Amphibian Reproduction

 

Metamorphosis of frogs

Sketch Courtesy of:: Stephanie Bergren

By: Tonia Traas, Grace Schellinger, Fran Uselman, and Stephanie Bergren

Amphibian Classification:

Kingdom: Animalia

Phylum: Chordata

Subphylum: Vertebrata

Superclass: Tetrapoda

Class: Amphibia

Amphibians are made up of four main groups of organisms: toads, frogs, newts/salamanders, and caecilians. Some common characteristics shared by all are that they are all ectothermic (cold-blooded), lay their eggs in fresh water, and all experience metamorphosis at some point in their life. All amphibians have four legs with the exception of caecilians, which are limbless. All experience metamorphosis where they go from water breathing to air breathing. Amphibians must lay their eggs in fresh water because their eggs are shell-less. In recent decades there has been a major decline in general amphibian population. Many amphibian species are now endangered or extinct, which is why learning more about them is so important. Endocrine disruptors effect amphibian reproduction and survival rates.

 

Normal reproductive functions and Structures of Amphibians

  Frogs Toads Salamanders/Newts Caecilians
Male Body Structure Urogenital Organs: Fat bodies, testicles, spermatic canal, adrenal gland, kidneys, ereters, bladder, cloaca  Same as male frog with the addition of a Bidder's Organ (hormone secreting organ located above each testis) Testis, Mullerian duct, Wolffian duct, cloaca Phallodeum
Female Body Structure Urogenital Organs:  Fat bodies, ovaries, adrenal gland, oviducts, kidneys, uterus, bladder, cloaca  Same as female frog Ovaries, oviducts, Wolffian duct, cloaca Cloaca
Copulation Males and Females get into the amplexus position; sperm and eggs are released simultaneously, fertilization occurs outside of the bodies of the both the male and the female Males and Females get into the amplexus position; sperm and eggs are released simultaneously, fertilization occurs outside of the bodies of the both the male and the female “Sperm packet” deposited by male in water and taken up by female for internal fertilization Internal fertilization
Gestation/Parturition Eggs are either left alone, buried on land or carried by the parent until it is ready to hatch (a few days to a few weeks); tadpole changes into a frog in a liquid environment through metamorphosis Eggs are laid in fresh water and hatch around 10 days post fertilization depending on the temperature of the environment  Lay eggs or development within the female for anywhere from 3 weeks to 2 years N/A
Mating Behaviors A male mounts any female he desires in the position of amplexus; Some males accidently mount other males and one would hear release call by the mounted male in this case Similarly to frogs the toad mounts a female using amplexus but he attracts the female through a mating call Male seeks out female and wafts glandular secretions toward her by fanning of his tail. N/A
 


Salamander/Newt Reproduction

Classification:

Kingdom: Animalia

Phylum: Chordata

Class: Amphibia

Subclass: Lissamphibia

Order: Caudata 

The salamander/newt group falls into the category of amphibians that reproduce by internal fertilization without actual sexual intercourse. The male salamander/newt has testis, a Mullerian duct, a Wolffian duct, and a cloaca while the female salamander/newt has ovaries, oviducts, a Wolffian duct, and a cloaca as well. The male newt or salamander seeks out the female, and then proceeds to waft glandular secretions toward her by fanning his tail. These secretions cause the female to approach the male. The male deposits a “sperm packet” consisting of a spermatophore capped with sperm in the water next to the female and the female takes it into her body through her cloaca. Once in the female’s body, the sperm cap is deposited in an internal pouch called the spermatheca and the eggs are then fertilized as they pass through the cloaca. The eggs are protected by a toxic, gel-like membrane. In some species of salamanders/newts, the fertilized eggs are expelled from the female and hatch externally and in other species of salamanders and newts, the eggs remain within the female anywhere from three weeks to two years, even going through metamorphosis within the female!   

Toad Reproduction

Classification:

Kingdom: Animalia

Phylum: Chordata

Class: Amphibia

Order: Anura 

Tadpoles stay together, feeding and growing for 40-70 days.   They have gills upon hatching, located posterior to their mouths, on the sides of their heads.  Twenty days into their development, tadpoles start to form hind legs, which grow slowly and continuously.  30-40 days into development, front legs emerge and gills disappear.  Toads complete their metamorphosis (reabsorbing their tails) between the last two to three days of development.  Depending on the temperature, newly-metamorphosed toads remain close to their pond for a few days.  They begin to live primarily on land.


Caecilian Reproduction 

Classification:

Kingdom: Animalia

Phylum: Chordata

Class: Amphibia

Order: Gymnophiona 


Caecilians are the only order of amphibians that use internal insemination. Males have a penis like organ called a phallodeum which is the extruding part of the cloaca.. This is inserted in the cloaca of the female for 2 to 3 hours.  Females have eggs and embryos similar to what fish have which lack the amnion and chorion structures that are present in mammals, reptiles, and birds. Caecilian embryos typically develop in a gelatinous egg case. About 25% of them lay eggs which the female guards. The state of young when they hatch varies among the different kinds of caecilians; some are already metamorphosed when they hatch and others hatch as larvae which are not fully aquatic and have to spend their time in the soil near the water and metamorphose into adults. The remaining 75% of caecilian young are born viviparous and develop inside their mothers feeding off the egg yolk. After the embryos use up all the egg yolk they begin to feed on “uterine milk” that is secreted by the oviducts. The embryos also use special scrapping teeth to feed on the oviduct cells while in utero.


Frog Reproduction

Classification:

Kingdom:Animalia

Phylum: Chordata

Class: Amphibia

Order: Anura 

The reproductive process of frogs begins with fertilization, and like all other amphibians except caecilians, they fertilize their eggs outside of the female’s body.  The male and female frogs place themselves into what is known as the amplexus position.  They can remain in this position from a few hours to days at a time.   This position involves the mounting of the male frog onto the back of the female frog and the male then wraps his forelegs around his partner.  This position allows for the best chance at the sperm fertilizing the egg outside of the body.  The male and female who are copulating, release the egg and sperm to be fertilized at the same time, and many then leave their eggs to develop on their own.  Other frogs take care of their eggs in different fashions such as:  carrying their eggs in their vocal sacs/abdomens or burying their eggs on land (they keep them wet through periodic addition of water or urine to the nest).  A tadpole, the point in an amphibian’s life style that is a solely aquatic larval stage, emerges from the egg a couple of days to a couple of weeks after it is fertilized.  This tadpole then grows and changes in a liquid environment through a process known as metamorphosis (process of drastic changes from one life stage to the next in the development of an organism).  The body of the tadpole changes in the following order:  the back legs of the frog begin to form, followed by the front legs, the internal organs prepare themselves for land life, lungs develop, the digestive changes to be able to process a frog’s diet, the tail almost completely disappears.  Once the tadpole has completely made all of its changes through metamorphosis, the animal begins a life on land and in the water as an adult frog and the cycle begins once again.

Endocrine Disruptors and Amphibians 

Endocrine disrupters are natural and man-made chemicals that can either mimic or disrupt the action of hormones. Atrazine is the most widely used pesticide in the United States and also acts as an endocrine disrupter in amphibian species. Atrazine and other endocrine disrupters have a feminization effect on amphibian species. The atrazine enters into the amphibian environment mainly by agricultural run-off into ponds where amphibians live, breed, and reproduce. Exposure to atrazine has been shown to have many effects on amphibians in the larval stage, some of these effects being strictly related to reproduction and development. For example, larval amphibians exposed to atrazine have shown increased hermaphrodism and demasculinized larynges. Larynges contain the vocal cords, and therefore are used for mating and release calls in some amphibian species, especially toads. Atrazine exhibits these endocrine-disrupting effects by inhibiting androgen receptors, leading to decrease
d testosterone binding. Atrazine also induces the enzyme aromatase, which converts androgen to estrogen, causing feminization effects. Studies have also shown that atrazine has different effects on amphibians in different life stages. For example, exposure to atrazine, and other endocrine disrupters, during critical times of development can lead to irreversible changes. Even though atrazine can cause these detrimental effects in amphibian and many other vertebrate species, the US refuses to ban atrazine even though it has been banned in many other countries around the world.


Amphibian Quiz Time

  1. True or False: All amphibians have four limbs and perform external fertilization.
  2. True or False: Endocrine disruptors tend to have defeminization effects on amphibians.
  3. True or False: Metamorphosis is a process that amphibians go through during the stage of early development.
 

References

  1. Cogger, Dr. Harold G., and Dr. Richard G. Zweifel. Encycolpedia of Reptiles and Amphibians. 2nd. San Diego, CA: Academic Press, 1998. 52-59. Print.
  2. Crump, Marty. Amphibians, Reptiles, and Their Conservation. North Haven: The Shoe String, 2002. Print.
  3. Gunzi, Christiane. Amphibians & Reptiles of North America. San Diego: Thunder Bay, 1995. Print.
  4. Hofrichter, Robert. Amphibians: the world of frogs, toads, salamanders and newts. 1st. Buffalo, NY: Firefly Books, 2000. 38-43. Print.
  5. O'Shea, Mark, and Tim Halliday. Reptiles and Amphibians. 1st. New York, NY: Dorling Kindersley, 2001. 209-10. Print.
  6. Patent, Dorothy H. Frogs, Toads, Salamanders, and How They Reproduce. Holiday House, 1975. Print.
  7. Stebbins, Robert C., and Nathan W. Cohen. A Natural History of Amphibians. Princeton: Princeton UP, 1995. Print.
  8. Storrs, Sara I., and Joseph Kiesecker. "Survivorship Patterns of Larval Amphibians Exposed to Low Concentrations of Atrazine." Environmental Health Prospectives 112.10 (2004): 1054-57. Web. 17 Nov 2009.
  9. Storrs, Sara I., and Jospeh M. Kiesecker. "Survivorship Patterns of Larval Amphibians Exposed to Low Concentrations of Atrazine." Environmental Health Perspectives 112.10 (2004): 1054-057. Web of Knowledge. Web. 22 Nov. 2009.
  10. Barlow, Maude. Blue Covenant. New York: The New, 2007. Print.

Ground Toads

Photo Courtesy Of: Grace Schellinger