Amphibians cover a fraction of the animal kingdom species and are often confused with other animals and organisms with similar traits. A question that comes up when talking about amphibians is whether they can live on both land and water.
Amphibians live and prosper in both water and on land. While some animals mimic this behavior, they are not regarded as amphibious due to the lack of specific organic, biological and physical differences. The most common mistake is to classify semi-aquatic mammals as amphibious, like hippopotami, otters, beavers, and platypi.
Differentiating between the animals that live on both land and in water can be a little confusing. Below, we’ll find out what exactly classifies an animal as an amphibian, how to correctly identify these Amphibia, and what other species mimic amphibians but do not belong to the same family.
What Classifies an Animal as Amphibious?
There are definitive telltale signs and distinctions which will inform you if an animal is truly amphibious. One is that they are biologically cold-blooded. However, there are many cold-blooded animals that are not considered amphibious and do not even like water, for that matter. For this reason, we will discuss and explain the distinctions and biological factors that an organism has to adhere to for it to be classified as amphibious.
Physiological Features of Amphibians
- Amphibians are vertebrates, which means they have a spinal column that runs along their body length. This spinal column usually features vertebrae that are ossified and interlock with each other. Their bones are generally hollow and very light. With the exception of some salamanders, most amphibians have four-toed appendages in the front and five-toed in the back. The length and muscular density of the appendages or limbs differ according to species. For example, in frogs that primarily use their limbs for swimming, the back legs are more muscular while the front legs are short. While salamanders mostly have short stubby legs.
- Cold-blooded organisms are animals that rely on external sources to heat the blood inside of their bodies. Cold-blooded amphibians are classified as both poikilothermic and ectothermic animals. Ectothermic animals have no build-in biological system to regulate their internal temperature and control it via an external heat source like basking in the sun. After gathering enough heat, amphibians will crawl into the shade or dive into the water in the case of most frogs. Poikilothermic means that the animal’s internal temperature varies greatly depending on the environment, but they can survive in more extreme temperatures as well.
- Every amphibian is carnivorous. From frogs to salamanders, they will prey upon other smaller organisms. This can vary from insects, mice, smaller frogs, reptiles, bats, snails, and even birds for frogs. For salamanders, this usually includes grubs, worms, crickets, and flies. Some frogs have tongues that are approximately a third of their body length and can flick them out, hit an insect, and tuck them back within 0,07 seconds. If that’s not enough, insects hit by a frog’s tongue experience 12 times the force of gravity. Salamanders are opportunistic predators running down and biting down on their prey with their proportionally strong jaws and teeth.
- Most amphibians have four legs as adults. Frogs, as a rule, do not have tails, but during their tadpole stage, they have tail appendages to help them swim. For the most part, salamanders have tails and four limbs. The siren species of salamanders are the exception, with only a tail and forelimbs. Different species of salamanders use their tails in various fashions. While some species use it as a weapon of defense or to move forward in small bursts, others use it as a divisionary tactic when confronted by predators. The fantastic thing about salamanders is that they can regenerate their limbs after losing them.
- All amphibians have what is known as primitive lungs. This means that during their metamorphosis from larvae, egg, or tadpole to adult, they have to develop their lungs as well. The lungs function as gills when underwater and in the larvae or infantile stage and grow to breathe oxygen through the skin when reaching the adult stage. There are some exceptions, such as axolotl, a type of salamander that retains its gills. For amphibians to breathe, however, their skin needs to remain moist. This type of breathing is called cutaneous respiration.
- Amphibians mostly reproduce using external egg fertilization. This means that some salamanders and most frogs usually lay their eggs in strings or clusters of gelatinous jelly-like substances and have the males fertilize them afterward. The majority of salamanders engage in internal fertilization, where the male deposits a spermatophore or a small packet of sperm on a gelatinous cone or a piece of land or in the water. The female then takes the packet of sperm with the lips of her cloaca, or her posterior orifice pushes it inside. The spermatozoa move to the cloaca’s roof, where it remains until the female begins to ovulate.
- Metamorphosis is a part of most amphibia. This means that most amphibians start their life as eggs, hatch, become larvae, and then grow into adults. Many factors play a role in the speed and growth of the larvae. For frogs, the larvae are called tadpoles. They develop from round, oval organisms with tails into miniaturized adult forms. Some frogs and salamanders have direct development, where the larvae grow to miniaturized adults inside the eggs. In the case of salamanders, the eggs hatch into larvae with semi-developed hind limbs and appendages next to their heads that keep the gills clean. They then develop front limbs and turn into adults.
Different Kinds of Amphibians
There are three main different types of amphibians. The Anura or Salientia class includes frogs and toads, the biggest group of amphibians.
Apoda is the smallest order of amphibians. They are the exception to some of the biological and physiological features typically associated with amphibians in general. These amphibians do not develop limbs and resemble worms. They are found in many wet tropical regions such as Southeast Asia, India, Bangladesh, West and East Africa, Central America, and South America.
They primarily live in the ground or the substrates of streams. Because they burrow into the ground, they have strong skulls with elongated snouts. Some species are viviparous, which means they give birth to already developed offspring.
Urodela is the class to which newts and salamanders belong. They have tails, as we mentioned before, and they generally have fewer bones in their skulls. They also have the fantastic ability to regenerate any limb.
Difference Between Semi-Aquatic Animals & Amphibians
As we mentioned before, people tend to mistake semi-aquatic animals for amphibians. This happens because these animals mimic and share some biological features with amphibians. Below we will list the differences between them and which animals are often mistaken for as amphibians.
Semi-aquatic is an inclusive term for animals and organisms that spend parts of their lives in water and on land, just like amphibians. However, they are not limited to vertebrates alone.
- They can be Arthropods, which are invertebrate animals that have exoskeletons and segmented bodies. Examples of these types of animals are lobsters, crabs, or even barnacles. Although some arthropods do go through metamorphosis, they are not considered amphibious because they are not vertebrates and tend to have more than four legs.
Here is an article I wrote about why lobsters cannot survive on land that will help you understand more interesting facts about them.
- Mollusks are another type of semi-aquatic animal confused with amphibians. They include snails, sea slugs, chitons, and saltwater clams. For the same reasons as arthropods, these creatures cannot be classified as amphibious. Namely, they are not vertebrates and do not have four limbs. Not all mollusks are carnivorous, which is another big difference.
- Mammals are probably one of the most prominent groups that are incorrectly identified or mistaken as amphibious. Otters, beavers, platypi, and hippopotami are mistakenly thought of as amphibious because they spend a lot of time in both water and land. Although they are vertebrates they do not go through metamorphosis, and they are fertilized internally.
- Reptiles are probably the group of animals that are the most mistaken as amphibious. This is because all reptiles are cold-blooded, a trait they share with amphibians. Freshwater turtles and certain semi-aquatic snakes like the Brazilian anaconda are often mistaken as amphibious. However, like mammals, reptiles do not go through metamorphosis, and many species are internally fertilized, unlike amphibians.
All amphibians are coldblooded and can live on land and in water. However, they are defined by much more than these facts. The easiest way to determine if an animal is amphibious is to observe whether they are: carnivorous, four-legged vertebrates that are coldblooded, go through metamorphosis, have primitive lungs, live on both land and in water, and that have their eggs externally fertilized.