Fishes of the Western NortJi Atlantic 



297 



Development and Growth. The eggs, about 3.5 mm in diameter after they are im- 

 pregnated, are demersal, but they are only slightly heavier than the water in which 

 they are deposited. As they are not adhesive, they drop loosely and singly to the bottom. 

 According to Ryder they hatch in 52 hours at an average temperature of 57.2° F, and 

 in less than 36 hours at an average of 74° F {ll2: 796). However, a longer incubation 

 period was reported by Leim {81 : 3 i), who said that eggs held under artificial conditions 

 hatched in 12-15 days at I2°C (53.6° F) and in 6-8 days at I7°C (62.6° F). 



The newly hatched fish, transparent and extremely slender, are about 9-10 mm 



Figure 72. Alosa sapidissima egg development, left to right: newly laid egg; embryo in natural position 

 within enveloping membrane of egg (photograph); egg with embryo nearly ready to hatch. After Leach. 



long. The yolk is absorbed in four or five days at a water temperature of 62.5° F and 

 in about seven days at 53.5° F; the fish have then attained a length of about 12- 

 15.5 mm {81: '},'^. In some preserved specimens that show the yolk almost absorbed, 

 the depth is only about 0.05 of the length, the large dark eyes protrude prominently, 

 the dorsal fin fold is somewhat developed, and the alimentary canal is discernible, 

 ending under the beginning of the posterior sixth of the body. During development, 

 the vent moves forward as in other Clupeidae. The fins are fully developed in 21- 

 28 days at a length of about 20 mm, when metamorphosis is nearly complete (j6: i 18). 

 For a more detailed account of the development of the larvae, see Leim {81 : 31-43). 



The young remain more slender than the adults until they are nearly grown 

 fish, but even among sexually mature fish there is great variation in depth, seemingly 

 not related to sex and only partly related to spawning conditions. In 25 young, '},Cj — 

 125 mm long, the depth varied between 26.5—30 "/(, of SL, and in a lot of 12, 460— 

 505 mm long, the depth was 32-48 "/o of SL. 



A gradual increase in the number of gill rakers, as already indicated, also takes 

 place with age and growth. For example, the number on the lower limb of the first 

 arch varied as follows: in 18 young 35 to 65 mm long, 26 to 31; in 21 examples 



