Ytz

(n) Spermatid

| Spermiogenesis (n) Sperm Sertoli cells

. ^ Lumen, continuous with

Figure 9. Spermatogenic cyst with several spermatogonia! cells.

Micropyle Fish Egg

fluid for nutrients and oxygen. The origin, quantity, and turnover rate of the ovarian fluid are unknown, although it could be a dilute ultrafiltrate of plasma.

Spawning

Spawning follows soon after ovulation. The range of spawning behavior as well as the spawning itself in fish is probably wider than that in all land vertebrates put together. This event is probably controlled by a combination of hormones and environmental factors. There may be a special spawning hormone acting directly on the central nervous system. Mammalian reproduction involves both the pituitary gonadotropins, LH and FSH, affecting the gonads and the ovary, which respond by producing estrogen and progesterone, whereas the LH hormone activity in fish predominates and FSH activity is absent or weak in some species.

Fertilization

Fertilization is also varied among species. Generally, one sperm enters the egg via a micropyle, which closes after entry (Fig. 10). This prevents multiple fertilization. Thereafter, the two nuclei unite. The chorion, the tough outer shell, or protective coating, of the egg swells by the egg imbibing water. This water hardening provides the egg with a protective shell permeable to dissolved substances, gases, and water. Water hardening produces a perivitelline space between the cell membrane and the external environment. Eggs and sperm survive only a short time if fertilization does not take place.

Development

Developmental time depends on environmental conditions. The important factors are temperature, salinity, oxygen, and light. All conditions have varying effects, depending on species. The following outline shows the major steps in the development of the fish, from embryonic to adult stages (Fig. 11). It is similar in pattern to the general pattern in vertebrate development.

Figure 10. Cross section of a salmonid egg.
Figure 11. Development stages of a salmonid. I = blastic cleavage; cell division is shown in A, B and C. Cell division continues. II = gastrulation, III = organogenesis, IV = hatching, V = parr, VI = smolt, VII = juvenile to adult.

I. Blastodisc cleavage (2 cells) II. Gastrulation

Specialization of cells

Body tissues develop and grow over the yolk Eyed stage

Basic body form is evident

III. Organogenesis Internal organs form Fins appear

Circulatory system develops connection between yolk and internal vasculature

IV. Hatching

Hatching glands develop around head of embryo and secrete chorionase, an enzyme that dissolves the tough chorion, or egg shell, for the embryo to escape.

V. PanGeneral body form resembles that of the adult Dark vertical bars along the lateral body wall VI. Smolt

Parr marks disappear Body coloration is silver

Body shape is slightly more elongate than smolt VII. Juvenile to adult

General body shape and color is like the smolt Size increases as it grows and matures at sea

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