of the macroplankton in the middle and at the west- 

 em end of the lake and nearer the American shore 

 during July, after which the percentages steadily de- 

 creased. In general it may be classed as a shallow 

 water cladoceran, preferring the aquatic vegetation 

 along shore, where it breeds in great numbers. Eaten 

 by the cisco and yellow perch. Bigelow (1922) ranked 

 it third in the list of organisms found in cisco food, 

 Daphnia being first and Leptodora second. 



82. Simocephalu s serrulatus (Koch). Found at 

 6 marginal stations on the Canadian shore and 5 on 

 the American shore; abundant at 3 stations on either 

 side. Eaten by the bullhead and the carp. 



83. Simocephalus vetulus ( O. F. Miiller). Found 

 at 4 marginal stations on the Canadian shore and 5 

 on the American shore; abundant at 2 of the latter. 

 Eaten by the bullhead and carp. 



Other Crustacea 



84. Mysis relicta Loven. Found only at the hot - 

 tom in the lacustric zone but present there in great 

 abundance throughout the entire year. Mysis was ob- 

 tained in goodly numbers at several stations during 

 June on both sides of the lake where the water was 

 only 20 meters deep, but during the 3 remaining 

 months it was found only in the Deep Hole. Here, 

 however, its percentages of the macroplankton were 

 very large and often above 50 percent and once or 

 twice reached 100 percent. During the summer it is 

 confined to the eastern end of the lake, but in the 

 winter it probably spreads into the middle and may 

 even reach the western end. 



85. Pontoporeia affinis Lindstrom. Dr. 

 Chancey Juday sent sjsecimens of the amphipod 

 species, which had been named Pontoporeia hoyi 

 byS. 1. Smith, to G. O. Sars, who after careful 

 examination declared them to be identical with 

 Pontoporeia affinis Lindstrom of the European lakes. 

 This is another species of the macroplankton that is 

 found only in the deepest portion of the lake and on 

 the bottom. Juday and Birge (1927) have reported that 

 in Green Lake it is rarely found above a depth of 10 

 meters during the summer. Green Lake has approx- 

 imately the same maximum depth (68 meters) as 

 Lake Erie (62 meters), but the thermocline here is 

 lower than in Green Lake, and probably these cold 

 water forms do not come as near the surface as 10 

 meters. 



Pearse (1921) found that Pontoporeia was an im- 

 portant source of food for the cisco in Green Lake 

 during the summer and it is probably eaten by them 

 in Lake Erie. The sturgeon also is a bottom feeder 

 and in Lake Nipigon it has been found that a consid- 

 erable pOTtion of its food consisted of Pontoporeia and 

 HyaleUa . At all events this amphipod helps to form 

 in the deepest portion of the lake a food supply of 

 considerable economic importance. 



Pontoporeia , Limnocalanus , and Mysis are 

 "marine relicts, " that is, they are species which 

 were originally marine but have become acclimated 

 to fresh water. Their presence in Lake Erie in con- 

 siderable abundance gives rise to the same question 

 that has presented itself in connection with their 

 appearance in European lakes, namely how did they 

 get there? 



In the larger lakes of Germany Mysis is reported 

 to come into the shallower water during the colder 

 months of the year and doubtless does the same in 

 Lake Erie. Kindle has reported it in Lake Ontario 

 down to depths of 258 feet, and that its abundance 

 in the deep water of that lake is one of the interest- 

 ing features of the bottom plankton. Reproduction 

 is said to take place at low temperatures and to be 

 most vigorous in winter when the temperature falls 

 to 3 or 4° C. It is reported to constitute quite a large 

 percentage of the food of cisco in other lakes and is 

 probably eaten by them here, but practically none 

 of the deep-water fish in Lake Erie have been ex- 

 amined for their food contents. 



Two theories have been advanced, one of which 

 supposes the lakes to have been filled originally with 

 salt water, forming an inlet or bay of the ocean. 

 Separated from the ocean by subsequent land move- 

 ments, and receiving sufficient freshwater drainage, 

 they have gradually changed into freshwater lakes. 



The other theory is that these crustaceans mi- 

 grated gradually from the ocean into the lakes after 

 the close of the glacial period. The fust of these 

 theories is automatically eliminated in connection 

 with Lake Erie, for although Lake Ontario is believed 

 to have been filled with salt water at the close of the 

 glacial period, Lake Erie was not, and there was be- 

 tween them the insurmountable barrier of Niagara Falls. 



171 



