

 of Great Britain, is no more than 110 mm long in Kiel Bay, no more than 

 40 mm off the Finnish coast, and only 20 to 25 mm at the far end of the Gulfs 

 of Bothnia and Finland. Off the Aland Islands Mytilus is no more than 

 37-5^mm long, while off Liban it is 38-5 mm. West of Bornholm it reaches 

 55-5 mm. The fluctuations in the maximum size within the limits of the Baltic 

 Sea proper are small, and a marked increase occurs only in the transitional 

 region*of the Belts and Oresund, this being related to changes in salinity. 

 The same is observed with Cardium and Mya. The maximum size of Cardium 

 edule at the northern boundary of its distribution is 18-5 mm; northwest of 

 the Aland Islands it is 23-7 mm, while in the North Sea its average size is 

 45 mm. In the North Sea and Kiel Bay the largest Mya arenaria is about 100 

 mm long ; off Gotland it is 58 mm, and at the eastern boundary of its distri- 

 bution in the Gulf of Finland it is 36-5 mm. In the case of many forms the 

 decrease in their size at the limits of their habitat, in the less saline sectors of 

 the Baltic Sea, is linked with the loss of reproductive power. The adult forms 

 exist, but either multiply very rarely or not at all. At a salinity below 6% the 

 normal sexual cells are formed in Amelia aurita, but the scyphistomae are not 

 developed (Sagerstrale, 1951). 



At the same time it is interesting to note that this rule of a decrease in size 

 associated with a fall in salinity does not hold good with certain forms. 

 Macoma baltica, for example, is 22 mm. long in the North Sea and retains this 

 length in the Baltic Sea. It is true there are some indications that at the extreme 

 limits of its distribution in the Gulfs of Bothnia and Finland the size of 

 M. baltica falls to 15 to 18 mm. According to K. Levander (1899), however, 



