Amphibian Reproduction - Anatomy & Physiology

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Introduction

Reproductive behaviour and physiology shows much placicity in amphibians. Gender and breeding season are controlled by genetics, photoperiod, temperature, humidity and food or mate availability.

Reproductive Traits

Amphibian reproduction can by classified by primitive traits that are shared by most fish (Oviparity, Anamniotic Eggs and a Cloaca). They also show derived traits (Metamorphosis, Ovoviviparity and Population Sex Modulation).

Oviparous

• Lay eggs with little or no embryonic development within the mother.
• Eggs laid singly,in clusters or in strands depending on the species of amphibian.

Ovovivparous

• Evident in a few species of frog and salamander.
• Ovoviviparous animals develop within eggs that remain within the mother's body up until they hatch or are about to hatch.
• It is similar to vivipary in that the embryo develops within the mother's body.
• Unlike the embryos of viviparous species, ovoviviparous embryos are nourished by the egg yolk rather than by the mother's body.
  • However, the mother's body does provide gas exchange.

Anamniotic Eggs

• No shell.
• Require moisture as there is only a single membrane around the embryo.
  • Amnion is absent.

Cloaca

• Shared urinary, reproductive and excretory channel.

Population Sex Modulation

Bisexual

• Both male and female must be present.
• Both sperm and oocyte contribute to the embryo.

Gynogenesis

• 99% female contribution.
• Mitosis occurs without separation and meiosis follows.
• Sperm nucleus does not enter the oocyte to form the diploid state.
  • Sperm contributes no genetic material to the embryo.
• Sperm are only required to activate the embryo.

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.

Gonads

Ovaries

• Produce 1-25,000 oocytes at a time.
• Under the influence of the pituitary gland.
• Oocytes squeeze out into the peritoneal cavity.

Female Ducts

• The oviduct (Mullarian Duct) covers the egg with a jelly-like material.
• Oviduct and utreter may be united through much of their length.
• Open into the cloaca.

Testes

• Located near the kidneys.
• Produce sperm under the influence of the pituitary gland.
• Often a rudimentary ovary nearby in the male.
  • Alows males to transform into females if the female population is scarce.

Species Differences

Caecilians (Gymnophiona)

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