Amphibian Reproduction - Anatomy & Physiology

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 (ceolom).
• Cilia in the ceolom direct the ova into the infundibulum of the oviduct.

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 (known as the Bidder's organ)nearby in the male.

Bidder's Organ

• Bidder's organ is a spherical, brownish organ in toads.
• The organ is located just in front of the kidney (mesonephros).
• It is formed at the cranial tip of the male and female gonad during the larval stage.
• Normally it is inactive and contains miniature follicles which have the capability to mature (becoming active).
• When the female population is scarce, the Bidder's organ enlarges and produces viable oocytes and then gonadotropins, which stimulate the growth of the Müllerian ducts to form uterus and oviducts.
• Its internal anatomy consists of two parts:
  • Central portion: consists of connective tissue and rich in blood vessels.
  • Periphery: consists of the cortex, which contains follicles in various stages of development.
  • Molecules of various proteins are present in the outer layer of the follicles, homologous to the zona pellucida of the ovum.

Male Ducts

• Mesonephric ducts carry both sperm and excretory waste.

Parental care

Paternal

Clutch Guarding

• Male stands nearby to guard the clutch until they hatch.

Limb Carrying

• Carry the clutch of eggs on their limbs.

Vocal Sac Brooding

• Eggs are laid and fertilised on land.
• The male frog guards the eggs until the tadpoles hatch.
• He then collects them in his mouth, without swallowing them, and passes them into his vocal sac.
• Fully developed froglets then escape the vocal sac and exit via the frog's mouth to the exterior.

Tadpole Carrying

• Carry tadpoles on their back.

Maternal

Internal Development

• Ovoviparous species.

Epidermal clutch carrying

• Eggs are carried in small pores in the skin (epidermis).
• Hatch by bursting through these pores to the exterior.

Gastric brooding

The Gastric-brooding frogs or Platypus frogs were a genus of ground-dwelling frogs native to Queensland in eastern Australia. The genus consisted of only two species, both of which became extinct in the mid-1980s. The genus was unique because it contained the only two known frog species that incubated the prejuvenile stages of their offspring in the stomach of the mother.

• Female ingests the fertilized eggs.
• In the jelly that surrounds each egg is a substance called prostaglandin E2 (PGE2).
• This substance has the ability to turn off the production of hydrochloric acid in the stomach. This source of PGE2 is enough to cease the production of acid during the embryonic stages of the developing eggs.
  • Once the eggs hatch, the tadpoles too create PGE2.
    • The mucus excreted from the tadpoles' gills contains the PGE2 necessary to maintain the stomach in a non-functional state.
    • These mucus excretions do not occur in tadpoles of most other species.
• During the period that the offspring are present in the stomach the frog does not eat.
• Tadpole development takes at least six weeks, during this time the size of the mother’s stomach continues to increase until it largely fills the body cavity.
• The lungs of the female deflate and breathing relies more upon gas exchange through the skin.
• Despite the mother's increasing size she still remains active.
• The birth process is widely spaced and may occur over a period of as long as a week.
  • However, if disturbed the female may regurgitate all the young frogs in single act of propulsive vomiting.
• The offspring are completely developed when expelled.

Species Differences

Frogs and Toads (Anurans)

Introduction

Once at the breeding ground, male frogs call to attract a mate, collectively becoming a chorus of frogs. The call is unique to the species, and will attract females of that species. Some species have satellite males who do not call, but intercept females that are approaching a calling male. The male and female then undergo amplexus. This involves the male mounting the female and gripping her tightly. Fertilization is external: the egg and sperm meet outside of the body. The female releases her eggs, which the male covers with a sperm solution. The eggs then swell and develop a protective coating. The eggs are typically brown or black, with a clear, gelatin-like covering. Most temperate species reproduce between late autumn and early spring. In the UK, most common frog populations produce frogspawn in February, although there is wide variation in timing. Water temperatures at this time of year are relatively low, typically between four and 10 degrees Celsius. Reproducing in these conditions helps the developing tadpoles because dissolved oxygen concentrations in the water are highest at cold temperatures. More importantly, reproducing early in the season ensures that appropriate food is available to the developing frogs at the right time.

Mating Calls

• Mating calls are crucial for most species.
• The call of a frog is unique to its species.
• Frogs call by passing air through the larynx in the throat.
• In most calling frogs, the sound is amplified by one or more vocal sacs, membranes of skin under the throat or on the corner of the mouth that distend during the amplification of the call.

Amplexus

• Amplexus is a form of pseudocopulation in which a male frog grasps a female with his front legs while she lays her eggs. At the same time, he fertilizes them with the fluid containing sperm.
• This mostly happens in the water, but some more terrestrial anurans like the disc-tongued frogs (Discoglossidae) perform amplexus on land.
• In more advanced anurans like the true frogs (Ranidae), the tree frogs (Hylidae) and the true toads (Bufonidae) the amplexus is axillary, while in less derived anurans (the Archaeobatrachia) and frogs in the Myobatrachidae family it is lumbar (abdominal, in front of the hindlegs).
• The Sooglossidae show inguinal amplexus where the male holds the female at the waist just anterior to her hind legs.
• Some species show cephalic amplexus where the head of the female is held while others show complete lack of amplexus.

Internal Fertilization

• Only occurs in tailed frogs.
• Male has a penetrating sperm delivery organ, the 'tail'.
  • Not a true tail as found in tadpoles and salamanders.

Spermatheca

• Organ of the female reproductive tract.
• Acts as a store of sperm in the female for later fertilization.
• Sperm can be stored for as long as years!

Salamanders and Newts (Caudata)

Primative salamander families have an Amplexus, but on the whole fertilization is internal. Paedomorphosis (the retention of juvenile traits) is common.

Spermatophore

• A packet of sperm with a pedestal to raise it above the ground in ponds.
• Deposited by the male.
• Picked up voluntarily by the female, depending on the success of the male's mating display.

Cloacal Kiss

• Some salamanders
• Press cloacae together for sperm transfer.
• Creates a water-tight seal so that the sperm is not compromised.

Caecilians (Gymnophiona)

• Caecilians are the only order of amphibians which only use internal insemination.
• The male caecilians have a penis-like organ, the phallodeum, which is inserted into the cloaca of the female for 2 to 3 hours.
• About 25% of the species are oviparous (egg-laying); the eggs are guarded by the female. For some species the young caecilians are already metamorphosed when they hatch; others hatch as larvae. The larvae are not fully aquatic, but spend the daytime in the soil near the water.
• 75% of the species are viviparous, meaning that they give birth to already developed offspring. The fetus is fed inside the female with special cells of the oviduct, which are eaten by the fetus with special scraping teeth.
  • The egg laying species Boulengerula taitanus feeds its young by developing a special outer layer of skin, high in fat and other nutrients, which the young peel off with similar teeth. This allows them to grow by up to ten times their own weight in a week. The skin is consumed every three days, the time it takes for a new layer to grow, and the young have only been observed to eat it at night.