Fish Reproduction - Anatomy & Physiology

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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

• Most species have paired, intra-abdominal testes.
  • May be partially or totally fused.
• Testicular duct is present except in Salmonids.

Female

Ovaries

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

• 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.

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

• 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.

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.

Polygyny

• 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 Polygyny.

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.