166 MARINE ANIMALS 



of a few meters. Stenohaline animals may live on the coasts away 

 from river mouths and below the level of tidal variation. Great depths 

 are also suitable for them. The colonial radiolarians, the reef corals, 

 the juvenile stages of the fish Fundulus, and many others, afford 

 examples of stenohaline animals. 



Euryhaline animals are, of course, able to live side by side with 

 the stenohaline forms, where they make no use of their ability to with- 

 stand variation of salt content, Other situations, however, are acces- 

 sible only to them, the more exclusively, the greater the limits of the 

 variation in this respect. These situations afford them (i.e., the eury- 

 haline forms) the advantage of freedom from competition, above all 

 from pursuit by stenohaline enemies. Thus one must count among the 

 front ranks of the euryhaline forms the coastal fauna which lives 

 between the tide levels, and is thus exposed to rain water at low 

 tide, and the animals near the mouths of rivers, where the extent of 

 the influence of the fresh water varies with the variable volume of 

 flow of the river. In this category fall the inhabitants of the salt 

 marshes, whose waters are concentrated by the summer sun, and 

 freshened by fall rains and melting snow in the spring, and the 

 fauna of the spray-pools on rock coasts. Examples of euryhaline 

 animals are such jellyfish as Aurelia aurita and Crambessa tagi, which 

 are driven into river mouths without harm; the edible mussel 

 (Mytilus) ; the lugworm (Arenicola) ; the green crab (Carcinides 

 maenas); the appendiculate Oikopleura dioica; some sharks which 

 range into fresh water; and numerous others. Closely related animals 

 behave differently in this respect. Among the chaetognathous worms, 

 Sagitta hexaptera is very sensitive to fresh water, while Sagitta bi- 

 punctata has an unquestionable adaptability to brackish water. 1 



The composition of the faunae of waters with varying salinity is 

 regulated by selection, as has been shown for the saline lakes and 

 for the Baltic in a later chapter (p. 283). This is also plainly shown 

 in the fauna of the Caspian, which is an impoverished one, like all 

 brackish-water faunae. Important groups of animals, widespread in 

 the ocean, are entirely absent, such as Anthozoa, scyphozoan medusae, 

 Ctenophora, Echinodermata, Tunicata, Brachiopoda, Pycnogonida, 

 Scaphopoda, as well as Cephalopoda and Selachia; and yet there 

 can be no doubt of the marine origin of the fauna. 



Adaptation to lowered salinity is frequently connected with reduced 

 size. Sea urchins wander into the mouth of the Rhone as larvae and 

 become mature, but do not reach more than a third of the normal 

 size. 2 The common mussel, which reaches a maximum length of 110 

 mm. in the Bay of Kiel, reaches 74 mm. in the eastern part of the 



