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Zoology Publications from Victoria University of Wellington—Nos. 63 and 64

Development of the Egg and Prolarva

Development of the Egg and Prolarva

The eggs are oval (mean dimensions of a standard sample of 100 are 1.81 × 1.48mm), dorsoventrally depressed and attached to the substrate by a flattened adhesive base. At an early stage of development the yolk is central and subspherical and has a mean diameter of 1.30mm. Development to hatching (Fig. 3, Nos. 1-11) takes 24 days at 11.5°C. No eggs were found earlier than the gastrulation stage, and these were estimated to be approximately two days old.

Second day (Fig. 3, No. 1). The blastodisc has spread half way around the yolk. Epiboly is not obvious. There are six main oil globules and about 150 smaller ones present in the yolk.

Third day (Fig. 3, No. 2). The blastodisc continues to expand over the yolk, covering about two-thirds of the yolk surface. The embryonic shield is present as a thickened strip of the blastodisc.

Fourth day. The blastopore is reduced to a small opening bordered by the thickened germ ring. The narrow embryonic shield lies notched into the yolk. Many of the small oil globules have coalesced to form large globules.

Fifth day (Fig. 3, No. 3). The closing of the blastopore is complete. The optic vesicles are present but rudimentary. The embryo is still deeply notched into the yolk, particularly in the head region, and the tail is flat.

Seventh day (Fig. 3, No. 4). The embryo has increased in length, encircling more than half of the yolk. There is a distinct pericardial cavity anterior to the head. The three main divisions of the brain are distinguishable. The lens tissue is separate from the ectoderm. Approximately 11 pairs of myomeres are present, arising posterior to the faint outlines of the auditory placodes. Kupffer's vesicle lies beneath the developing tail bud.

Eighth day (Fig. 3, Nos. 5 & 6). The brain, from the dorsal aspect, shows clearly defined ventricles, each covered by a thin roof. The optic cups surround the spherical lens tissue, and the chorioid fissure has not yet closed. The anterior myomeres are chevron-shaped, and the tail bud begins to lift clear of the yolk.

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Tenth day (Fig. 3, No. 7). The heartbeat is regular, sending blood slowly through the major blood vessels. The head has expanded laterally, and the brain ventricles, especially that of mesencephalon, have increased in size. The chorioid is slightly pigmented and the fissure has closed. Two small otoliths are present in each round otic vesicle. The gut is thin and tubular.

Fourteenth day. The head is broad and has lifted from the yolk, exposing the chambers of the heart and the developing lower jaw. The sinus venosus is a thin expanded sac lying within the pericardial cavity forward of the head, and receiving blood from the large vitelline vessels. The brain lobes are rounded and larger. The chorioid fissure appears as a faint white line on the ventral aspect of the eye. The eyes are prominent and the lenses can still be seen through the chorioid pigment. The upper parietal peritoneum has scattered melanophores. Pectoral fin buds are present and the tail has turned to lie parallel with the head and trunk. A single large oil globule and ten smaller ones are present in the yolk.

Sixteenth day (Fig. 3, No. 8). Very little change has taken place, except that the jaws appear more definite and slightly fleshy.

Twentieth day (Fig. 3, Nos. 9 & 10). The olfactory pits appear as shallow depressions anterior to the telencephalon. The tail overlaps the head and there is an increase in the number of peritoneal melanophores. The embryo alters position frequently. A single large oil globule remains in the yolk.

Twenty-fourth day (Fig. 3, No. 11). The yolk is reduced considerably and the ventral aspect of the embryo faces upwards. Pigmentation of the chorioid appears complete, the silvery appearance of the eye being due to the presence of iridiophores. The embryo is cramped within the egg and the tail completely overlaps the head and turns on itself. The mouth, olfactory bulbs and pectoral fins are well formed. The gut is long and convoluted. The liver and the green spherical gallbladder lie behind the yolk. The upper peritoneal pigment is arranged approximately as two rows of 10 large stellate melanophores, extending from above the liver to the vent. In some individuals two rows of 3-4 stellate melanophores are present on the lower parietal peritoneum extending back from the yolk. Just prior to hatching the embryo becomes active and begins to flex its tail. As a result, the chorion is ruptured and the prolarva is released.

Prolarva (Fig. 4, Nos. 1 and 2). Prolarval length on hatching ranges from 5.35mm - 6.10mm. Like D. puniceus there was some variation in the amount of yolk present in each prolarva immediately after hatching. The yolk contains a single reduced oil globule, and the heart lies on the anterior margin of the yolk sac. The gut is long and convoluted and extends past the mid-length of the body. Upper peritoneal pigmentation is scattered and extends from above the liver to the vent. Some individuals retain the lower peritoneal pigmentation. A yellow tinge lies within the muscles directly above the peritoneal pigment, and extends beneath the otic capsules and the hind-brain. Two stellate melanophores are present beneath each pectoral fins, but no pigment is present on the head.

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The measurements of prolarvae are as follows:
Number of fish 23
Standard length (mm) 5.70 (5.35-6.10)
Head length (mm) 1.04 (0.95-1.10)
Eye length (mm) 0.43 (0.40-0.45)
Snout to vent (mm) 3.43 (3.20-3.60)
Greatest depth (mm) 1.02 (0.80-1.20)