746 FRESH-WATER BIOLOGY 



Similar conditions prevail in the ocean, where Copepoda form an 

 essential part of the plankton, which there, too, is an important 

 element in the food, not only of fishes, but of some of the great 

 ocean mammals. Our fresh- water Copepoda are descendants of 

 salt-water forms, and the elucidation of the lines of descent forms 

 a most interesting problem, towards the solution of which very 

 little has been done. 



The distribution of the Copepoda in our lakes is a matter of 

 great interest. Certain species are characteristic of distinct regions 

 of the lakes. For example, Cyclops bicuspidatus, Diaptomus sicilis, 

 Diaptomus minutus, and Diaptomus ashlandi are characteristic of 

 the limnetic regions of the Great Lakes. Cyclops prasinus is espe- 

 cially characteristic of limnetic regions, Cyclops albidus and Cyclops 

 fuscus are more commonly littoral, while Cyclops bicolor and Cyclops 

 phaleratus are more usually found in pools. Others, especially at 

 certain seasons, may be found only in the deeper waters, or are 

 "abyssal" in habitat. This is true of Limnocalanus macrurus, 

 which is rarely found at the surface in the summer season, but 

 almost entirely in the region below the thermocline. Generally 

 speaking, the Diaptomi in lakes are characteristic of the limnetic 

 regions, but it does not follow that all Diaptomi are limnetic; for 

 there are many species that confine themselves strictly to the 

 extremely shallow waters of pools, like Diaptomus sanguineus, which 

 occurs widely through the temperate regions in the temporary 

 pools of spring. It should not be inferred, however, that these 

 distinctions between littoral, limnetic, abyssal, etc., are absolute. 

 In many cases, species commonly littoral may adapt themselves to 

 a limnetic habitat, or those commonly found in limnetic regions 

 may become littoral, and flourish in those regions, thus forming 

 part of what is sometimes known as the tycholimnetic or tycholittoral 

 fauna. Cyclops bicuspidatus, for example, while ordinarily limnetic, 

 may become a part of the littoral fauna. In other cases, species 

 like Diaptomus oregonensis and Diaptomus minutus may seem to live 

 equally well in deep or shallow waters. Deep lakes and shallow 

 lakes have their characteristic copepod faunas, but this distinction 

 does not always hold rigidly; for frequently the species show a 

 great deal of elasticity in adapting themselves to changed conditions. 



