Arxiu d'etiquetes: lungs

How to breathe without lungs, lissamphibian style

Even though most terrestrial vertebrates depend on lungs for breathing, lissamphibians also present cutaneous respiration, they breathe through their skin. Even if this may seem a handicap, because they must always keep their skin moist enough, in this entry we’ll see the many benefits that cutaneous respiration gives them and how in some groups, it has completely replaced pulmonary respiration.


Terrestrial vertebrates use lungs to perform gas exchange. While our aquatic ancestors breathed using gills, these are of no use on land, as gravity would collapse them and cause them to lose their form. As lungs are found inside the body, they can keep their form in a habitat with much higher gravity. Both gills and lungs have highly branched structures to increase their diffusion surface, and this way facilitate gas exchange (in a larger surface there’s more exchange).

Giant_Mudskipper_(Periophthalmodon_schlosseri)_(15184970133)Specimen of giant mudskipper (Periophthalmodon schlosseri), a fish from southeast Asia which is able to get out of water due, in part, to cutaneous respiration. Photo by Bernard Dupont.

We can find a third form of gas exchange in vertebrates. Even if it’s not as widespread as gills or lungs, cutaneous respiration is found in several groups of animals, such as lunged fish and some marine reptiles (turtles and sea snakes). Yet the lissamphibians are the group that has brought their specialization in cutaneous respiration to the ultimate level.


Present day lissamphibians are the group of tetrapods with the highest diversity of breathing strategies. Apart from cutaneous respiration present in all species, most lissamphibians are born in an aquatic larval stage with gills. After metamorphosis they develop lungs to breathe on land.

The larvae of urodeles and apods present external, filamentous and highly branched gills which allow them to breathe underwater. These must be constantly moved for gas exchange to occur. Some neotenic salamanders maintain their gills during adulthood. On the other hand, anuran tadpoles present internal gills covered by gill pouches.

Salamander_larva_closeupPortrait of a salamander larva in which the branched filamentous gills can be appreciated. Photo by Brian Gratwicke.

Most terrestrial lissamphibians present a pair of simple lungs with few ramifications and large alveoli. These have a low gas diffusion rate compared with amniote’s lungs. Also, while amniotes ventilate their lungs using the expansion of the thoracic cavity and the diaphragm, lissamphibians must force the air to their lungs using a buccal-pump system.

Four_stroke_buccal_pumpingScheme of the system of pulmonary respiration of lissamphibians. In the buccal-pump system, the buccal cavity is filled with air and then, elevating the mouth floor, this air is forced to the lungs. Image by Mokele.

Apart from gill and pulmonary breathing, lissamphibians take oxygen to their blood by cutaneous respiration. The skin of lissamphibians is very thin and has a high concentration of capillaries (it’s got a great number of blood vessels). As a result, it has a great capacity of diffusion of gas molecules, allowing cutaneous respiration using a countercurrent system.

600px-ExchangerflowModified scheme of a countercurrent exchange system. In this, deoxygenated blood (with CO2) circulates in the opposite direction that air does (full of O2) and between both fluids the gas interchange happens, in an attempt to equalize the concentration of both gases. Modified image by Joe.

Lissamphibian skin is different from that of amniotes in that it doesn’t present scales, feathers or fur. This makes lissamphibian skin much more permeable to both gases and water (which makes them great bioindicators of the health of their environment, as their skin takes up many different kinds of soluble substances). That’s why lissamphibians must keep their skin relatively moist for the gas exchange to take place.

KammolchmaennchenMale northern crested newt (Triturus cristatus) in its nuptial phase. Its wide tail crests increase the surface of skin also increasing gas diffusion. Photo by Rainer Theuer.

Lissamphibians live constantly in a delicate equilibrium in which the skin must be kept moist enough to allow gas exchange, but not too permeable as to lose water, dehydrate and die. They acheive this living in wet environments, or creating layers of moist skin to create an aqueous ambient around them.

Bombay_caecilianPhoto of a Bombay caecilian (Ichthyophis bombayensis) a lissamphibian which lives in swamps and other humid habitats. Photo by Uajith.

Many lissamphibians present a large quantity of skin, which increase the respiratory surface. Some examples are the vascular papillae of the hairy frog (Trichobatrachus robustus), the skin folds of the frogs of the Telmatobius genus or the wide caudal fins of many newts.

TrichobatrachusGreenDrawing of the hairy frog (Trichobatrachus robustus) where the papillae which gives it its name can be seen. Image extracted from Proceedings of the Zoological Society of London (1901).

Even though most frogs get most of their oxygen from their lungs during summer, during the colder months (when their metabolism is slower) many species hibernate at the bottom of frozen lakes, conducting their gas exchange solely through their skin.

6887057816_d68fccf4f4_oMany subarctic lissamphibians hibernate underwater, using their skin to extract oxygen from water and expel carbon dioxide from blood. Photo by Ano Lobb.

Adult urodeles present a much higher diversity of breathing strategies, and among them there is one family that is the only group of terrestrial vertebrates that has no trace of lungs.


Inside the suborder of the salamandroideans we find the Plethodontidae family. These animals are popularly called lungless salamanders because, as their name implies, they have no lungs and depend exclusively on their skin to conduct gas exchange.

Kaldari_Batrachoseps_attenuatus_02California slender salamander (Batrachoseps attenuatus) photographed by Kaldari. This is a perfect example of the long and thin bodies of plethodontids which facilitate gas diffusion.

These urodeles are distributed mainly through the Americas, with some species in the island of Sardinia and the Korean Peninsula. The most surprising fact about plethodontids is that, like most salamandroids, they are mainly terrestrial animals and do not present an aquatic larval stage. Even though some species present gills during their embryonic development, these are lost before hatching and lungs are never developed.

Northern_red_salamander_(Pseudotriton_ruber)Photo of a red salamander (Pseudotriton ruber) a plethodontid endemic from the Atlantic coast of the USA. Photo by Leif Van Laar.

It is believed that this family evolved in fast-flowing mountain streams. The presence of lungs would have made them float too much, and this would have made moving much more difficult in such habitats. The cold waters of alpine rivers are rich in oxygen, making cutaneous respiration more than enough for these small animals.

Video by Verticalground100 in which we can see some plethodontid species.

A thin and vascularized skin (facilitates diffusion) and the evolution of long and slender bodies (facilitates the transport of O2 through all the body) made lungs useless for plethodontids. Currently, lungless salamander are the most numerous of all urodele families, and they represent more than half the animal biomass in many North American ecosystems. Also, they are much more active than most lissamphibians, with highly developed nervous and sensory systems, being voracious predators of arthropods and other invertebrates.

3679651745_d678454a1b_oOzark zigzag salamander (Plethodon angusticlavius) a curious lungless salamander common in the state of Missouri. Image by Marshal Hedin.

As you can see lissamphibian cutaneous respiration allows them to make things few tetrapods are able to do. Passing a whole winter underwater and living on land without lungs are some of the incredible feats reserved to a small group of animals. Maybe lissamphibians still depend on the aquatic medium to survive, but as we have seen, they are far from being slow or primitive, as they present some of the most impressive physiological adaptations found on the animal kingdom.


The next sources have been consulted during the elaboration of this entry:


How do fish breathe?

It’s probable that you know that most of the fishes that inhabit in the Earth breathe due to the presence of gills. However, this is not the only respiratory system present in fishes. In this post, we will review different types of breathing in fishes. 


The respiratory system of fishes have to be adapted to two important limitations of underwater life. On the one hand, the amount of dissolved oxygen is smaller in the water than in the air: at 23ºC, air has 210 ml of oxygen per litre of air, while in freshwater is about 6,6 ml/l and in salt water is 5,3 ml/l. On the other hand, water is much more dense and viscous than air. These limitations explain the adaptations in the breathing of this group of animals.


The oral cavity of teleostei fishes (modern ray-finned fishes) is communicated with the exterior through the mouth and pharyngeal pouches, lateral openings present in the pharynx in which the gills develop. Thanks to the opercle (or gill cover), a hard structure placed in each side of the head, gills are protected.

The structure of the gills is complex. From branchial arches, curved structures that pierce through pharyngeal pouches in each side of the head, two gill filaments grow forming a V. These filaments produce the gill lamellas, folds of the wall’s filaments with a perpendicular disposition. In each side of the filament, we may find between 10 to 40 lamellas per mm. So, it is in these lamellas where the gas exchange happens because they are a very thin wall of tissue and are well supplied with blood.

Structure of the fishes' gills (Picture: AS Biology Ms Timms).
Structure of the fishes’ gills (Picture: AS Biology Ms Timms).

So, the oxygenated water that passes through the mouth cross the gills and finally abandon the oral cavity through the opercle, while the blood flows in the opposite direction across lamellas to catch the oxygen.

The larva of many fishes have external gills in each side of the head. In the rest of phases, gills become internal. Fishes with a respiration with gills are hagfishes, lampreys, elasmobranchii and bony fishes.

Les mixines són peixos amb respiració branquial (Foto: Natureduca).
The hagfish is a species with a breathing with gills (Picture: Natureduca).


About 400 bony fish species are known to have the ability of breathing from air, most of them living in freshwater ecosystems. Anyway, most of them have both gills and lungs. These species with the two mechanisms usually use the air in certain occasions:

  • When the oxygen level in the water goes down.
  • When the temperature increases, so the higher the temperature is, the higher are the oxygen necessities.

Lungfishes (Dipnoi) are among the species with the most advanced system. Their lungs have crests and septums similar to those in the lungs of amphibians. The Australian lungfish (Neoceratodus) can breathe with both gills and a lung. African lungfishes (Protopterus) and the South American lungfish (Lepidosiren) breathe with a complex lung and single gills. These fishes need to compulsorily breathe air, as in the contrary they die.

Peixos pulmonats: Peix pulmonat australià (Neoceratodus forsteri), africà (Protepterus annectens) i sud-americà (Lepidosiren paradoxa) (Foto: Encyclopaedia Britannica).
Lungfishes: Australian lungfish (Neoceratodus forsteri), African lungfish (Protepterus annectens), South American lungfish (Lepidosiren paradoxa) and Devonian lungfish (Dipterus) (Picture: Encyclopaedia Britannica).


Many fishes have the capacity of breathing through the skin, specially when they are born because they are so small that they do not have specialised organs. As the animal is growing, gills or/and lungs are developing because the diffusion through the skin is not enough. Anyway, skin may be responsible of a 20% or more of gas exchange in some adult individuals. Others can do it through the mouth, the pharynx, the oesophagus, the intestine or the rectum, as is the case of Hoplosternum.

El peix Hoplosternum té la capacitat de respirar a través del tub digestiu (Foto: Free Pet Wallpapers).
Hoplosternum has the ability of breathing through the digestive tube (Picture: Free Pet Wallpapers).

Some species have developed cavities beyond the gills, the suprabranchial chambers, which can be filled of air. In other, complex organs developed from a very irrigated branchial arch can be formed and act as a lung. This is the case of the catfish and Electrophorus .

Some fishes have the ability to breathe air without a specific adaptation. This is the case of the American eel (Anguilla rostrata), that cover the 60% of the oxygen requests through the skin and the 40% swallowing air from the atmosphere.


  • Notes of the subject Chordates of the Degree in Biology (University of Barcelona).
  • Hickman, Roberts, Larson, l’Anson & Eisenhour (2006). Principios integrales de Zoología. Ed. McGraw Hill (13 ed)
  • Hill, Wyse & Anderson (2006). Fisiología animal. Ed. Medica Panamericana


The evolution of amphibians: the conquest of the land

Amphibians were the first group of vertebrates to develop limbs and to be able to leave the water to conquer the land. Even if they are seen as simple and primitive animals by most people, amphibians show a wide diversity of survival strategies which have allowed them to occupy most terrestrial and fresh-water habitats. On this entry we’ll explain some of the aspects related to their evolution, explaining how our ancestors managed to get out of the water.


Current amphibians, together with reptiles, birds and mammals are found within the superclass Tetrapoda (“four limbs”), the vertebrate group that abandoned the sea to conquer the land. These first tetrapods were amphibians and they evolved around 395 million years ago during the Devonian period from lobe-finned fish named sarcopterygians (class Sarcopterygii, “flesh fins”) within which we find the coelacanth and the current lungfish.

Specimen of coelacanth (Latimeria chalumnae) a sarcopterygian fish, photo by smerikal.

This group of fish is characterized by its fins which, instead of being formed by rays like in most bony fish, they have a bony base that allowed the subsequent evolution of the limbs of the first amphibians. Within the sarcopterygians, the nearest relatives of the tetrapods are the osteolepiformes (order Osteolepiformes) a group of tetrapodomorph fish that got extinct about 299 million years ago.

Eusthenopteron_BWRestoration of Eusthenopteron, an extinct osteolepiform, by Nobu Tamura.


The conquest of land was not done from one day to the other; it was possible with the combination of multiple adaptations. Some of the most important characteristics that allowed the first amphibians to leave the water were:

  • Evolution of lungs, which are homologous to the gas bladder that allows fish to control its buoyancy. Lungs appeared as an additional way to get oxygen from the air. In fact, there is actually a sarcopterygian family the members of which have lungs to get oxygen from the air, for they live in waters poor on oxygen.
  • Lungs_of_Protopterus_dolloiDissection of Protopterus dolloi a sarcopteryigian fish with lungs.
  • Development of the choanaes, or internal nostrils. While fish present a pair of external nostrils at each side of its snout through which water passes on while swimming, the ancestors of the tetrapods only had one external nostril at each side connected to the internal nostrils, the choanae, which communicated with the mouth. This allowed them to get air through their noses using lung ventilation and this way to smell outside of water.
  • Apparition of the quiridium-like limb. The quiridium is the tetrapod’s most basic characteristic. This limb is known for having the differentiated parts: the stylopodium (one bone, the humerus or the femur), the zeugopodium (two bones, the radius or tibia and ulna or fibula) and the autopodium (fingers, hands, toes and feet). While the stylopodium and zeugopodium derived from the sarcopterygian’s fins, the autopodium is a newly-evolved structure exclusive from tetrapods.
Simplified drawing of the structure of the quiridium, by Francisco Collantes.

In short, the relatives of the osteolepiformes developed the tetrapod’s typical characteristics before ever leaving water, because they probably lived in brackish, shallow waters, poor in oxygen and that dried out quickly and often.


Probably the creature known as Tiktaalik is the closest animal to the mid-point between the osteolepiformes and the amphibians. The first recorded amphibians were labyrinthodonts meaning that their teeth had layers of dentin and enamel forming a structure similar to a maze.

Labyrinthodon_MivartCross-section of a labyrinthodont tooth, form "On the Genesis of Species", by St. George Mivart.

There were four main groups of primitive amphibians, each characterized by: a group that includes the first animals that were able to get out of water, a second group which contains the ancestors of the amniotes (reptiles, birds and mammals) and two more groups, both candidates to be the ancestors of modern amphibians.

Order Ichthyostegalia

Ichthyostegalians were the first tetrapods to be able to leave the water. They appeared at the late Devonian period and they were big animals with large wide heads, short legs and an aquatic or semi aquatic lifestyle (they probably were pretty clumsy on land). They moved around using mainly their muscular tail with rays similar to that of fish.

5212816060_da1a11e94e_oFossil and restoration of Tiktaalik. Photo by Linden Tea.

Similarly to current amphibians, they presented a lateral line (sensory organ that allows fish to detect vibrations and movement underwater) and were able to breathe through their skin (they lost the cosmoid scales of their ancestors). Also, the eggs were laid in the water, from which the tadpoles emerged and later on, they suffered a metamorphosis process to become adults just like current amphibians. Subsequently ichthyostegalians gave rise to the rest of amphibian groups.

ichthyostega(1)Skeletons of Ichthyostega and Acanthostega, two typical ichthyostegalians.

Clade Reptiliomorpha

Reptiliomorphs were the ancestors of amniotes and appeared about 340 million years ago. Most of them were usually large and heavy animals, which presented more advanced adaptations to live on land (laterally-placed eyes instead of dorsally-placed ones and a knobby more impervious skin). Even though, reptiliomorphs still laid their eggs in the water and had larval-stages with gills. It wouldn’t be until the late Carboniferous period when the first amniotes (animals that could lay their eggs on dry land) would emancipate completely from water.

Diadectes_phaseolinusMounted skeleton of Diadectes a large herbivorous reptiliomorph from the American Museum of Natural History, photo by Ghedoghedo.

Order Temnospondyli

This group is one of the possible candidates to being the ancestors of modern amphibians. This is the most diverse group of primitive amphibians and it survived until the early Cretaceous period, about 120 million years ago. The temnospondyls varied greatly in shape, size and lifestyle.

Eryops1DBRestoration of Eryops megacephalus a large temnospondylian predator, by Dmitry Bogdanov.

Most of them were meat-eaters, but some were terrestrial predators, some were semi aquatic and some had returned completely to water. Even though, all species had to return to water to breed for the fertilization was external; while the female was laying clutches of eggs in the water, the male released the sperm over them.

Mounted skeleton of Koskinonodon a 3 metres long temnospondyl, from the American Museum of Natural History, photo by Lawrence.

Within the temnospondyls we can find some of the biggest amphibians that ever lived, such as Prionosuchus, with an estimated length of 4,5 meters and about 300 kilograms of weight. Also, even though their skin was not covered with scales, it wasn’t completely smooth like in modern amphibians.

Prionosuchus_DBRestoration of Prionosuchus by Dmitry Bogdanov.

It is believed that this group could be the sister-taxon of modern amphibians, even though there’s one last group which could be a candidate to that post.

Order Lepospondyli

Lepospondyls were a small group of primitive animals which appeared at the early Carboniferous and disappeared at the late Permian period. Even though lepospondyls were not as numerous and smaller than the temnospondyls, they presented a wide range of body shapes and adaptations.

Diplocaulus_BWRestoration of Diplocaulus magnicornis, of about 1 metre long was the biggest of all lepospondyls, by Nobu Tamura.

The first lepospondyls looked superficially like small lizards, but subsequently lots of groups suffered processes of limb reduction or loss.

Pelodosotis1DBRestoration of Pelodosotis, an advanced lepospondyl, by Dmitry Bogdanov.

The relationship of the lepospondyls with the rest of tetrapods isn’t very clear. Different hypothesis go from some authors arguing that they are a group separated from the labyrinthodonts, some thinking that they are the ancestor of current amphibians and reptiles, and some even saying that they are the ancestors of only a portion of modern amphibians.

LysorophusRestoration of Lysorophus, a Permian lepospondyl, by Smokeybjb.

As we can see, the classification of primitive amphibians can be an extremely complex thing. On this entry I tried to make a summary of the most important groups of ancient amphibians and, on the next one, we’ll center on the evolution of modern amphibians, the so-called “lissamphibians”, and we’ll look in more detail all the controversies surrounding these curious animals.


The following sources have been consulted in the elaboration of this entry: