Fish Reproduction and Embryo Development
I. Fish Reproduction
The reproductive organs of fish, the gonads, resemble sacs where eggs or sperm (milt) develop. Immature fish have small, seemingly empty gonads. As the fish mature, these gonads fill with germ cells. Sexual products are released through a short canal behind the anus. As fish approach sexual maturity, they seek specific spawning grounds. These spawning grounds require particular conditions, often related to water temperature. For example, cod spawn in waters between 4°C and 6°C, while mackerel require temperatures around 12°C. Typically, eggs and sperm are released into the water while the parents remain side-by-side, sometimes belly-to-belly. However, some fish, like sharks and rays, possess copulatory organs, enabling internal fertilization. Egg color varies widely, from transparent and green to red and ochre. Some eggs float to the surface, while others sink or attach to vegetation. Due to high egg mortality, fish produce vast quantities of eggs; the sunfish, for instance, lays over 300 million.
1. Gonads
When examining a fish, the gonads are typically visible only when the animal is ready to spawn. These paired, strap-like structures appear as two strands in sexually mature fish outside of the spawning season. During spawning season, they become more prominent. In males, the gonads are two white, triangular strips. In females, they are orange, reflecting the color of the egg yolk. The female gonad can reach over half the fish’s body weight, while the male gonad is considerably smaller, up to a tenth of the body weight. Passages connect the gonads to the urinary tract, which may be shared in some species.
2. Ovarian Development
Primordial germ cells, present in the gonads from the earliest stages, differentiate into oogonia. Vitellogenesis, the process of yolk formation, marks the beginning of sexual maturation. The liver synthesizes vitellogenin (a phospholipid) which is transported via the blood to the gonads and oocytes. Within the oocytes, vitellogenin forms yolk granules and globules. This meiotic process is the longest stage of egg development.
3. Maturity and Sexual Reproduction
Sexual maturity is the ability to spawn. Fish reach this stage when their gonads develop and produce eggs or sperm, marked by visible morphological changes. First maturity refers to the initial attainment of sexual maturity, which varies among species. Smaller, shorter-lived species mature earlier than larger ones. The onset of sexual maturity is also influenced by food availability. Determining the stages of gonad maturation is crucial for understanding reproductive cycles.
4. Reproduction Modes
Oviparous Fish
Over 97% of fish species are oviparous. Following external fertilization, the female lays eggs that develop and hatch outside her body. Embryos are nourished by the yolk within the egg. While most oviparous fish utilize external fertilization, a small percentage, including sharks and rays, employ internal fertilization. Spawning, the release of eggs, can be categorized into two types for reef fish: pelagic spawners and demersal spawners. Pelagic spawners release eggs into the water column, where currents disperse them. Demersal spawners produce denser eggs that sink and adhere to the bottom. These eggs are larger and have longer developmental periods. Some oviparous species, such as wrasses, damselfish, and flying fish, exhibit parental care, guarding their eggs until hatching.
Ovoviviparous Fish
After internal fertilization, eggs remain within the mother’s body. Each embryo develops inside its egg, relying on the yolk for nourishment, and is born live.
Viviparous Fish
Viviparous fish also employ internal fertilization. However, the embryos receive nutrients directly from the mother’s body, not from the yolk, and are born fully developed.
5. Fish Sex
Determining the sex of a fish can be challenging, as the gonads are internal. In some species, fins have evolved into copulatory organs, allowing for internal fertilization, as seen in the Peciliidae family. In other species, secondary sexual characteristics, such as color, body size, fin shape, and behavior, can help distinguish males from females. However, in some species, sex determination remains difficult, if not impossible.
II. Obtaining Fish Embryos
1. Preparation of Broodstock for Spawning
Getting Broodstock
Successful fish reproduction requires healthy, sexually mature individuals of both sexes, known as broodstock. These can be obtained by capturing wild fish and transporting them to a farm, or by breeding them on the farm, allowing for selective breeding and improved stock.
Management of Breeding Ponds
Breeding ponds must be suitable for the specific fish species, with regulated temperature and well-oxygenated water. Fish require abundant natural food, supplemented with protein-rich feed if necessary. Population density should be appropriate for the food supply, but generally low. Ponds should be easily accessible but secure against poaching.
Selection of Good Broodstock
Careful selection of broodstock is essential. Males should release milt when the abdomen is gently squeezed, while females should have a swollen, reddish/pink genital opening and a rounded, soft abdomen, indicating developed gonads.
Urochordates
In hermaphroditic urochordates, fertilization is typically external, with gametes released through the exhalent siphon. The resulting zygote develops into a tadpole-like larva. Larvaceans have simple gonads with a duct opening for external fertilization. Hatching may occur within the atrial siphon, and development is indirect, involving a tadpole larva and metamorphosis. In the dioecious species Oikopleura, individuals are either male with one testis or female with one ovary. Other appendicularians are hermaphrodites, possessing a single ovary and testis, or a pair of testes, in the trunk. Mature eggs are released when the trunk ruptures, resulting in the animal’s death. Sperm is released through a duct. Appendicularian development resembles that of sea squirts, with some differences in early differentiation.
Class Ascidiacea
Reproductive
Asexual Reproduction
All ascidians regenerate effectively, but only colonial ascidians reproduce asexually, through budding. The budding mechanism is complex and variable. The blastozooid, originating from the oozooid (zooid developed from a fertilized egg), forms the bud. Bud location and development vary among species. The most primitive budding occurs in Perophora, where buds arise from the stolon. In other families, buds develop on the abdomen, post-abdomen, or even in larval stages, as in Diplosoma.
Sexual Reproduction
The ovary, located in the visceral cavity near the intestine, consists of a vesicle with a germinal epithelium wall. An oviduct connects the ovary to the atrial cavity, opening near the anus. The testis comprises numerous branched channels on the ovary and intestine surface, leading to an efferent duct parallel to the oviduct. Fertilization is usually external in solitary ascidians or internal within the atrium in colonial ascidians. Embryos may be released into the environment or retained in the atrium. After a planktonic phase, the larva attaches to a substrate and undergoes metamorphosis. The notochord and neural tube are reabsorbed, the pharynx develops into a branchial basket for feeding, and a tunic, primarily composed of tunicin, covers the body. Colonial species can reproduce asexually by budding, enabling rapid proliferation under favorable conditions.
Class Thaliacea
Reproduction is primarily through budding by blastozooids. The blastozooid, a free-living individual, develops from an embryonic stage and regresses later. Early embryonic development in salps occurs within the mother’s body. In perisomids, eggs are held in the gill chamber, while in other salps, they are retained in the ovary, specifically within the follicle. The follicle forms a lump in the cloacal chamber and develops an epithelial support structure for nourishing the embryo, forming a placenta-like connection.
Cephalochordata
Cephalochordates have separate sexes and external fertilization. Larvae are initially asymmetric swimmers, later developing bilateral symmetry. Synapomorphies, such as segmented axial muscles and metameric organization of visceral arches, suggest a close relationship between Cephalochordata and Craniata (vertebrates). Reproduction is sexual, with separate sexes and no sexual dimorphism. Numerous, segmentally arranged gonads release eggs and sperm into the atrial cavity, exiting through the atriopore. Fertilization is external. The small, yolk-poor eggs undergo holoblastic cleavage. Larvae mature rapidly into the adult form. Branchiostoma has 26 pairs of gonads, while Asymmetron has gonads only on the right side. Amphioxiformes have separate sexes, external fertilization, and eggs and sperm released through the atriopore. Branchiostomidae have a double row of gonads and asymmetric metapleural folds. Epigonichtidae have gonads only on the right side and asymmetric metapleural folds that may surround the anal area.