PETERSON ET AL.: EARLY LIFE STAGES OF ATLANTIC TOMCOD 



decrease with time until a minimum is attained at 

 27 d postfertilization. The sp. gr. then rose again, 

 as for eggs reared at and 10%o. The decrease in 

 egg solids' sp. gr. from fertilization to day 27 at 20 

 and 30%o may be due to more efficient salt elimina- 

 tion as the embryo grows. After 27 d the decrease 

 due to salt elimination is more than balanced by 

 increases due to factors postulated above for FW 

 eggs. Apparently, eggs reared at 20%o were suc- 

 cessful in eliminating salt, because the sp. gr. of 

 their egg solids tends to converge with that for 

 eggs reared at lower salinities. At 30%o, however, 

 the embryos appeared to be unable to eliminate 

 excess salt successfully because the solids' sp. gr. 

 never approached those for eggs reared at lower 

 incubation salinities. The 30%o eggs hatch earlier 

 than those incubated at lower salinities, perhaps 

 in response to high salt concentrations. As men- 

 tioned previously, they were abnormally de- 

 veloped. 



The higher sp. gr. of egg solids at 30%o would 

 require that 15% of the solids be excess salt (using 

 the formula 1.27a + 1.8(1 -a) = 1.35; where a = 

 proportion of egg solids that is not salt; 1 .8 = sp. gr. 

 of salt, 1.27 = sp.gr. of FW egg solids, 1.35 = sp.gr. 

 of 30%o egg solids). This amounts to about 0.08 mg. 

 This is reasonably close to the increase in dry 

 weight of 30%o eggs over FW eggs (0.07 mg). The 

 decrease in sp. gr. of 30%o egg solids to its 

 minimum of 1.33 would require the loss of 0.02 mg 

 of salt. 



Newly hatched larvae in freshwater that did not 

 have access to the water surface had an sp. gr. of 

 1.032, which is nearly identical to that of eggs 

 incubated in freshwater. If larvae were permitted 

 access to air at the water surface, the sp. gr. de- 

 clined within 7 h to 1.01, coincident with ingestion 

 of air into the swim bladder. Newly hatched larvae 

 were observed swallowing air at the surface. The 

 filling of the swim bladder was investigated 

 further with a Cartesian diver technique. Larvae 

 that had access to air floated at a flotation pressure 

 (Saunders 1965) of 154 mm Hg (130-170, n = 6) 

 (0.8 atm). Those that had been denied access to air 

 for 24 h failed to float at 675 mm Hg (645-685, n = 

 15), corresponding to 0.1 atm (the greatest vac- 

 uum attainable with the apparatus). No air was 

 observed in the swim bladder of these larvae. 

 These larvae were then allowed access to the sur- 

 face overnight. When tested subsequently, none 

 floated at 675 mm Hg, nor was air observed in the 

 swim bladder. These latter larvae, unlike larvae 

 with air in the swim bladder (that spend most of 



their time near the surface), stayed on the bottom 

 of the container. 



Newly hatched larvae were photopositive as 

 tested in a half-blackened Petri dish. In two trials, 

 78% (39/50) and 64% ( 16/25) larvae were observed 

 in the lighted (unpainted) half of the Petri dish. 

 When the dish was kept in darkness, 46% (23/50) 

 and 36% (9/25) larvae were observed in the 

 unpainted half. Larvae were commonly observed 

 to aggregate near the lighted sides of rearing 

 containers. 



GENERAL DISCUSSION 



The changes that occurred in eggs reared in 

 various salinities will first be summarized: 



Eggs reared in freshwater consisted of 2.8 mg 

 water, sufficient for the embryo's needs, being con- 

 stant throughout development. The egg sp. gr. was 

 also constant despite decrease in solid materials 

 (ca. 0.1 mg) — the egg diameter should therefore 

 decrease slightly (about a 1.7% decrease is re- 

 quired), although this was not observed, as it is 

 within experimental error. 



Eggs reared in 107oo salinity have about 2.2 mg 

 water for the first month of development, but take 

 up an additional 0.5-0.6 mg in the later stages of 

 development, due to the greater water require- 

 ments of embryonic tissue. Some of this uptake 

 may also be associated with formation of fluid 

 filled body cavities (Zotin 1965). This water up- 

 take was associated with a decreased egg sp. 

 gr. The 10%o eggs may have a slight salt load 

 which is probably eliminated in the later de- 

 velopmental stages. The egg dry weight declined 

 by only 0.07 mg, and newly hatched 10%o larvae 

 may have larger yolks than do those in 0%o (Figure 

 9). 



Eggs reared in 20%o salinity had a water content 

 equal to or greater than that of 10%o eggs in the 

 early stages of development, but had to tolerate a 

 higher salt load (ca. 0.04 mg/egg) as a result. Egg 

 sp. gr. declined throughout development due to 

 salt elimination as the embryo developed and to 

 accumulation of about 0.2 mg water during the 

 later developmental stages. Advanced embryos 

 eliminated much of the initial salt load as the egg 

 solid sp. gr. of late stage eggs is nearly identical to 

 that of eggs reared at lower salinities. The concept 

 of salt elimination seems reasonable, but is subject 

 to some uncertainty in these experiments because 

 the solids associated with the chorion and 

 perivitelline fluid are included in the estimates of 



155 



