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Spawning and Development of the New Zealand Sprat, Sprattus Antipodum (Hector)

II. Eggs from the plankton

II. Eggs from the plankton.

The developing eggs collected from the plankton tow ranged in size from 0.81 to 1.10mm, with a mean diameter of 0.98mm. They were spherical with a smooth transparent vitelline membrane, and (initially) page 4
Fig. 2: Unfertilised egg from Sprattus antipodum. Figs. 3-7: Stages in the development of sprat eggs—3, at capture, blastodermal cap stage; 4, 12-14 hrs after capture, blastula formation; 5, 45-48 hrs, blastopore closure; 6, 55-60 hrs, somites developing; 7, 72 hrs, tail separation.page 5

Fig. 2: Unfertilised egg from Sprattus antipodum. Figs. 3-7: Stages in the development of sprat eggs—3, at capture, blastodermal cap stage; 4, 12-14 hrs after capture, blastula formation; 5, 45-48 hrs, blastopore closure; 6, 55-60 hrs, somites developing; 7, 72 hrs, tail separation.

page 6 a small (0.005 - 0.01mm) perivitelline space. The yolk was large, segmented, and there was no oil globule.

During development the yolk decreased in size and the perivitelline space widened. The living eggs sank to the bottom of the containers, indicating a high specific gravity.

Stages in the development of the eggs are illustrated in Figs. 3-9. The earliest stage present in the plankton sample already possesses a many-celled blastodermal cap which forms a prominent dome on the yolk (Fig. 3). Other stages present show the eccentric development of the blastocoele and the surrounding of the blastoderm by the germ ring (Fig. 4), culminating in the formation of the embryonic keel and closure of the blastopore (Fig. 5). Figures 6 and 7 show that as the embryo lengthens and thickens, somites develop in the middle part of the body, and the optic vesicles become prominent. As the tail begins to separate from the yolk the dorsal and ventral fin folds form. A characteristic clupeoid feature appears with the formation of the intestine—a posteriorly placed anus, which can be seen as a small indentation in the ventral fin fold near the tail. With the increasing size of the embryo the diphycercal tail lengthens and begins to curl around the yolk (Figs. 7 and 8), and by the time hatching is imminent, it has curled beyond the head parallel to the body axis (Fig. 9). In the late embryonic stages somites develop posterior to the anus, and sensory neuromast organs appear as slight protruberances along the flanks towards the tail. At hatching the egg capsule is ruptured by the flexing embryo, which then escapes headfirst (Fig. 9).

Eggs at the blastodermal cap stage were separated from the rest of the sample immediately they were recognised, and their development monitored. Twenty-four hours after capture the blastoderm had covered about two-thirds of the yolk and the embryonic shield was just visible. At 48 hrs the blastopore had closed, and the head and tail regions of the embryo had become differentiated from the surrounding tissue. After 72 hrs the embryo was well formed, with somites present in the middle of the body, and the tail had separated from the yolk and begun to curl towards the head. The embryos began hatching at about 90 hrs, and by 98 hrs all had hatched.

Assuming that the blastodermal cap-stage eggs were only a few hours old, it then appears that the sprat takes about 4 days to hatch at temperatures between 11° and 13°C.