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.
BREATHING AIR OR WATER
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).
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.
HOW DO LISSAMPHIBIANS BREATHE?
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.
Portrait 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.
Scheme 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.
Modified 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.
Male 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.
Photo 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.
Drawing 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.
Many 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.
LIVING WITHOUT 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.
California 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.
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.
Ozark 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:
- Cover image by Michael Righi.