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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 egg and yolk are completely spherical, and the mean chorion diameter of 80 eggs was 1.16mm (range 1.12-1.23mm). The yolk early in development is slightly granular, and contains 25-35 medium sized oil droplets closely associated with 15-25 pigment spheres. The eggs attach to the substrate by a mass of tendrils arising from one side of each egg. Each tendril consists of a matted series of fine filaments that arise from a single point on the chorion. These tendrils are larger than those of T. capito.

Development to hatching (Fig. 2. Nos. 1-12) took 22 days at a water temperature ranging from 11°C-13.5°C.

Two hours (Fig. 2. No. 1). A single large mass of protoplasm appears at one end of the yolk.

Fifteen hours (Fig. 2, No. 2). One hundred and twenty-eight angular cells form a low cap on one side of the yolk.

Thirty hours (Fig. 2, No. 3). The blastula is well formed and consists of a semi-circular mass of cells (blastoderm) overlying a wide blastocoel.

Fifty hours (Fig. 2, No. 4). The blastodisc covers two-thirds of the yolk. The embryonic shield is clearly defined and lies slightly embedded in the yolk, particularly in the anterior region.

Third day (Fig. 2, No. 5). The embryonic axis (neural keel and somite precursors) is obvious although still flattened against the yolk. Poorly defined optic vesicles are visible. Twelve to 20 large oil globules are present and the pigment spots have decreased in number (to 5-10). The blastopore is closed.

Fourth day (Fig. 2, No. 6). The embryo is prominent above the yolk surface and encircles more than half of the yolk sac. The lenses are formed and have partially separated from the ectoderm. Eight myomeres lie well forward either side and Kupffer's vesicle has appeared directly beneath the tail bud. A group of tiny melanophores lies anterior and posterior to each eye. A further two clusters are laterally positioned within the myomeres half-way along the embryo.

Fifth day (Fig. 2, No. 7). The brain is lobed and the ventricles are visible. The auditory placodes appear as faint depressions behind the eyes. Approximately 15 stellate melanophores lie scattered over the page 6surface of the yolk sac. The relative position of the melanophores is unchanged although there is an increase in the chromatophore number. A further group of melanophores has developed behind each otic vesicle.

Sixth day (Fig. 2, No. 8). The heart beats slowly and faintly but no blood flow is visible except very close to the heart. The pericardial cavity has not yet expanded and the heart is embedded in the yolk directly beneath the head. The chorioid fissure has not closed. Each auditory placode appears as a double ringed structure. Four pairs of chromatophore groups are present, with one group anterior and another posterior to each eye, one behind each auditory placode and one two-thirds back along the body of the embryo. The melanophores in these groups form a diffuse network making accurate counting impossible. The tail bud is still attached firmly to the yolk surface.

Seventh day. The heart beats regularly and strongly, and blood flows through the main vessels and across the yolk tributaries. The heart lies deep within the pericardial cavity slightly forward of the head. The sinus venosus is the only clearly visible chamber. The oil globules are reduced further (to 5-10), and all pigment spheres deep in the yolk have dispersed. The anterior myomeres are chevron shaped.

Eighth day. The yolk is dense and granular. The embryo alters position about once every minute.

Eleventh day (Fig. 2, No. 9). Pigmentation has appeared in the chorioid of the eye and the chorioid fissure has closed. The eyes have shifted laterally broadening the head and bringing the otic vesicles closer to the eyes. Several stellate melanophores are scattered above the brain and along the mid-ventral and mid-lateral aspects of the body and tail. The tail overlaps the head and the rudimentary pectoral buds are present.

Thirteenth day (Fig. 2, No. 10). Three otoliths lie within each otic vesicle. The chorioid pigment is darker although with transmitted light the lenses are still visible. Individual melanophores throughout the embryo are generally larger, although the size varies according to changes in light intensity. The melanophore clusters that were located two thirds back along the body have dispersed. Fewer pigment spots are present on the yolk surface. There is a large melanophore in the upper peritoneum slightly forward of the anus and 8 smaller ones above the gut in the region of the pectoral fins. The tail is long and passes over the head and returns to lie parallel with the body. Narrow fin folds are visible.

Sevententh day (Fig. 2, No. 11). The head is lifted slightly from the yolk, exposing the chambers of the heart and the poorly developed lower jaw. The lenses are no longer visible through the eye pigmentation. The gallbladder is visible in the mid-gut region. Two to 8 oil globules are present within the yolk.

Twentieth day. The external nares appear as two shallow depressions anterior to the telencephalon. A row of 9-10 stellate melanophores runs along the mid-ventral line past the vent. Fin folds and pectoral fin buds are well developed.

Twenty-second day (Fig. 2, No. 12). The yolk is reduced considerably and the ventral side of the embryo faces upwards. Numerous iridiophores are present in the chorioid of the eye. A considerable reduction in the amount of pigment present around the brain, above page 7the gut and in the lateral mid-line of the tail has taken place. Two to 3 chromatophores lie on the yolk surface. The mouth, external nares and pectoral fins are well defined. The embryo twists violently before hatching. The tail is flexed rapidly, as a result of which the chorion is ruptured and the prolarva is released. Immediately following this release the prolarva swims to the surface.

Prolarva (Fig. 3, Nos. 1 and 2). Prolarval length on hatching ranges from 5.70mm-6.10mm s.l. All melanophores about the brain have faded. As with T. capito the size of the yolk sac varies according to the degree of premature hatching. Three to 4 oil globules are present in the yolk. The upper gut peritoneum has two large melanophores, one slightly behind the yolk and the other slightly anterior to the anus. The yolk is bordered anteriorly by the heart and posteriorly by the liver and the prominent gallbladder. From 5-15 stellate melanophores are present along the ventral mid-line behind the anus and from 2-4 are situated along the dorsal mid-line of the tail. One or two melanophores lie beneath the pectoral fins on either side of the yolk. The prolarvae are positively phototropic.

The average measurements of 25 prolarvae of each species are as follows:
T capito T. robustum
Standard length (mm) 4.90 5.95
Total length (mm) 5.17 6.20
Head length (mm) 0.76 0.84
Eye length (mm) 0.33 0.37
Snout to anus (mm) 1.90 2.50
Mid-ventral melanophores 21(16-26) 7(5-15)
Mid-dorsal melanophores 2(2-5) 2(2-4)
Lateral yolk melanophores 2 1(1-2)