Amphibians – Definition, Examples, Characteristics


Amphibians Definition and Examples

Amphibians are a fascinating and diverse group of animals that play a crucial role in many ecosystems. These creatures live both in water and on land. Let’s explore their characteristics, life cycle, evolutionary history, and classification.

What Is an Amphibian?

Amphibians are ectothermic (cold-blooded) vertebrates that belong to the Class Amphibia. A defining characteristics is their ability to live both in aquatic and terrestrial environments. The term “amphibian” is derived from the Greek words “amphi” (both) and “bios” (life), reflecting their dual lifestyle. They lack scales, instead having moist skin.

Characteristics of Amphibians

Amphibians share several characteristics that distinguish them from other animals:

  1. Dual Life Cycle: Most amphibians have a life cycle that includes both aquatic and terrestrial phases.
  2. Skin: Amphibians typically have moist, permeable skin that assists in respiration.
  3. Cold-blooded: Amphibians are ectothermic, meaning they regulate their body temperature through external sources.
  4. Reproduction: Amphibians usually lay eggs in water, and most have an aquatic larval stage (e.g., tadpoles in frogs).
  5. Respiration: Amphibians breathe through their skin, lungs, and gills at different life stages.

Characteristics Shared by Amphibians and Other Vertebrates

Amphibians, like other vertebrates, share a range of characteristics that define their biological classification. Here’s a list of key characteristics that amphibians share with other vertebrates:

  • Vertebral Column: Amphibians have a spine or vertebral column, which is a defining characteristic of all vertebrates. This provides structural support and protection for the spinal cord.
  • Endoskeleton: Amphibians have an internal skeleton (endoskeleton) made up of bone and cartilage. This structure provides support and facilitates movement.
  • Brain Encased in a Skull: Their brains are encased in a hard skull, providing protection to this vital organ, a common feature among vertebrates.
  • Bilateral Symmetry: They exhibit bilateral symmetry, meaning their body can be divided into two identical halves along a central axis.
  • Complex Organ Systems: They possess complex organ systems, including a heart and circulatory system, lungs (in adult stages for most species), a digestive system, and a nervous system.
  • Closed Circulatory System: Amphibians have a closed circulatory system with a heart that pumps blood through their body, which is a common feature in vertebrates.
  • Sexual Reproduction: They generally reproduce sexually, with the male and female producing gametes (sperm and eggs, respectively).
  • Development from Embryos: The development of amphibians, like other vertebrates, begins with embryos. Amphibians typically lay eggs, which is common in many vertebrate groups.
  • Ectothermic Metabolism: Like reptiles and fish, amphibians are ectothermic, meaning they rely on external sources to regulate their body temperature.

Table Comparing Amphibians With Other Vertebrates

Here’s a comparative table that highlights key differences and similarities among amphibians and other major groups of vertebrates: mammals, reptiles, birds, and fish. Note that there are exceptions within each class.

FeatureAmphibiansMammalsReptilesBirdsFish
SkinMoist, glandular, lacks scalesTypically covered with hair or furDry, scalyFeathers cover most of the bodyScales
Temperature RegulationEctothermic (cold-blooded)Endothermic (warm-blooded)EctothermicEndothermicEctothermic
ReproductionMostly oviparous, external fertilizationMostly viviparous, internal fertilizationOviparous or ovoviviparous, mostly internal fertilizationOviparous, internal fertilizationOviparous or ovoviviparous, external or internal fertilization
DevelopmentMetamorphosis (aquatic larva to terrestrial adult)Direct development, no metamorphosisDirect development, no metamorphosisDirect development, no metamorphosisMostly direct development, some with larval stage
RespirationLungs, skin, and gills (in larval stage)LungsLungsLungs with air sacsGills (some also have lungs)
Circulatory SystemDouble-loop, 3-chambered heartDouble-loop, 4-chambered heartDouble-loop, 3-chambered heart (except crocodilians with 4-chambered)Double-loop, 4-chambered heartSingle-loop, 2-chambered heart
LimbsTypically 4 limbs (except legless caecilians)4 limbs (except aquatic mammals like whales)Typically 4 limbs2 legs, 2 wingsFins
SkeletonBony with cartilageBony with cartilageMostly bony with cartilageBony with hollow bonesBony or cartilaginous
EggsSoft and jelly-like, usually laid in waterHard-shelled or live birthSoft-shelled or hard-shelled, usually laid on landHard-shelled, laid in nestsSoft or hard-shelled, laid in water
ExamplesFrogs, toads, salamandersHumans, dogs, whalesSnakes, lizards, turtlesEagles, sparrows, penguinsSalmon, sharks, goldfish

Examples of Amphibians and Non-Amphibians

Examples of Amphibians

  • Frogs and Toads (e.g., the American Bullfrog, the Common Toad)
  • Salamanders (e.g., the Tiger Salamander, axolotls)
  • Newts (e.g., the Eastern Newt)

Animals That Are Not Amphibians

  • Reptiles (e.g., snakes, lizards)
  • Mammals (e.g., humans, cats)
  • Birds (e.g., eagles, sparrows)
  • Fish (e.g., eels, sharks)
  • Invertebrates (e.g., crabs, earthworms)

For examples, turtles, eels, lizards, and fish are not amphibians. Fish (which include eels) live in water, but they have scales and do not have lungs. Lizards resemble salamanders and newts, but they have scales and have lungs all their lives. Like lizards, turtles are reptiles.

Amphibian Life Cycle

Frog Life Cycle

The life cycle of amphibians has three stages:

  1. Egg Stage: Amphibians lay eggs, usually in water.
  2. Larval Stage: Larvae (e.g., tadpoles) live in water and breathe through gills. Most species undergo a metamorphosis into an adult form.
  3. Adult Stage: Adult amphibians typically live on land and breathe with lungs and through their skin. There are exceptions, like the axolotl, which has lungs, but lives in water and usually breathes through its gills.

Evolutionary History of Amphibians

Amphibians evolved from fish around 370 million years ago during the Devonian period. This transition from water to land was a significant evolutionary step, leading to the development of limbs and lungs. Amphibians were the first vertebrates to adapt to a terrestrial lifestyle, paving the way for reptiles, birds, and mammals.

Classification of Amphibians

The Class Amphibia has three main orders:

  1. Anura (Frogs and Toads)
    • Characteristics: Tailless, long hind legs for jumping, smooth or warty skin.
    • Examples: American Bullfrog, Common Toad
  2. Caudata (Salamanders and Newts)
    • Characteristics: Elongated bodies, tails, and smooth, moist skin.
    • Examples: Tiger Salamander, Japanese Giant Salamander
  3. Gymnophiona (Caecilians)
    • Characteristics: Legless, worm-like, mostly burrowing.
    • Example: Common Caecilian

Interesting Amphibian Facts

Here’s a collection of interesting and unusual facts about them:

  1. Amazing Regenerative Abilities: Some species of salamanders, like the axolotl, regenerate lost body parts, including limbs, tail, heart, and parts of their brain and spinal cord.
  2. Biofluorescence: Certain species of frogs and salamanders exhibit biofluorescence, where they absorb light and re-emit it as a different color. This phenomenon is especially visible under ultraviolet light.
  3. Diverse Communication: Beyond the well-known croaking of frogs, amphibians use various forms of communication, including visual signals, body postures, and chemical signals.
  4. Extreme Survival: The wood frog of North America can survive being frozen solid during winter. Its heart stops, and ice crystals form in its blood, yet it revives in the spring. Other amphibians living in cold climate experience brumation, which is a period of dormancy.
  5. Estivation: Meanwhile, some amphibians undergo estivation, a state of dormancy during hot and dry periods. They burrow underground and remain inactive to conserve moisture.
  6. Poisonous Species: The skin of some amphibians, like the poison dart frog, contains potent toxins. Indigenous tribes have used these toxins for centuries to poison the tips of their darts.
  7. Unique Breeding Strategies: For example, the Surinam toad carries its eggs on its back where they develop. Once the development is complete, the young toads emerge fully formed from pockets in the skin.
  8. Variation in Size: Amphibians vary greatly in size. The smallest, like the Paedophryne amauensis (a frog from Papua New Guinea), measures just 7.7 mm, while the largest, like the Chinese giant salamander, grows over 1.8 meters long.
  9. Sensory Perceptions: Some amphibians have highly developed sensory systems. For example, caecilians, which are mostly blind, have tentacles that help them detect chemicals in their environment.
  10. Respiration Variations: While many amphibians develop lungs, some retain gills throughout their lives. Others, like the lungless salamanders, rely entirely on skin and mouth lining for gas exchange.
  11. Longevity: Some amphibians have surprisingly long lifespans. For instance, the European common toad can live for more than 40 years.
  12. Color Changes: Many amphibians change their skin color for various reasons, including camouflage, temperature regulation, and mood expression.

References

  • Dorit, R. L.; Walker, W. F.; Barnes, R. D. (1991). Zoology. Saunders College Publishing. ISBN 978-0-03-030504-7.
  • Lamb, Jennifer Y.; Davis, Matthew P. (2020). “Salamanders and other amphibians are aglow with biofluorescence”. Scientific Reports. 10 (1): 2821. doi:10.1038/S41598-020-59528-9
  • San Mauro, Diego; Vences, Miguel; Alcobendas, Marina; Zardoya, Rafael; Meyer, Axel (2005). “Initial diversification of living amphibians predated the breakup of Pangaea”. The American Naturalist. 165 (5): 590–599. doi:10.1086/429523
  • Schoch, Rainer R. (2014). Amphibian Evolution: The Life of Early Land Vertebrates. John Wiley & Sons. ISBN 9781118759134.
  • Wells, Kentwood D. (2010). The Ecology and Behavior of Amphibians. University of Chicago Press. ISBN 9780226893334.