Fish Reproduction - Anatomy & Physiology

Introduction

There is great diversity in the reproductive systems of fish. Some produce eggs and sperm for external fertilization, while others copulate with the discharge of either fertilized eggs or young fish.

Male

Testes

• Most species have paired, intra-abdominal testes.
• Suspended by a mesentery from the dorsal abdominal wall adjacent to the swim bladder.
• May be partially or totally fused.
• Vary greatly in size with age and season.
• Lie within a thin tunica albuginea.
• Made up of a series of semniferous tubules or blind-ending sacs, lined with spermatogenic epithelium either along their whole length (salmonids and cypprinids) or only at their distal ends.
• Caudal section of the testes in ictulurids and some cyprinids is lined with non-germinal secretory epithelial cells.
  • May be involved in sperm storage, nutrition or may contribute to the ejaculate.
• Testicular duct is present except in Salmonids.
• No epididymis or ductus deferens present in Salmonids
• Spermatozoa released into the body cavity before leaving via the genital opening.
• Interstitial fibrous tissue contains hormone secreting cells (analagous to Leydig cells of mammals).
• No lymphatics of the type seen in mammals.

Female

Ovaries

• Paired
• Suspended from the dorsal abdominal wall by a mesentery.
• Enclosed by a tunica albuginea containing fibrous tissue and smooth muscle.
• Appear as a small cluster of orange/white spheres in the immature fish.
• Parenchyma consists of a well vascularised connective tissue stroma containing germinal and folicular epithelium.
• Hormones are produced by follicular tissue.
• Primary ovarian cells line hollow cavities with folded walls.
  • Oogonia are shed into this cavity, gaining a layer of epithelial cells in the process.
  • Become granulosa cells, responsible for yolk formation in the developing egg.
• Ovarian follicle grows.
  • Hyaline zone forms between the oocyte and the epithelial cells - the zona pellucida.

Oviduct

Gymnovarian

• Primative
• Oocytes are released directly into the coelomic cavity
• Enter the ostium
• Travel through the oviduct and are eliminated.

Secondary Gymnovarian

• Salmonids and a few other teleosts.
• Ovaries shed ova into the coelom
• Go directly into the oviduct.

Cystovarian

• Most teleosts
• Oocytes are conveyed to the exterior through the ovarian duct (not oviduct).
  • Ovarian lumen is continuous with the ovarian duct.

Post-Oocyte release

• Postovulatory follicles are formed after oocyte release
  • Do not have endocrine function
  • Wide irregular lumen
  • Rapidly reabosrbed in a process involving the apoptosis of follicular cells.
• A degenerative process called follicular atresia reabsorbs vitellogenic oocytes not spawned.
  • This process can also occur, but less frequently, in oocytes in other development stages.

Oviparity

• 97% of fish
• Mostly external fertilisation, with the male and female fish shedding their gametes into the surrounding water.
• A few oviparous fishes practise internal fertilisation, with the male using an intromittent organ to deliver sperm into the genital opening of the female.
  • Species that pracitce internal fertilization include oviparous sharks, such as the horn shark, and oviparous rays, such as skates.
  • In these cases, the male is equipped with a pair of modified pelvic fins known as claspers.
• The newly-hatched young are called larvae.
  • Usually not well developed, carry a large yolk sac (from which they gain their nutrition) and are very different in appearance to juvenile and adult specimens of their species.
  • The larval period in oviparous fish is relatively short, usually several weeks.
  • Larvae rapidly grow and change appearance and structure (metamorphosis) to resemble juveniles of their species.
    • During this transition larvae use up their yolk sac and must switch from yolk sac nutrition to feeding on zooplankton prey.

Ovoviviparity

• Examples include guppies, angel sharks, and coelacanths.
• Eggs develop inside the mother after internal fertilization.
• Receive little or no nutrition from the mother.
• Depend on yolk.
• Each embryo develops in its own egg.

Viviparity

• Very rare
• Mother retains the eggs
• Embryos receive nutrition from the mother.
  • Usually have a structure analogous to the placenta seen in mammals connecting the mother's blood supply with the that of the embryo.
• The embryos of some viviparous fishes exhibit a behaviour known as oophagy where the developing embryos eat eggs produced by the mother.
• Intrauterine cannibalism is a rarely seen form of viviparity where the largest embryos in the uterus will eat their weaker and smaller siblings.
  • Found in some sharks.

Stages of Reproduction

Oocyte Development

• Gonadotrophin independent
• Dependent on body size

Vitellogenesis

• Production of the yolk
• Longest phase of reproduction

Spawning

• Release of eggs and sperm
• Release into the optimum environment for fertilization to occur
• Mating behaviour displayed

Recovery

• Body condition restored
• New oocyte developement

Breeding Cycles

• Vary from 4 weeks - many years.
• Can breed once - many times a year.
• Some species breed continuously in the spring and summer.
• Some species only breed once in a lifetime (Pacific Salmon).

Reproductive Endocrinology

In fish, as with all higher animals, hormones play a critical role in the reproductive process. Hormones are chemical messengers released into the blood by specific tissues, such as the pituitary gland. The hormones travel through the bloodstream to other tissues, which respond in a variety of ways. One response is to release another hormone, which elicits a response in yet another tissue. The primary tissues involved in this hormonal cascade are the hypothalamus,pituitary gland, and gonads.

• Reproduction occurs under environmental conditions that are favorable to the survival of the young.
• Long before spawning, seasonal cues begin the process of maturation.
  • In many fish, this can take up to a year.
• When the gametes have matured, an environmental stimulus may signal the arrival of optimal conditions, triggering ovulation and spawning.
  • Examples of environmental stimuli are:
    • Changes in photoperiod
    • Temperature
    • Rainfall
    • Food availability.
• A variety of sensory receptors detect these cues, including the eye, pineal gland (an organ in the dorsal part of the forebrain that is sensitive to light), olfactory organs, taste buds, and thermoreceptors.
• The hypothalamus, located at the base of the brain, is sensitive to signals from sensory receptors and releases gonadotropin releasing hormones (GnRH) in response to environmental cues.
• GnRH travels from the hypothalamus to the pituitary gland.
• Gonadotroph cells of the pituitary receive GnRH and release gonadotropic hormones into the bloodstream.
  • GTH-I has FSH-like activity.
  • GTH-II has LH-like activity.
• The gonadotropic hormones travel to the gonads, which synthesize steroids responsible for final maturation of the gametes.
  • GTH-I binds to theca and granulosa cells of follicles. This induces testosterone production by theca cells. Testosterone then travels to the granulosa cells, where it is converted to oestrogen (as in mammals). Oestrogen induces vitellogenin (egg yolk precursor protein) production in the liver.
  • GTH-II binds to granulosa cells and induces production of progestins.
• Maturation of the egg is a long process that involves complex physiological and biochemical changes. One important step, vitellogenesis, is a process in which yolk proteins are produced in the liver, transported to the ovary, and stored in the egg, resulting in tremendous egg enlargement. The yolk is important as a source of nutrition for the developing embryo.
• Also critical are germinal vesicle migration and germinal vesicle breakdown (GVBD).
  • Before it migrates, the germinal vesicle, or nucleus, is located at the center of the egg in an arrested stage of development.
  • At this stage, the egg is physiologically and genetically incapable of being fertilized, even though it has the outward appearance of a fully mature egg.
  • When conditions are appropriate for final maturation, nuclear development resumes, and the germinal vesicle migrates to one side.
  • Finally, the walls of the germinal vesicle break down, releasing the chromosomes into the cell.
• After the egg has matured, prostaglandins are synthesized.
  • Stimulate ovulation
• The egg is then released into the body cavity or ovarian lumen, where it may subsequently be released to the outside environment.
• Following ovulation, the viability of the eggs can decrease rapidly.

Sex Determination

Genetic

Sex Chromosome Dependent

• 10% of fish
• Male is XY
• Female is ZW
• Some variations to this may be seen.

Autosome Dependent

• Autosomes have sex determining factors

Temperature Dependent

• Some species of fish

Dioecism, Parthenogenesis & Hermaphrodism

Dioecism

• The majority of fish are Dioecious (a species that possesses both males and females in separate bodies).

Parthenogenesis

• An asexual form of reproduction found in females where growth and development of embryos occurs without fertilization by males.
• 100% female contribution.
• Self-activated oocytes require no contribution from sperm.
• The offspring produced by parthenogenesis almost always are female in species where the XY chromosome system determines gender.
• Occurs in very few fish species.

Hermaphrodism

Hermaphroditic species can be either simultaneously hermaphroditic or sequentially hermaphroditic.

Simultaneously Hermaphroditic Species

• Sea Bass
• Each individual is both male and female at the same time
• Ripening of the gonads may be sequential to prevent unnecessary self fertilization.

Sequentially Hermaphroditic Species

• Any given individual is only one gender at a time, but can change gender when necessary.
• In sequentially hermaphroditic species all individuals are born as the same gender, this can be either male or female, but is fixed for the species.
• The gender change follows environmental cues that normally reflect the reproductive state of other nearby individuals of the same species.
• Species that are born male and change to female are called Protandrous (andros = male, proto = first)
• Species that are born female and change to male are called Protogynous (gyne = female, proto = first).
• Gender change normally accompanies a change in size to a larger individual.
  • If the species is protandrous then the females are larger than the males
  • If the species is protogynous then the males are larger than the females.
• Examples include Wrasses:
  • Social heirarchy present consisting of a dominant male and many females.
  • If the dominant male is removed, the most dominant female will become male.

Monogamy, Polygyny, Polyandry & Polygynandry

Monogamy

• One male and one female form a pair bond for life, or at least for one reproductive season.
• Rare in fish
• Mostly seen in Anemone and some Cichlid species.

Polygymy

• One male and two or more females
• Common when males form a harem, protecting the females that make up the harem from other sexually active males.
• Polygyny may also occur as a result of Lekking.
  • Lekking is where all the males of an area gather together in a 'Lek' to display. Females visit the Lek, chose a male and leave with him to mate. The male returns to the 'Lek' after mating, and may then be chosen by another female. Thus lekking often results in Polygymy.

Polyandry

• One female and more than one male
• Very rare in fish

Polygynandry

• Most common
• No bond between mating pairs.
• Female may have her eggs fertilized by many males and a male may fertilize the eggs of many females.

Pelagic & Benthic Spawners

Mass spawning is the most common method of reproduction in fish. It involves all the members of a particular species in a particular area getting together in a single place to release sperm and eggs together. Number of sperm produced by a single male fish is normally billions, to increase the chance of fertilization occuring. Sperm can survive in the environment of the water for a period of time to allow external fertilization. Species that use this method include Tuna, Sardines, Pilchards, Cod, Mackeral, Pollack, Hake, Tailor, Halibut, Eels, Herring and Menhaden.

Pelagic Spawners

• Eggs released into surrounding water.
• Eggs are carried along by the currents.

Benthic Spawners

• Pike and many Carp
• Eggs are released and usually fall to the bottom of the substrate e.g. sea floor.
• Fertilized by males on the bottom of the substrate.
• Eggs are not carried away by the currents.
  • May be sticky and adhere to plants or rocks.
  • May be small and rapidly take on water, trapping themselves in small crevasses.

Bearers, Guarders and Non-Guarders

Bearers

• One parent carries the eggs until they hatch.

Guarders

• Guard the eggs

Non-Guarders

• Supply no aftercare to the eggs.
• Reproductive cycle involves very little, or no courtship.

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