FISHERY BULLETIN: VOL. 73. NO. 1 



Table 5. — Normal development of bairdiella eggs. Designation 

 of stages in general follows Ahlstrom ( 1943), and times required 

 to reach various stages are given for eggs in 33%o water at 25°C. 



Figure 6. Series A showred similar patterns, but 

 due to poorer survival the data are less complete 

 and are not shown. In Figure 6 the cumulative 

 percentage hatched has been plotted on a proba- 

 bility scale against time on an arithmetic scale; a 

 straight line in this type of plot indicates a normal 

 distribution (Sokal and Rohlf 1969), which is to be 

 expected if differences in hatching time are due 

 simply to random individual variation. At 30°C 

 hatching was normally distributed for all 

 salinities, but this was not true at the lower tem- 

 peratures. At 27°C there was a plateau at 25°/oo, 

 indicating that the hatching of certain eggs was 

 delayed. At 24°C, hatching was distributed ap- 

 proximately in a normal fashion at 20, 40, and 

 50%o, but at 30%o there was an inflection, the 

 rate of hatching being slower after 23 h than be- 

 fore. At 21°C, hatching was distributed normally 

 for 15, 35, and 45%o, but at 25"/oo hatching took 



place in two phases separated by a 3-h period dur- 

 ing which no hatching took place. 



The time required for 50% of the larvae to hatch, 

 estimated by graphical interpolation, decreased 

 from 35.2 h at 21°C-25%o to 16.0 h at 27°C-25%o. 

 The estimated time at 50% hatching was slightly 

 later at 30°C than at 27°C, although hatching 

 began 2 h earlier in the former (see Figure 6). No 

 clear-cut effect of salinity on median hatching 

 times is discernible, but Figure 6 shows that 

 hatching was completed more rapidly at the 

 higher salinities (35%o and above). The duration 

 of hatching (the time between the appearance of 

 the first and last hatched larvae) tended to be 

 greater at the lower salinities and temperatures. 



Embryonic Mortality 



In certain treatments some surviving embryos 

 failed to hatch but continued to develop wathin the 

 chorion. Alderdice and Forrester (1971b) intro- 

 duced the apt term, "postmature unhatched eggs" 

 to describe such cases. Almost without exception, 

 the postmature unhatched embryos were de- 

 formed in some way, usually bent and abnormally 

 small. Often in such eggs part of the chorion was 

 eventually digested away (Figure 7f), presumably 

 by hatching enzymes, but the weak embryo was 

 incapable of breaking completely free. Postma- 

 ture unhatched eggs were most common at the low 

 salinities, and the greatest proportion occurred at 

 30°C-20%o (Table 6). 



Eggs in Series A showed much higher mortality 

 than those in Series B, especially at the higher 

 temperatures and salinities (Table 7). The follow- 

 ing description of embryonic mortality refers 

 primarily to the eggs in Series B, which are consid- 

 ered more representative of normal, healthy 

 eggs. The higher mortality in Series A usually 

 showed up very early in embryonic development 

 (prior to stage V); otherwise the two series showed 

 similar trends. 



No eggs hatched at 18°C and nearly all died 

 during stage lie (blastula). After the second cleav- 

 age at 18°C the blastomeres assumed a clover- 

 leaf appearance which was not seen at higher 

 temperatures (Figure 7; cf. Figure 5). Subsequent 

 cleavages at 18°C were rather irregular, and dur- 

 ing the blastula stage much of the cytoplasm 

 gathered into isolated clumps, and the periblast 

 became unusually large (Figure 7). Nearly all 

 eggs stopped developing at this stage. 



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