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

Development of the Egg and Larvae

Development of the Egg and Larvae

Most of the eggs within the clusters are laid so close together that they impinge on each other and so deform the chorion (Fig. 1, No. 1). The consequent irregularities in the shape of most eggs make accurate measurement of the egg diameter difficult.

Eggs that make no contact with their neighbours, however, are spherical when viewed dorsally, and have a mean diameter of 1.80mm. The yolk is sub-spherical and in a standard sample of 100 eggs has a mean diameter of 1.30mm. During early development the yolk is dark purple and contains a single large oil globule (mean diameter 0.45mm) and 20-30 smaller ones. The eggs are dorso-ventrally depressed and are attached to the substrate by a flattened adhesive base.

Development to hatching (Fig. 1, Nos. 1-12) takes 24 days at 11.57°C. No eggs were found earlier than the gastrulation stage, and these were estimated to be nearly two days old.

Second day (Fig. 1, No. 1). At this stage the blastodisc has spread halfway around the yolk. Epiboly is not obvious, except for a slight thickening of the germ ring.

Third day (Fig. 1, Nos. 2 & 3). The embryonic shield is well defined and lies deeply notched into the yolk. The blastodisc continues to expand over the yolk, covering about two-thirds of the yolk surface. Separated eggs retain the ring-like shallow depressions that were produced by contact with other eggs.

Fourth day (Fig. 1, No. 4). The blastodisc is reduced to a small opening through which the yolk bulges. The neural keel (future central nervous system) is well defined, encircling almost half of the yolk. The optic vesicles are present but rudimentary.

Fifth day. The three main divisions of the brain are distinguishable. The closing of the blastopore is complete. Three to four myomere blocks are present on either side of the neural keel.

Sixth day (Fig. 1, No. 5). The embryo has increased in length and is well defined. The optic cups can be seen surrounding the spherical lens tissue. Approximately 13 pairs of myomeres are present, arising posterior to the faint outlines of the auditory placodes. Kupffer's vesicle is present as two small sacs beneath the tail bud.

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Seventh day (Fig. 1, No. 6). The lobes of the brain are well defined and from the dorsal aspect the ventricles of the prosencephalon, mesencephalon and the rhombencephalon are conspicuous. Each ventricle is covered by a thin roof. The optic cups enclose the lenses but as yet the chorioid fissure has not closed. The anterior myomeres are chevron-shaped, and the tail bud has begun to lift clear of the yolk.

Ninth day (Fig. 1, Nos. 7 & 8). There is a distinct pericardial cavity beneath, and extending forward of, the head. The heart beats faintly, but there is no visible circulation, and the crimson-purple of the yolk obscures any sign of blood-island formation. The head is expanded laterally, and the brain ventricles, especially that of the mesencephalon, are relatively large. Pigmentation of the chorioid of the eye has begun. The gut is tubular, and there are scattered, stellate melanophores in the trunk muscle directly above it.

Eleventh day (Fig. 1, No. 9). The heart beats regularly and strongly, and a flow of blood through the dorsal and ventral blood vessels is obvious. The sinus venosus is pronounced, lying within the pericardial cavity slightly forward of the head, and receiving blood from the large vitelline vessels. The chorioid fissure has closed completely and appears as a faint white line on the ventral aspect of the eye. Chorioid pigmentation has increased. The brain lobes are more rounded and two otoliths are present in each otic vesicle. The tail bud is less rounded and extends well clear of the yolk, and the upper parietal peritoneum is covered by scattered melanophores. A single large oil globule is present in the yolk.

Fourteenth day (Fig. 1, No. 10). The head is broad and has lifted from the yolk, exposing the chambers of the heart and the rudimentary lower jaw. The eyes are prominent and the lenses can still be seen through the chorioid pigment. Pectoral fin buds are present and the tail is turned to lie parallel with the body.

Sixteenth day (Fig. 1, No. 11). The external olfactory pits appear as shallow depressions anterior to the telencephalon. The tail overlaps the head and there is an increase in the peritoneal and segmental pigmentation. The peritoneal melanophores extend forward beneath the myelencephalon. The embryo changes position frequently.

Twenty-fourth day (Fig. 1, No. 12). The yolk is reduced considerably and the ventral aspect of the embryo faces upwards. Pigmentation of the eye appears complete with many small iridiophores present. The embryo is cramped within the chorion and the tail completely overlaps the head and turns on itself. The mouth, olfactory bulbs and pectoral fins are well formed. The gut is large and convoluted. The liver lies posterior to the yolk and contains a green spherical gallbladder. Just prior to hatching the embryo becomes agitated and the tail begins to flex. As a result, the chorion is ruptured and the prolarva is released.

Prolarva (Fig. 2, Nos. 1 & 2). Prolarval length on hatching ranges from 5.00mm - 6.05mm s.l. There is considerable variation in the amount of yolk present in each prolarva immediately after hatching. Large amounts of yolk in some prolarvae may be accounted for by premature hatching, induced by disturbance. The yolk contains a single reduced oil globule. The heart is prominent and lies on the anterior yolk margin. The gut is long and convoluted and extends beyond the mid-length of the body. Numerous melanophores (30-50) are present on the myomeres page 5of the body and tail, the first of which is placed behind the pectoral fin base; this series extends to the fifth myomere past the vent. Occasional specimens have as few as 6 melanophores on the myomeres. A yellow tinge is present deep in the muscle tissue directly beneath the melanophores of the body and tail. Numerous melanophores line the upper parietal peritoneum. The head is broad and blunt, and there are two clusters of stellate melanophores beneath the myelencephalon. The prolarvae are positively phototropic.

Eight day larva (Fig. 2, Nos. 3 & 4). 6.20mm s.l. The yolk is almost completely absorbed and the oil globule is very small. Pigmentation is unaltered except for the development of several melanophores at the base of each pectoral fin. A yellow tinge surrounds the base of the brain, particularly the myelencephalon, and extends posteriorly within the myomeres above the gut.

Fifteenth day larva (Fig. 2, Nos. 5 & 6). 7.70mm s.l. The overall shape of the larva has altered slightly. There is an increase in the depth of the tail and the relative size of the gut. The pigment pattern is unchanged. Sucker buds are visible at the base of the pectoral fin. Myomeres have the double chevron pattern and the notochord is slightly upturned in the tail. Six rudimentary caudal rays are present. Larval and prolarval measurements in millimetres are as follows:

Prolarvae 10 days 15 days
Number of Fish 25 25 1
Total length (mm) 5.67(5.40-6.60) 7.12(6.95-7.30) 7.90
Standard length (mm) 5.28(5.00-6.05) 6.70(6.55-6.90) 7.70
Head length (mm) 1.10(1.00-1.15) 1.30(1.25-1.40) 1.70
Eye length (mm) 0.52(0.50-0.54) 0.54(0.51-0.56) 0.65
Snout to vent (mm) 3.32(3.20-3.51) 4.41(4.15-4.55) 5.00
Greatest depth (mm) 1.15(1.00-1.21) 1.02(0.90-1.10) 1.35