Spawning and Development of the New Zealand Sprat, Sprattus Antipodum (Hector)
Results
Results
I. Eggs from the adult females.
Eggs stripped from the sprat ovaries were spherical and between 0.90 and 1.00mm in diameter. They were almost completely filled with translucent, closely packed yolk globules, giving the whole yolk a coarsely granular or segmented appearance. No oil globule was present. Samples dissected from the ovaries consisted of two kinds of cells: small (0.1-0.3mm), irregularly shaped, translucent cells, and large (0.5-1.0mm), rounded, yellowish cells. The small cells were very numerous and formed interstitial packing between the larger cells. The large cells contained yolk, and some were translucent like those stripped from the fish at sea (Fig. 2). Evidently the largest cells were eggs about to be spawned, for their diameters were almost the same as the fertilised eggs collected from the plankton. Ovary samples of the large eggs showed a progression of egg diameter modes for sprats at slightly different stages of maturity—samples from three fish are plotted in Fig. 1 for comparison with the diameters of the planktonic eggs.
page 3Fig. 1: Frequency of diameters for samples of Sprattus antipodum eggs from ovaries of three specimens (solid-line polygons, 1-3), and from plankton tow (broken-line polygon, 4).
Counts of large eggs indicated that between 3,000 and 4,000 were present in each pair of ovaries.
Following the July catch, sprats with well developed ovaries and testes (stages 4-5, see Baker, 1972, p.19) were taken in the Sounds again in August, September, and October of 1966; and in January, 1967, one ripe/running female sprat was caught in Pelorus Sound. This indicates that the spawning season is a lengthy one, extending over seven months. Fishes with long spawning seasons are often fractional spawners, releasing their eggs intermittently throughout the season. Many clupeids spawn in this way (see Ivanova, 1949 and Naumov, 1956) and their ovaries are characterised by the presence of very small, non-developing eggs, and eggs in several different phases of development. Measured samples from such ovaries show multimodal egg diameter frequencies.
The New Zealand sprat, however, has only two size-groups of eggs in the ovaries–usually a characteristic of total spawning over a short period. While it is possible that the small size-group of eggs may develop to maturity after the large size-group and be released later in the spawning season, it is more likely that the long spawning season is due to different individuals, or broods of individuals, becoming mature at different times. The New Zealand sprat is, therefore, probably not an extended fractional spawner (nevertheless, the fishes could spawn intermittently over a short period of perhaps several days).
II. Eggs from the plankton.
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.
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.
III. Development of the larvae.
Figs. 8-9: Final stages of development and hatching of the sprat embryo—8, 80-86 hrs, tail lengthening; 9, 90-98 hrs, hatching. Fig. 10: Sprat yolk-sac larva measuring 4.0mm b.l. Figs 11-13: Sprat larva measuring 4.7mm, 8.5mm, and 12.0mm b.l., respectively.
Fig. 14: Post-larval sprat measuring 19.5mm b.l. Fig. 15: Late post-larval sprat of 31.0mm b.l. at beginning of metamorphosis. Fig. 16: Juvenile sprat of 43.0mm b.l. in advanced metamorphosis. Fig. 17: Adult sprat measuring 125.0mm b.l.
By 7.0 - 9.0mm body length (Fig. 12) the yolk sac has been completely absorbed and the mouth has assumed its functional shape. The dorsal fin is prominent by 8.0mm, and the intestine is tightly convoluted, with a row of superficial melanophores above its dorsal side. There is also a single melanophore just above the origin of the pectoral fin. The notochord turns dorsally and the hypurals develop at about 12mm. The anal fin also appears at this length, and the tail assumes a slightly heterocercal shape (Fig. 13). At 19.0mm pelvic fins arise alongside the pyloric region of the gut (Fig. 14), and the full complement of vertebrae (44-46) are evident in stained specimens. The tail is now homocercal.
Fig. 18: The changing proportions of the sprat body during metamorphosis 30-50mm b.l.) and subsequent growth. Triangles represent greatest depth of body, dots represent greatest thickness of body (n = 134).
During the colour change the body deepens markedly, becomes laterally compressed (Fig. 18), and the ventral edge of the belly becomes noticeably curved and armed with enlarged, pointed scales which give it a serrated appearance (Fig. 17). There is also considerable migration of fins and the anus along the long axis of the body during late post-larval life; the pelvic fins migrate backward over two myotomes (17th-18th), the dorsal fin migrates forward over seven myotomes (25th-18th), and the anus migrates forward over four myotomes (35th-31st).