According to the Penguin Dictionary of Biology (Abercrombie et al., 1966), Amphibia are a class of vertebrates represented by three orders: Anura (Frogs and Toads), Urodela (Newts and Salamanders) and Apoda (strange burrowing worm-like creatures called Caecelians, pronounced seh-SILL-yuns).
The word Amphibian is derived from the greek words amphi, which means “two” and “bios” means lives. This refers to the fact that they spend part of their lives under water (breathing with gills) and the remainder on land (breathing with lungs). There are about 5,500 known species of amphibians.
Amphibians are cold-blooded (ectothermic), meaning that they cannot internally regulate their body heat like mammals. They rely completely on warmth from the sunlight for warmth and a burrow, other form of shade or water to cool down. Amphibians don’t drink. Instead, they absorb water and much of the oxygen they need through their skin.
Aquatic amphibians lay their eggs in a big mass, outside the body. Fertilization occurs by the male squirting his sperm over these eggs (external fertilization). Amphibians that reproduce this way need water or moist soil for breeding. Without water, their eggs (which are not protected by a shell) would quickly dry out and the young would die before they even had a chance to develop. But, there are also many species of amphibians that fertilize their eggs internally and many that do not require water fertilization.
There are people who think that amphibians all undergo metamorphosis and that they all hatch and start off as tadpoles. This is true with the aquatic amphibians, but there are a lot of species that do not undergo the free-swimming tadpole stage. Rather, they undergo direct development in burrows to adulthood.
Some amphibians avoid the drying effects of the sun by being active only at night (nocturnal). Others shelter in moist habitats under logs, rocks, leaves, mosses and ferns.
Characteristics of Amphibians:
- Circulatory system has a 3 chambered heart with a separate blood circuit through the lungs.
- Skin must remain moist. It is highly vascularized because it acts as a respiratory surface (sometimes the only respiratory surface).
- Some amphibians, the caecelians, have internal fertilization. Most amphibians deposit eggs in water where they are externally fertilized.
- All amphibians are at least in part, dependent on environmental water.
Amphibian or Reptile?
If you see an animal and you can’t tell whether it’s an amphibian or reptile, examine the skin. If it’s hard and scaly with scutes or bony plates, it’s a reptile. An amphibian’s skin is soft and smooth or warty-looking, and it may also be moist.
What kind of amphibian is it?
If it has legs and a tail, it’s a salamander or newt. If it doesn’t have a tail but has legs, it’s a frog or toad. And it’s easy to tell Caecelians from other types of amphibians because they look like worms.
The largest amphibian is the Japanese giant salamander (Andrias japonicus), at 6 feet long (1.8 meters) and 140 pounds (63 kilograms), and the smallest is an Izecksohn’s toad (Brachycephalus didactylus) that weighs just a few grams.
Around a third of amphibian species are threatened with extinction. Traditional threats to species and ecosystems (such as habitat loss and pollution) are being compounded by new threats such as climate change and emerging disease.