For this project I will be investigating how salamanders respires and specifically looking at an axolotl, a unique, critically endangered salamander which is indigenous to the lakes of Mexico. Why did I choose the axolotl? I chose the Axolotl as it has an incredibly unique feature in that it does not undergo metamorphosis when coming in to maturity, this means it keeps its most iconic feature, its feathery gills that our outside its body. The feathery gills are present in many juvenile salamanders as they do not tend to leave the water in their early life. This causes much confusion as there are few distinguishing features for any juvenile salamanders.
- The most prominent way that axolotls oxygenate their blood is through their skin. Their skin, like all salamanders they have very thin, moist skin. Whilst moistness is irrelevant for an axolotl who will be in the water anyways, in both cases the gas is dissolved in a liquid which makes for more efficient gas exchange. They also have thin walls which means the gases can passively diffuse quickly into blood vessels which are very close to the skin. These blood vessels, capillaries, take the gases that diffuse through the blood to cells where they are needed for respiration. By quickly taking the gases away the blood at the skin maintains a lower concentration of oxygen and a higher concentration of carbon dioxide. This means diffusion, which is along the concentration gradient, can happen quickly and efficiently. The carbon dioxide is also expelled from the body through the skin. These features of the skin that make it a good medium for gas exchange also suit carbon dioxide diffusing into the water
- The second and most physically apparent way the axolotl oxygenates its blood is through the feathery gills on head. They are feathery as to have an increased surface area to allow for a larger volume of gas to be exchanged. This gas can be diffused into the blood stream straight from the water. It also has gill rakers, these make sure, due to the blood being so exposed that no impurities can get into the blood stream from the water, for example parasites
- The final way the axolotl breathes is using its lungs. It is the least volume of oxygen the axolotl receives of the three ways it oxygenates the blood. The axolotl only really uses its lungs in oxygen lacking water where it cannot get sufficient oxygen from its gills and skin which pull from the water’s oxygen content. The lungs are described as simple and sac-like. Most salamanders that have gills and lungs, have lungs that fit this description but it is even more profound as they do not go through metamorphosis. When the axolotl takes a breath the air will go to his lungs which has a large number of folded pockets encircled by blood vessels. The oxygen in the air inhaled will diffuse through the sac which is a very thin membrane, it will then go into the blood stream which will circle the body and be used in respiration. This oxygen will be diffused into capillaries which surround the outside of the lungs. These capillaries run into vein (called the pulmonary vein in humans) which takes the oxygenated blood to the heart. The heart of an animal in the Caudata family (that houses newts, salamanders and frogs) has three chambers. The three chambered heart consists of two atria and one ventricle. Blood leaving the ventricle will be split by a fork in the artery into either the aorta or the pulmonary arteries. This sounds like you could pump de oxygenated blood around the body which is a waste of energy. This is counteracted by the oxygen diffused directly into the blood stream through the skin and the gills. This compensates for the lack of oxygen in the lungs
- The oxygenation of the blood is what all the processes do, this blood is then taken to the cells which need it as a key part of respiration. This respiration gives the cells energy to keep continuing with their specific purpose. This purpose could be anything, every process is vital and so that is why getting oxygen to respire is so crucial. The blood also takes away CO2 a waste product of respiration which is detrimental if it is allowed to remain at the site of respiration. It is taken back to the lungs where it is diffused out of the body and on the way it is diffused out of the skin. Both ways are along the concentration gradients as there is a higher concentration of carbon dioxide in the axolotl then in the water
The first difference I will talk about is the number of different ways the axolotl can oxygenate its blood, compared to a human. An axolotl can oxygenate its blood in three ways, compared to the one way a human can. The axolotl uses its porous skin to diffuse oxygen straight into the blood stream. This would give axolotls the anatomical advantage as the diffusion of oxygen is passive, whereas an animal taking breath is not. They need to contract the diaphragm and the ribs move up and out. This takes energy which means that you would have less efficient respiration and you need to use more energy to inhale the air. However the lungs of a human are much more developed then an axolotls. A human’s lungs have a clearly defined structure with bronchiole, bronchi and alveoli. This is necessary in a human as a human is much larger than a axolotl, with the biggest being about 40cm. This means the blood needs to be sufficiently oxygenated to provide enough oxygen for all the cells. For an axolotl maybe have underdeveloped small lungs is a good thing. If an axolotl was the same size as a human the lungs would still be much smaller than a humans. This is a good thing as if axolotls are not dependant on their lungs completely for oxygenation of the blood they do not need large lungs and can use the space they save from not having big lungs towards other necessary systems.
Axolotl also do not have ribs, they have costal grooves. This do give the impression of ribs but are used in thermoregulation and help keep the skin most, allowing respiration to remain efficient. The rib cage in a human is used to protect vital organs however this is not so important in an axolotl as they have no natural predators so would be unlikely to experience trauma that could damage their organs. In any case the only vital organ a rib cage would protect in an axolotl is the heart as they can oxygenate their blood through skin and gills but cannot pump the blood without the heart.
A human has a four chambered heart whereas an axolotl has a three chambered heart. A three chambered heart puts less focus on getting all the blood to the lungs before being passed to the body which means leaving the heart the blood may be of a lower oxygen concentration which can therefore mean the oxygen can diffuse through the skin and in through the gills. This means the human heart has to beat twice (blood to the lungs and blood to the body) whereas the axolotl heart beats once as the blood goes to the lung or body from the same contraction of the heart muscle. It obviously takes more energy to beat twice for every beat of an axolotl heart. This means although the three chambered heart seems more rudimentary and simplistic, for something like an axolotl in which the blood is also oxygenated subcutaneously (under the skin).
How is an Axolotl specialised?
The one clear unique feature an axolotl has that all other salamanders do not have, is neoteny, or the failure to induce metamorphosis. However is this feature an advantage? This property of axolotls originates from failure to produce thyroid stimulating hormone. This hormone is then meant to make the thyroid produce thyroxine which then transforms the juvenile salamander into an adult one. Over the years there has been great speculation by scientists studying the abystoma mexicanum about whether neoteny is a positive trait or a negative one.
For the axolotl being neotenous means always have gills but, in the flip side, having under developed lungs. In practical terms this means the axolotl will be seen above water very little compared to normal salamanders as it will become lethargic very quickly as their blood would oxygenate at a very poor rate. It means most of the axolotls life is underwater, it mates and lays eggs underwater. This would not necessarily be a bad thing until humans introduced other fish, the African tilapia and the Asian carp which compete for the same resources as the axolotl. If it were to lay eggs above water other fish would not matter however they cannot so this is leading to a crash in the population. This is a key example of a specialised feature of the axolotl being a negative for the organism.
A study published under the name; Aerial and Aquatic Respiration in Axolotl and Transformed Abystoma tigrinum, goes into detail on this particular topic. They compare an axolotl to an adult tiger salamander which does not have feathery gills and has developed lungs. They set up their experiment at two different temperatures; 15 and 25 degrees with 14 hours of light then 10 hours of dark per day. This was done to ensure the experiment was as fair as possible. They then using a desiccator and a magnestir. The desiccator measured the oxygen content in the water while the magnestir measured the oxygen content in the air. All tanks were oxygenated 30 minutes prior to the experiment and then sealed. Spectators recorded and maintained the temperature and also took readings and noted the number of times both species would break the surface of the water.
The scientists then used the separate readings from the water and air and notes from the spectators to create a table comparing the overall oxygen intake to how much was from the water / air and how many times and for long the Abystoma were at the surface. By doing this the scientists were allowing for a deeper analysis and can confirm a hypothesis as to why the tiger salamander has a larger oxygen intake. We can see from table 1 that transformed salamanders surfaced less frequently but stayed surfaced for longer at both temperatures. This is because when the axolotls surface they take a small gulp of air, as their lungs are poor they cannot inhale a large volume of air. The salamanders, having developed lungs can stay at the surface for long periods of time without having their oxygen levels fall like an axolotls would.
The Axolotl’s gills are lined with filaments which increase surface area which is a benefit as an increased surface area directly correlates to volume of gas that is exchanged. These gills also have many branches which, like the filaments increase surface area however the branches can be seen, the filaments cannot. The gills also have gill rakers attached at the base which are thin pieces of cartilage which act as a filter. This allows the capillaries to be extremely close to the gills whilst being protected by these rakers. This means the concentration of oxygen is always higher in the water allowing for efficient diffusion which is a passive transport mechanism meaning the Axolotl can respire with wasting energy.
Axolotls, like mentioned earlier, also ‘breathe’ through their skin. This allows them to diffuse oxygen directly through their skin in to their blood vessels. This is not unique to axolotls, as all salamanders can, some solely breathe through their skin. This is specialised as the skin is thin meaning quick diffusion of gases. The skin is also moist which dissolves the gases, more important for land but this means the gases diffuse more efficiently. The blood vessels are also close to the skin meaning quick diffusion but also that they maintain a good concentration gradient.
Despite these gill adaptions, I can therefore conclude that the Axolotl is not well specialised and the only real difference between it and a tiger salamander is not a benefit but a hindrance as it is proven to be less efficient then lung breathing. Salamanders overall are, however, quite specialised but Axolotl does not have any positive specialised features over a generic salamander.
References & Citations
Campbell’s “biology” 4th Edition pg 822
Lopez, Carl H., and Edmund D. Brodie. 1977. “The Function of Costal Grooves in Salamanders (amphibia, Urodela)”. Journal of Herpetology 11 (3). Society for the Study of Amphibians and Reptiles: 372–74. doi:10.2307/1563252.
Whitford, Walter G., and Robert E. Sherman. 1968. “Aerial and Aquatic Respiration in Axolotl and Transformed Ambystoma Tigrinum”. Herpetologica 24 (3). Herpetologists' League: 233–37. http://www.jstor.org/stable/3891017.
Gregory, E. H. (1897). “ORIGIN OF THE ELASTIC FIBRES IN THE HEART AND AORTA OF THE AXOLOTL AND THE SALMON TROUT.” Journal of the Boston Society of Medical Sciences, 2(3), 18–20.
Kardong, Kenneth V. (2009). Vertebrates: Comparative Anatomy, Function, Evolution (5th ed.). McGraw-Hill. ISBN 978-0-07-304058-5.