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. With regard to physiology, there are several categories which can be used to divide fish into reproductively similar groups as follows:

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.