342 ANIMALS IN INLAND WATERS 



The fish fry typically are plankton feeders; of adult fishes, the Sal- 

 monidae are especially dependent upon the plankton. 



Fishes living on the lake bottom show a number of adaptations 

 which appear convergently in members of groups of widely different 

 taxonomic relationships. In many of these forms the search for food 

 in the turbid water near the muddy lake floor is facilitated by means 

 of beard-like feelers thickly seat with nerve buds. These are exhibited 

 by the sturgeon and the sterlet, the carp from among the Cyprinidae, 

 the Cobitidae, many catfishes, which are typically inhabitants of the 

 bottom, and the eelpout {Lota) among the Gadidae. The mouths of 

 the sturgeon and carp and of the American suckers (Catostomidae) 



Fig. 101. — Sheatfish or wels, Silurus glanis, a mud-inhabiting catfish of central 

 Europe, with elongate anal fin. After Brauer. 



can be protruded for the purpose of gathering food from the bottom. 

 The long extended trunk of the African Mormyridae is of service in 

 searching through the mud. All these fishes which feed on the bottom 

 mud possess, like the plankton feeders, a fine gill filter. Another 

 characteristic common to many bottom-living fishes is the extension 

 of the anal fin, which, with its wave-like serpentine undulations, lifts 

 the posterior end of the fishes upward and depresses the anterior end: 

 e.g., the European catfish (Silurus glanis) (Fig. 101), the eelpout, and 

 the electric eel of South American swamps. 



The depths of lakes are also inhabited by certain fish which are 

 partly limited to the deep waters, and partly also occur in shallow 

 water; these are partly pedonic and partly limnetic forms. In the 

 Great Lakes there is evidence of stratification on the part of the fishes. 

 Among the coregonid fishes, there are groups of bottom-feeding forms 

 which belong primarily to the shoal waters, others apparently are most 

 often taken in mid-depths, while still another group of species belong 

 chiefly in the deeper waters. Similar relations hold during the spawn- 

 ing. The lake herring and the common whitefish spawn in shoal water 



