specimens of this copepod taken in the large Norwegian 

 lakes in summer were fully grown, but that the ovarian 

 tubes of the females were empty, which is exactly the 

 condition in Lake Erie. From this he inferred that the 

 breeding season was at a different time of the year 

 from that of the other copepods, perhaps in winter. 



Ekman (1920) has shown that in the Baltic lakes 

 Limnocalanus has but a single breeding season late in 

 the autumn. The copepodid stages are reached in 

 March and the adults are fully grown by May; then 

 there is a resting stage of 5 months, during which the 

 sex organs remain undeveloped. Sexual maturity comes 

 in the autumn and breeding begins in November at a 

 temperature of 7° C. The adults of the previous year 

 die off in the spring and have all disappeared by May, 

 their places being taken by the young of the year. 

 Marsh (1889) found in Green Lake that this copepod had 

 2 maxima during the year. May and November, the 

 spring maximum showing the larger numbers. During 

 February, March, and April most of the specimens ob- 

 tained were immature, and judging from the speci- 

 mens obtained the first of June this is probably the case 

 in Lake Erie. 



Limnocalanus is repelled by bright light and high 

 temperature, and hence retreats more and more com- 

 pletely to the bottom through the summer. The max- 

 imum temperature of the water in which it can thrive 

 is 14° C. , but the water must be considerably colder 

 than this before it will come freely to the surface. Its 

 diurnal migrations are more pronounced in cold weath- 

 er and throughout the summer it stays below the 

 thermocline. 



Limnocalanus was much more abundant at the 

 eastern end of the lake in the littoral zone on the Can- 

 adian side during 1928, and was entirely absent from 

 more than half of the stations on the American side. 

 It still showed a decided preponderance along the Can- 

 adian shore during June and July 1929, was more uni- 

 formly distributed in August, and found only in the 

 deepest part of the lake in September. In reality, the 

 species is lacustric, not littoral. 



It is known to be eaten by Cisco, white bass, and 

 yellow perch, and doubtless serves as food for many 

 fishes of the lacustric zone, whose stomach contents 

 have not thus far been examined. 



Ekman (1913, p. 371) has pointed out that Limno- 

 calanus macrurus is a descendent of L. grimaldii , the 



two species differing chiefly in the shape of the head. 

 The dorsal contour of the forehead of L. grimaldii is 

 much less than that of L. macrurus ; the former lives 

 in salt or brackish water, the latter is strictly a fresh- 

 water form. In general the greater the salinity the 

 flatter the forehead, but as the salinity decreases the 

 form of the head approaches the semi-cfrcular contour 

 of L. macrurus. The transformation thus becomes 

 greater the longer the life in fresh water has lasted, 

 and may serve as a sort of criterion of the length of 

 time that has elapsed since the habitat changed from 

 salt to fresh water. The form found in Lake Erie di- 

 verges from the extreme form of L. grimaldii much 

 less than some of the European races, and would be, 

 according to Ekman, of more recent origin. 



The Lake Erie race of Limnocalanus is one of the 

 largest known, greatly exceeding any other freshwater 

 race and surpassed only by salt-water races of North Si- 

 beria and Finland. The length of the caudal rami in 

 proportion to the length of the body varies greatly 

 also in the different freshwater races. Gurney (1923) 

 in his discussion of this copepod in the EngUsh Lake 

 District said "The published information is too scanty 

 to admit of a definite conclusion as to whether there 

 is any connection between the shortness of the fuica 

 and the length of sojourn in fresh water. " He gave the 

 length of the caudal rami in American specimens sent 

 to him by Dr. Juday as 13. 3 percent of the body length 

 in both sexes from Lake Erie, 13. 8 to 14 percent from 

 Green Lake, and 15 percent from Lake Canandaigua. 

 These measurements were all notably shorter than those 

 of the L. grimaldii form from the Caspian, but they do 

 not agree with measurements made during the present 

 survey. The average for 25 specimens, including both 

 sexes and varying from 2. 10 to 2. 87 mm. in length was 

 15. 6 percent, varying from 13. 8 to 16. 9 percent. This 

 brings the average of the Lake Erie race much closer to 

 that of the Caspian and Swedish forms and again indi- 

 cates that the former are of recent origin, as is ex- 

 plained farther on under Pontoporeia . 



Parasitic Copepods 



22. Achther es ambloplitis Kellicott. Parasitic in 

 the mouth and on the gills of the smallmouthed black 

 bass; only one infested fish found during the present sur- 

 vey. 



23. Achtheres coregoni (S. I. Smith). Parasitic 

 on the fins and gill arches of the Cisco in the Niagara 

 River near Lake Erie, and on white fish in the other 



166 



