Amphibians are vertebrate animals characterized by having bare, permeable skin without scales.
In this article we will explain all the secrets of these animals, starting with amphibian reproduction , the types of amphibians that exist, some examples and other interesting facts that you will surely find very useful.
Reproduction of amphibians
Being oviparous, amphibians reproduce by laying eggs. Reptiles and mammals reproduce through internal fertilization (inside the female), while in most amphibians, especially anurans (frogs and toads), external fertilization predominates.
Amphibian fertilization typically occurs in freshwater because this type of water protects the eggs during their development and compensates for their lack of a shell and appendages like the amnion. Therefore, the eggs must remain in humid environments to prevent them from drying out, which explains the group's permanent connection to water.
Fertilization in anurans, being external, follows a characteristic process: the male grasps the female by amplexus , and as she releases the eggs, the male fertilizes them with his sperm . The eggs remain in the water, forming clusters or strings, or attached to vegetation . Aquatic larvae emerge from them.
In both fish and amphibians, where external fertilization predominates, the eggs have a thin, gelatinous covering to allow the sperm to pass through. Therefore, they are usually deposited in the water, stuck together and forming large clusters or strings , depending on the species.
Amphibians are born as aquatic larvae that move with a tail and breathe through gills . When the larva, called a tadpole, has grown large enough, it undergoes a profound metamorphosis . Except for some species of rainforest frogs that exhibit direct development , these larval characteristics disappear, and lungs and limbs develop as the tadpoles grow.
This class of amphibian vertebrates includes frogs, toads, salamanders, and caecilians . They have the ability to live both in and out of water, although they need humid environments , since their skin plays an essential role in respiration and water balance.
External fertilization, internal fertilization and amplexuses
In anurans, the general rule is external fertilization with amplexus, but in urodeles (newts and salamanders) internal fertilization is common , using a spermatophore deposited by the male and collected by the female with her cloaca. Amplexus also varies among species: it can be axillary (the male grasps the female behind her front legs), inguinal (at the hips), or cephalic . This diversity of behaviors improves reproductive efficiency in different habitats.
Why is water essential for reproduction?
Amphibian eggs do not have a waterproof shell like those of reptiles and birds; their outer covering is gelatinous and permeable , so they lose water easily. An aquatic environment or environments with constant humidity are essential to prevent desiccation. In addition, water provides dissolved oxygen and buffers against sudden temperature changes during embryonic development.
Many species have evolved strategies to minimize risks such as predation or desiccation: laying eggs in foam or bubble nests, eggs on hanging leaves that fall into the water when they hatch, underwater chambers excavated by the parents, or even choosing microhabitats such as bromeliads that accumulate water in their leaves.
Diversity of reproductive modes in amphibians
Amphibian reproduction is extraordinarily varied . Recent research has proposed classification systems that consider dozens of reproductive modes based on traits such as the spawning site (running water, ponds, bromeliads, damp soil), the presence or absence of metamorphosis before or after hatching, or the existence of structures such as foam nests . In total, more than seventy variants have been described among the different orders.
Some illustrative examples: in certain frogs of the Pipidae family , females carry the eggs on their backs until they hatch; in tree frogs of humid forests, eggs are laid in bromeliads or bamboo cavities , where the larvae complete their development; some species build underwater chambers to deposit and protect their eggs; others exhibit direct development (the young are born as miniature adults), common in some leaf-litter frogs . Even in caecilians, unique maternal behaviors have been observed in which the young feed on the mother's nutrient-rich skin.
This diversity also includes plasticity within a single species: some frogs can choose between different nesting sites (ponds, damp soil, or bromeliads) depending on local conditions. Such strategies increase their chances of success in the face of environmental changes, while other species are strictly dependent on a single method.
In urodeles, in addition to internal fertilization , some salamanders retain the eggs in the oviduct until they give birth to fully formed live young , and in a number of tropical amphibians reproduction occurs on land with prolonged parental care to maintain moisture and avoid predators.
Amphibian animals, what are they?
In Latin, the word amphibian has a peculiar meaning; it literally refers to "two lives." This is a distinctive characteristic of these animals, capable of adapting and carrying out their biological functions in two different ecosystems : land and water. However, let's delve a little deeper into the meaning of amphibian.
Amphibians are part of that large family of living beings classified as vertebrates (they have an internal skeleton), anamniotes (their embryo develops without amnion or allantois, so they need external moisture ), tetrapods (with four limbs, ambulatory or manipulative) and ectothermics (their body temperature depends on the environment).
They undergo a period called metamorphosis (a transformation they experience during their development that affects their morphology, functions, and lifestyle). Among the notable changes are the transition from gills to lungs , as well as the development of limbs and the reabsorption of the tail in most anurans.
Types of amphibians
Triton
Within this large family that makes up amphibians, we can make a small classification based on three orders: anurans , caudates or urodeles and apoda or gymnophiona.
Anurans include all those amphibians commonly known as frogs and toads. But be careful: frogs and toads are not the same species. They are grouped together due to their morphological and behavioral similarities, but they exhibit great internal diversity . They typically lack a tail in adulthood, have powerful hind legs for jumping , and generally reproduce externally.
Urodeles are distinguished by their long tails and elongated bodies. Their eyes are not particularly well-developed and may be covered by a thin layer of skin. This group includes newts , salamanders , olms , and mermaids . In many cases, they exhibit internal fertilization and locomotion based on undulations and quadrupedal walking.
Finally, there are the legless amphibians , which are the most peculiar of all due to their appearance. They resemble worms or earthworms because they lack limbs and have elongated, cylindrical bodies. Most live underground and possess senses adapted to this environment.
Amphibian characteristics
As we said, amphibians are vertebrate animals and have the "privilege" of being the most primitive among the tetrapods that inhabit the Earth, with a lineage that goes back hundreds of millions of years.
They have four limbs: two front and two hind. These limbs are known as chiridia , with a morphology similar to the human hand: four fingers on the front feet and five on the hind feet, although there are variations depending on the group. Many amphibians also have a functional tail (especially salamanders).
Being cold-blooded (ectotherms), their body temperature depends on the environment, since they cannot self-regulate their heat . This condition drives their activity during humid or nighttime hours and their close association with bodies of water.
They are oviparous , meaning they hatch from eggs. The female is responsible for laying them, always in an aquatic or very humid environment. Therefore, young specimens breathe through gills and, in the case of anurans, go through an aquatic larval stage.
The skin is permeable and can be penetrated by molecules and gases. Many species secrete toxic substances as a defense mechanism, and the skin actively participates in cutaneous respiration and water balance. Its moist, scaleless nature allows it to absorb water and oxygen, but also makes it vulnerable to dehydration when ambient humidity is low.
The circulatory system has a three-chambered heart composed of two atria and one ventricle, with closed, double, and incomplete circulation . This design allows for the partial separation of oxygenated blood from deoxygenated blood.
The eyes are usually large and bulging , providing a wide field of vision ideal for hunting. Although it may not seem so, many amphibians do have teeth (especially small ones in the jaws), useful for holding prey. Their sticky tongue , particularly in frogs and toads, is a key tool for capture. The digestive system includes a tubular stomach , intestine, two kidneys, and a urinary bladder.
Skin, chromatophores and poisons
Amphibian skin contains mucous and even granular glands that secrete defensive substances. Chromatophores (pigment cells) produce colors for camouflage or warning (aposematism). Toxic skin compounds help deter predators and can inhibit harmful fungi and bacteria on the skin.
Differences between amphibians and reptiles
- Eggs: in reptiles they have a waterproof shell; in amphibians they are soft and gelatinous.
- Hatchlings: Reptiles are born in the form of adult miniatures; in amphibians there are metamorphosis from larva to adult.
- Fertilization: reptiles, intern; amphibians, generally external in anurans and intern in urodeles.
- Skin: amphibians, moist and permeable; reptiles, dry and waterproof, covered in scales.
- Breathing: reptiles, pulmonary; amphibians, branchial, pulmonary and cutaneous according to phase.
- Activity: amphibians often nocturnal and dependent on humidity; many reptiles, more daytime.
Examples of amphibians
Salamander
Currently, around 3.500 amphibian species are cataloged . However, scientists estimate that the total number may be around 6.400 or more, given the ongoing description of new species.
When thinking about amphibians, a frog or a toad usually comes to mind , but we also find newts and salamanders , as well as the discreet caecilians.
These are just a few examples of amphibians, although, logically, there are many more :
Anderson salamander (Ambystoma andersoni)
This type of salamander is also known as the axolotl or Purépecha achoque. It is an endemic species, meaning it exists only in a specific location. In this case, it inhabits only the Zacapu Lagoon, located in the state of Michoacán, Mexico. It is characterized by a robust body , short tail, and external gills . Its orange or red color , along with black spots along its body, makes it unmistakable.
Marbled Newt (Triturus marmoratus)
It is found mainly in European territory, in northern Spain and eastern France. It has a striking mottled greenish coloration and a reddish dorsal line running along its back, a characteristic feature of the group.
Common toad (Bufo bufo)
Common throughout much of Europe and Asia. It prefers habitats with stagnant water , irrigated areas, etc. Its tolerance of poor water quality has facilitated its spread. It has a brownish coloration and skin with noticeable warts .
Vermilion frog (Rana temporaria)
Widespread across Europe and Asia. Although it prefers damp places , it spends much of its time on land. It exhibits variation in color pattern , tending towards brown tones and spots, and has a very distinctive pointed snout.
Other interesting examples
- Midwife toad (Alytes spp.): the male carries the laying coiled around its hind legs until it is released into the water when the embryos are ready.
- Saint Anthony's Frog (Hyla spp.): arboreal, with adhesive discs on the fingers and high-pitched edge; typical of temporary ponds.
- Axolotl (Ambystoma mexicanum): example of neoteny, maintains larval features (external gills) in the adult state.
- Proteus (Proteus anguinus): European cave-dwelling urodele, underground life completely aquatic and vestigial eyes.
- cecilias (gymnophiona): legless amphibians of life fossil, often with reproduction Earth and parental care.
Metamorphosis and the life cycle
Metamorphosis is a key biological process in the life cycle of amphibians. The typical cycle comprises three stages : embryonic (egg), larval (tadpole or gill larva) , and adult. The duration and characteristics vary among species and environments.
- Egg: After fertilization, the embryo grows inside a gelatinous casing permeable, absorbing water and swells, agglutinating the eggs into masses or cords.
- Larva: feeds, grows and breathes by gillsIn anurans, it develops hind legs, then anterior legs, reduces the tail and restructures its digestive system towards a carnivorous diet.
- Adult: breathe for lungs and skin, disperses, looks for a mate and repeats the cycle. In urodeles, metamorphosis can be incomplete or even absent (neoteny) if the aquatic environment is stable and favorable.
There are species with direct development in which there is no external aquatic larval stage: the embryos complete development inside the egg and the young are born as miniature adults , a strategy that reduces dependence on open bodies of water.
The song of the amphibians
In anurans, song is an essential communication tool. Males vocalize to attract females , mark their territory , and coordinate reproduction. Each species has its own unique acoustic pattern that facilitates species recognition, and in many frogs, a vocal sac acts as a resonator.
Females usually position themselves on the periphery of the ponds , listening to the males and choosing a mate based on the quality of their song (rhythm, intensity, frequency), a trait linked to physical condition and the ability to defend the territory.
Feeding of amphibians
Adult amphibians are predominantly carnivorous . They consume live prey such as arthropods (insects, arachnids), earthworms , snails , and small vertebrates if size permits. The protrusible, sticky tongue of many frogs and toads allows for lightning-fast captures.
Amphibian larvae exhibit diverse diets: some are herbivores or filter-feeding detritivores, while others are carnivores . Cannibalism among tadpoles is not uncommon , especially when resources are scarce or population density is high.
Conservation status and threats
Amphibians are among the most threatened groups on the planet; the threat to the American bullfrog exemplifies how one species can disrupt ecosystems and impact the reproduction of other populations. Habitat loss and fragmentation , water pollution, climate change , and emerging diseases (such as chytrid fungus) have triggered population declines in numerous regions.
Many reproductive modes depend on very specific microhabitats (temporary ponds, bromeliads, shaded streams). The disappearance of a pond or a decrease in local humidity can disrupt the reproduction of entire populations. Species with reproductive plasticity can adapt better, choosing different spawning sites depending on the conditions, while specialists are more vulnerable.
Conservation efforts include protecting wetlands , restoring ponds, controlling invasive species, monitoring health, and breeding and reintroduction programs for critical species. Environmental education and citizen science are also essential for detecting and mitigating threats early.
Origin and evolution of amphibians
The amphibian lineage originated from de peces freshwater with fins lobed, capable of supporting their weight and moving on shallow substrates. Over time, these fins gave rise to limbs with fingers, paving the way for the colonization of subaerial environments. Among the earliest tetrapods, forms such as Acanthostega o ichthyostega They show intermediate features between aquatic and terrestrial life.
The transition from water to land is linked to the exploration of new resources and the response to changing environmental conditions (droughts, variations in bodies of water). Skin and cutaneous respiration, metamorphosis , and ectothermic physiology were key to expanding the presence of amphibians across much of the planet , except for extremely cold or arid regions.
After learning how they reproduce, what types exist, and what their most important characteristics are, it becomes clear why amphibians are such a versatile and fascinating group : their dual connection to water and land, the diversity of their reproductive strategies , metamorphosis, and the sensitivity of their skin make them early indicators of ecosystem health and indispensable players in global biodiversity.