AMPHIBIANS: CHARACTERISTICS, TYPES, SYSTEMS, REPRODUCTION - BIOLOGY - 2025

The amphibians are a class of vertebrates that do not have the ability to regulate body temperature. The class is comprised of nearly 6,000 species of frogs, toads, salamanders, and caecilians. This last group is not popularly known and they are amphibians similar to a snake, since they have degenerated their limbs.

The term “amphibian” refers to one of the most outstanding characteristics of the group: its two ways of life. Amphibians generally have an aquatic larval stage and a terrestrial adult.

An anuran. Source: pixabay.com

Therefore, its reproduction is still linked to bodies of water. The reproductive event results in an egg that lacks the amniotic membranes, so it must be deposited in ponds or in humid environments. Frogs have external fertilization, and salamanders - and probably caecilians - have internal fertilization.

Amphibian skin is very thin, moist, and glandular. Some species have modifications for the production of venom to defend themselves against potential predators. Although some species have lungs, in others they have been lost and respiration occurs entirely through the skin.

We find amphibians in a wide diversity of ecosystems, across tropical and temperate regions (with the exception of oceanic islands).

Herpetology is the branch of zoology in charge of the study of amphibians - and also reptiles. The professional who develops professionally in this science is known as a herpetologist.

characteristics

Bone characteristics

Amphibians are vertebrate, tetrapod and ancestrally quadruped animals. Its skeleton is composed mainly of bone and the number of vertebrae is variable. Some species have ribs that may or may not be fused to the vertebrae.

The skull of salamanders and frogs is generally open and delicate. In contrast, caecilians exhibit enormous compaction in their skull, transforming it into a heavy and solid structure.

General morphology of the body

The morphology of the body presents three basic dispositions, which correspond to the taxonomic ordering of the class: the fused body, plump, without neck and modified forelimbs for the jumping of frogs; the graceful build with a defined neck, long tail and limbs of equal size to salamanders; and the elongated, limbless shape of the caecilians.

Metamorphosis

The life cycle of most amphibians is characterized by being biphasic: an aquatic larva hatches from the egg that transforms into a sexually mature terrestrial individual that lays eggs and thus closes the cycle. Other species have eliminated the aquatic stage.

Skin

Amphibian skin is quite unique. It is characterized by being very fine, humid and with the presence of multiple glands. In species lacking lungs, gas exchange can occur through the skin. There are modifications of structures that release toxic substances to combat predators.

On the leather, the striking colors stand out - or the ability to camouflage. Many of them are intended to alert or hide from predators. In fact, amphibian coloration is more complex than it appears; It is made up of a series of cells that store pigments called chromatophores.

Types (classification)

The class Amphibia is divided into three orders: Order Gymnophiona (Apoda), made up of the caecilians; Order Urodela (Caudata) commonly called salamanders, and Order Anura (Salientia) formed by frogs, toads and related. Below we will describe each of the characteristics of each class:

Order Gymnophiona (Nickname)

Eocaecilia micropodia, an early caecilian from the Lower Jurassic, pencil drawing. Nobu Tamura (http://spinops.blogspot.com)

The gymnofions or caecilians make up an order of 173 species of organisms with a very elongated body, without limbs and with subterranean ways of life.

Superficially, they resemble a worm or a small snake. Its body is covered by small dermal scales and they are nullified. The caecilian skull is significantly ossified. In the few aquatic forms that exist, the ring pattern is not as marked.

Most of the species are found in the tropical forests of South America, buried in the ground. However, they have also been reported in Africa, India and some regions of Asia.

As in most species with subterranean habits, the eyes are very small and in some species they are completely useless.

Order Urodela (Caudata)

Salamander salamander. Source: pixabay.com

The urodelos are made up of almost 600 species of salamanders. These amphibians inhabit diverse environments, both temperate and tropical. They are abundant in North America. From an ecological point of view, salamanders are very diverse; they can be totally aquatic, terrestrial, arboreal, underground, among other environments.

They are characterized by being organisms of small sizes - rarely a specimen exceeds 15 cm. The exception is the giant Japanese salamander, which reaches more than 1.5 meters in length.

The extremities form a right angle with the trunk of the cylindrical and thin body that they possess. The hindquarters and forelimbs are the same size. In some aquatic and subterranean forms, the members have suffered a considerable reduction.

Order Anura (Salientia)

Pelophylax perezi

The Anura order is the most diverse among amphibians, with almost 5,300 species of frogs and toads, divided into 44 families. Unlike salamanders, anurans lack a tail. Only frogs of the genus Ascaphus possess one. The name of the Anura order alludes to this morphological characteristic.

The group's alternative name, Salientia, highlights the group's adaptations for locomotion, by jumping thanks to its powerful hind legs. Their body is plump and they lack a neck.

Is it a frog or a toad?

Sometimes when we see an anuran, we usually wonder if the specimen corresponds to a "toad" or a "frog". Generally, when we speak of a toad we refer to an anuran with leathery skin, prominent warts and a robust body, while a frog is a graceful animal, brightly colored, striking and with glandular skin.

However, this differentiation is merely popular and has no taxonomic value. In other words; there is no taxonomic range called toads or frogs.

Digestive system

1-right atrium, 2-orth artery, 3- eggs, 4- colon, 5-left atrium, 6-ventricle, 7- stomach, 8- left lung, 9- spleen, 10-small intestine. CloacaPicture taken by Jonathan McIntosh

Adaptations of the cephalic region

The tongue of amphibians is protrusible and allows them to catch the small insects that will be their prey. This organ has various glands that produce sticky secretions that seek to ensure the capture of food.

Tadpoles have keratinized structures in the oral region that allow them to scrape off the plant matter they will consume. The arrangement and number of these buccal structures have taxonomic value.

Adaptations of the intestines

Compared to other animals, the digestive tract of amphibians is quite short. Throughout the animal kingdom, a digestive system made up of short intestines is typical of a carnivorous diet, as they are relatively easy to digest food materials.

In larvae, the gastrointestinal system is longer, a characteristic that presumably favors the absorption of plant matter, allowing fermentation.

Diet

Most amphibians have a carnivorous diet. Within the anuran menu we find multiple species of insects, spiders, worms, snails, millipedes and almost any animal that is small enough for the amphibian to consume it without much effort.

Gymnofions feed on small invertebrates that they manage to hunt in underground environments. Salamanders have a carnivorous diet.

In contrast, most of the larval forms of the three orders are herbivorous (although there are exceptions) and feed on the plant matter and algae found in bodies of water.

Circulatory system

Didactic model of an amphibian heart. Wagner Souza e Silva / Museum of Veterinary Anatomy FMVZ USP

Amphibians have a heart with a venous sinus, two atria, a ventricle, and a cone arteriosus.

The circulation is twofold: it passes through the heart, the pulmonary arteries and veins supply the lungs (in the species that possess them), and the oxygenated blood returns to the heart. Amphibian skin is rich in small blood vessels.

1 - The internal gills / point where the blood is reoxygenated. 2 - Point where the blood is depleted of oxygen. 3 - Two-chamber heart. Red - Oxygenated Blood. Blue - blood depleted of oxygen. Opellegrini15

The larval form presents a different circulation than that described for the adult forms. Before metamorphosis, the circulation is similar to that found in fish (remember that larvae have gills and the circulatory system must include them in its path).

In larvae, three of the four arteries that start from the ventral aorta go to the gills, and the remaining one communicates with the lungs in a rudimentary or very underdeveloped state.

Nervous system

The nervous system is made up of the brain and the spinal cord. These structures are derived embryologically from the neural tube. The anterior portion of this structure widens and forms the brain. Compared to the rest of the vertebrates, the amphibian nervous system is quite small, simple and rudimentary.

In amphibians there are 10 pairs of cranial nerves. The brain is elongated (not a round mass as in mammals) and is structurally and functionally divided into an anterior, middle and posterior region.

The brain is similar in all three groups of amphibians. However, it is usually a shorter structure in frogs and more elongated in caecilians and salamanders.

Respiratory system

Frog skin section. A: mucous gland, B: chromophore, C: granular poison gland, D: connective tissue, E: stratum corneum, F: transition zone, G: epidermis and H: dermis. Jon houseman

In amphibians, there are various structures that participate in the respiratory process. The thin, glandular and highly vascularized skin plays an important role in the gas exchange of many species, particularly those lacking lungs.

The lungs in amphibians have a particular mechanism; Unlike the air intake from the lungs of other animals, ventilation occurs by positive pressure. In this system, air is forced into the trachea.

The larval forms - which are aquatic - breathe through gills. These external respiratory organs achieve efficient extraction of dissolved oxygen in the water and mediate exchange with carbon dioxide. There are salamanders that can have only gills, only lungs, both structures, or neither.

Some species of salamanders that live their entire adult lives in bodies of water have the ability to evade metamorphosis and preserve their gills. In evolutionary biology, the phenomenon of preserving a childlike appearance in adult and sexually mature forms is called pedomorphosis.

One of the best known representatives of salamanders that manages to preserve the gills in their adult state is the axolotl or Ambystoma mexicanum.

Vocalization

When we think of frogs and toads it is almost impossible not to allude to their nocturnal songs.

The vocalization system in amphibians is of great importance in anurans, since the songs are an important factor in the recognition of the pair and in the defense of the territory. This system is much more developed in males than in females.

The vocal cords are located in the larynx. The anuran is capable of producing sound thanks to the passage of air through the vocal cords, between the lungs and through a pair of large sacs located on the floor of the mouth. All these structures mentioned are in charge of orchestrating the production of sound and the songs so particular to the group.

Excretory system

The excretory system of amphibians consists of kidneys of the mesonephric or opisthephric type, the latter being the most common. The kidneys are organs in charge of removing nitrogenous waste from the bloodstream and maintaining a water balance.

In modern amphibians, a holonephric kidney exists in embryonic stages, but never becomes functional. The main nitrogen waste is urea.

Reproduction and development

Given their inability to regulate body temperature, amphibians seek to reproduce at times of the year when the temperature of the environment is high. Since the reproductive strategies of the three orders are so disparate, we will describe them separately:

Gymnofions

The literature related to the reproductive biology of this order of amphibians is not particularly rich. Fertilization is internal and males have a copulatory organ.

Generally, the eggs are deposited in humid places with nearby bodies of water. Some species have the typical aquatic larvae of amphibians, while in others the larval stage occurs inside the egg.

In certain species, the parents exhibit the behavior of sheltering the eggs in the folds of their body. A significant number of caecilians are viviparous, being a common event within the group. In these cases, the embryos feed on the walls of the oviduct.

Urodelos

The eggs of most salamanders are internally fertilized. Female individuals are capable of taking structures called spermatophores (a packet of sperm produced by a male).

These spermatophores are deposited on the surface of a leaf or a trunk. Aquatic species lay their eggs in clusters in the water.

Anurans

Series of images of tadpoles (common toad - Bufo bufo). The images show the last two weeks of larval development ending with metamorphosis. Bufo_metamorphosis.jpg: CLauterderivative work: Cwmhiraeth

In anurans, males attract females through their melodic (and species-specific) songs. When the couple goes to copulate, they engage in a kind of "hug" called the amplexus.

As the female lays her eggs, the male discharges the sperm onto these gametes to fertilize them. The only exception to the external fertilization event in anurans are organisms of the genus Ascaphus.

The eggs are laid in humid environments or in the body of water directly. They agglomerate in masses with multiple eggs and can anchor themselves to patches of vegetation. The fertilized egg develops rapidly, and a small aquatic tadpole emerges when ready.

This little tadpole will undergo a dramatic change event: metamorphosis. One of the first modifications is the development of the hind limbs, the tail that allows them to swim is reabsorbed - like the gills, the intestine shortens, the lungs develop and the mouth takes on adult characteristics.

The time frame of development is highly variable among amphibian species. Certain species are capable of completing their metamorphosis in three months, while others take up to three years to complete the transformation.

Evolution and phylogeny

Artistic representation of Triadobatrachus massinoti. Pavel.Riha.CB

The evolutionary reconstruction of this group of tetrapods has experienced several difficulties. The most obvious is the discontinuity of the fossil record. Furthermore, the methods used to reconstruct phylogenetic relationships are constantly changing.

Living amphibians are descendants of the first terrestrial tetrapods. These ancestors were the lobe-finned fish (Sarcopterygii), a very particular group of bony fish.

These fish appear when the Devonian period was ending, about 400 million years ago. The group experienced adaptive radiation to both fresh and salt water bodies.

The earliest tetrapods retained a lateral line system in their juvenile forms, but absent in adults. The same pattern is seen in modern amphibians.

Amphibians have been a group that have successfully exploited a wide diversity of terrestrial environments, which are linked to bodies of water.

First tetrapods

There are a number of fossils that are key to the evolution of tetrapods, including Elginerpeton, Ventastega, Acanthostega, and Ichthyostega. These now extinct organisms were characterized by being aquatic - a trait that is inferred from the anatomy of their body - and by having four limbs.

Members of the genus Acanthostega were organisms that had formed limbs, but these structures were so weak that it is unlikely that the animals had the ability to walk freely out of the water.

In contrast, the genus Ichthyostega featured all four limbs and, according to evidence, could stay out of the water - albeit with awkward gait. A striking characteristic of both genders is the presence of more than five digits in both the hindlimbs and the forelimbs.

At one point in the evolution of tetrapods, pentadactyly was a characteristic that was fixed and remained constant in the vast majority of tetrapods.

Phylogenetic relationships between current groups

The relationships between the three current amphibian groups remain controversial. Tentatively modern groups (modern amphibians are grouped under the name of lisanfibios or Lissamphibia) together with extinct lineages are grouped into a larger group called temnospondyls (Temnospondyli).

Most molecular and paleontological evidence supports the phylogenetic hypothesis that groups anurans and salamanders as sister groups, leaving the caecilians as a more distant group. We emphasize the existence of several studies that support this phylogenetic relationship (for more information see Zardoya & Meyer, 2001).

In contrast, using ribosomal RNA as a molecular marker, an alternative hypothesis was obtained. These new studies designate caecilians as the sister group to salamanders, leaving frogs as a distant group.

Current state of conservation

Today amphibians are exposed to different factors that negatively affect populations. According to recent estimates, the number of amphibians that is threatened with extinction is at least a third of all known species.

This number far exceeds the proportions of threatened species of birds and mammals.

Although it has not been possible to pinpoint a single cause that is directly related to the massive decline in amphibians, scientists propose that the most important are:

Habitat destruction and climate change

The main forces that threaten amphibians include: habitat degradation and loss, and global warming. Since amphibians have very thin skin and are so dependent on bodies of water, fluctuations in temperature and stages of drought affect them considerably.

The increase in temperature and the decrease in pools available for egg laying seems to be an important factor in this phenomenon of local extinction and a very marked population decline.

Chytridiomycosis

The rapid spread of the infectious disease chytridiomycosis, caused by the fungus of the species Batrachochytrium dendrobatidis, greatly affects amphibians.

The fungus is so harmful because it attacks a very important aspect of the amphibian's anatomy: its skin. The fungus damages this structure, which is essential for thermoregulation and for the accumulation of water.

Chytridiomycosis has caused huge declines in amphibian populations in large geographic regions, including North America, Central America, South America, and in localized areas of Australia. Until now, science does not have an effective treatment that allows to eliminate the fungus of the species.

Introduction of exotic species

The introduction of species in certain regions has contributed to the decline of populations. Many times, the introduction of exotic amphibians negatively affects the conservation of endemic amphibians in the area.

References

1. Divers, SJ, & Stahl, SJ (Eds.). (2018). Mader's Reptile and Amphibian Medicine and Surgery-E-Book. Elsevier Health Sciences.
2. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology. McGraw – Hill.
3. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution. McGraw-Hill.
4. Llosa, ZB (2003). General zoology. EUNED.
5. Vitt, LJ, & Caldwell, JP (2013). Herpetology: an introductory biology of amphibians and reptiles. Academic press.
6. Zardoya, R., & Meyer, A. (2001). On the origin of and phylogenetic relationships among living amphibians. Proceedings of the National Academy of Sciences of the United States of America, 98 (13), 7380-3.