EGGS AXD YOLK-SAC LARVAE OF YELLOWFIN MENHADEN 



99 



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Figure 12. — Thirty-eight-hour stage with early embryo. 



Figure 13. — Forty-one-hour .stage with tail separating 

 from volk. 



Figure 14. — Forty-six-hour stage with late embryo. 



Figure 15. — Forty-si.x-hour stage hatching. 

 LATE- STAGE EGG 



The embryo had grown so large that the tail, 

 free of the yolk, fell just short of toucliing the 

 head (fig. 14). The somites were visible except 

 near the end of the tail. The embryo was ver}'' 

 active, exhibiting convulsive movements at fre- 

 quent intervals. The tail was outlined with a 

 distinct finfold. Pigmentation generally was lim- 

 ited to one to three sniall chromatophores along 

 the tail and three to eight in the head or anterior 

 region. Pigmentation of embryos was variable, 

 and no pattern or concentration of chromatophores 

 was discernible. Hatcliing occurred with the 

 rupture of the external membrane, and the larva 

 emerged, head first (fig. 15), 46 hours after 

 fertilization. 



YOLK-SAC LARVA 



Yellowfin menhaden, like many other fishes 

 with pelagic eggs (Ahlstrom and Counts, 1955), 

 hatched in a relatively undeveloped condition. 

 The mouth had not formed, and the eyes were 

 unpigmented. Fin rays had not developed, and 

 the pectoral fin buds were not visible. However, 

 the anus had formed and was discernible as a 

 tube passing through the finfold. 



The early larva (figs. 16 and 17) floated ventral 

 side up, with the yolk and oil globule uppermost, 

 except dm-ing brief, convulsive swimming. During 

 initial swimming, the larva oriented dorsal side 

 up and then, in a head-down position, would move 

 towards the bottom. Bodj- movement would stop 

 after a few seconds and the larva would turn 

 ventral side up and float towards the surface. 

 Such behavior was most marked during the first 

 24 hours. As the larva grew and the yolk dimin- 

 ished, swimming increased and b}' 48 hours was 



