Most frogs (not all), start their lives as tiny tadpoles adapted for a fully aquatic life. Over time, the tadpoles will grow legs, lose their tails, and metamorph into adult frogs.
Many tadpoles have lungs that they can use to breathe air in as little as 3 days post-hatch. Air breathing is necessary for survival in waters with low levels of dissolved oxygen where tadpoles can not get sufficient oxygen via their gills and cutaneous respiration alone.
Many tadpoles experience hypoxic waters for some if not most of their lives. This problem is worsened by the fact that most frogs avoid laying eggs in open, more oxygenated waters due to the presence of predators such as fish and turtles.
Many Tadpoles Have Lungs
It was previously believed that tadpoles lacked lungs throughout most of the larval period, relying on gills and cutaneous respiration for gas exchange until lungs and air breathing developed during metamorphosis in preparation for adult life.
However, with new scientific research, it is now widely known that the development of lungs in many tadpoles occurs well before metamorphosis.
A study of the tadpoles of five frog species representing three families found that they all began to inflate their lungs and breathe air within 3 or 4 days of hatching.
In addition, another study confirmed that Gray tree frog (Hyla versicolor) tadpoles develop lungs and begin breathing air within 3 days of hatching.
Many tadpoles show air-breathing behavior at minute body sizes, as small as 3 mm body length.
As earlier mentioned, air-breathing is essential for survival in water with low oxygen levels – where their gills (and skin) cannot absorb sufficient oxygen.
Tadpoles often live in water with low levels of dissolved oxygen for some if not most of their lives.
How Tadpoles Breathe Air
When they first hatch, the tadpoles are too feeble to break through the water’s surface for a gulp of air. To get around this, they “bubble-suck”.
To do this, they first attach their open mouths to the underside of the water’s surface. They then open their jaws wide and suck at the water’s surface. Doing this creates an air pocket that they can pinch off by quickly closing their jaws.
After they close their mouth, a portion of the fresh air bubble is forced down into their lungs;
Since the tadpoles are tiny, they usually cannot absorb the entire air bubble, and the excess is burped out as a small bubble, after which it floats back up to the water’s surface.
Once the tadpoles get bigger, they will be strong enough to break through the water’s surface for a gulp of air (‘breach breathing’).
However, the tadpoles of some frog species such as the Gray tree frog (Hyla versicolor) continue to bubble-suck throughout the larval period and well into metamorphosis, despite growing large and fast enough to breach the surface.
Air-Breathing/Lung Filling at Very Early Stages May Be Non-respiratory
Histological analysis of Gray tree frog (Hyla versicolor) tadpoles showed that their lungs do not become highly vascularized until sometime later in development – when the tadpoles grow to be approximately 6 mm snout to vent length (SVL).
Although the tadpoles of other frog species have been poorly studied in this regard, it may be true for other species, as well.
For this reason, researchers have theorized that air-breathing/lung filling in tadpoles at very early stages may be non-respiratory.
If air-breathing at early stages is non-respiratory, inflated lungs may have functions other than respiration.
A Study by John H. Gee and Ruth C. Waldick showed that Gray tree frog (Hyla versicolor) tadpoles use inflated lungs to promote neutral buoyancy, which may help reduce the energetic cost of holding position in the water.
2. Normal Lung Development
Air-breathing may also be necessary for normal lung development in some species.
An experiment on African clawed frog (Xenopus laevis) tadpoles found that those that were denied access to air after hatching had lungs one-half the size of those that had access to air.
It was also found that the longer tadpoles were denied access to air, the longer they took to metamorphose, and their probability of successful metamorphosis significantly decreased.
One tadpole raised throughout its larval life without access to air successfully metamorphosed but had abnormal, solidified lungs and an enlarged heart.
This experiment demonstrated that lung use during early larval development in African clawed frogs is not necessary for survival through metamorphosis, but its absence severely harms growth.
Tadpoles Delay Lung Inflation to Different Stages
Although the tadpoles of many frog species start using lungs soon after hatching, it’s important to note that delay tadpoles’ lung inflation (meaning the initial act of inflating their lung rudiments with air) to different stages after hatching – depending on the species.
Not all tadpoles breathe using their lungs shortly after hatching.
For example, the tadpoles American bullfrogs (Lithobates catesbeianus) do not inflate their lungs until mid-tadpole stages. Before the start of lung use, the tadpoles strictly breathe using their gills, and skin.
Other tadpoles such as those of the coastal tailed frog (Ascaphus truei), Lace-eyed Tree Frog (Litoria dayi), Goose frogs (Megophryidae), European spadefoot toads (Pelobatidae), North American spadefoot toads (Scaphiopus), and bufonid toads – delay lung inflation until metamorphosis.
Tadpoles Have Gills in Addition to Lungs
Apart from lungs, tadpoles also have gills for gas exchange. When first hatched, tadpoles have external gills, often in pairs, on either side of their heads. They use these gills to breathe in the water, just like fish.
The gills have thin membranes and a network of blood vessels. These membranes absorb dissolved oxygen from the water and move it into the bloodstream.
At the same time, carbon dioxide from the bloodstream passes out through the membranes, and into the water.
When tadpoles are bout 4 – 6 weeks old, opercular folds grow backward from the hyoid arch of each side covering the external gills and gill slits.
In this way, a gill cover known as an “operculum” is formed enclosing the external gills and gill slits – turning the external gills into internal gills. These internal gills are vented by ventrolateral openings, known as spiracles.
Depending on the species, there can be:
- Two spiracles on both sides of the body,
- A single spiracle on the underside near the vent, or
- A single spiracle on the left side of the body
Spiracles are tubular structures leading out of the opercular chamber. Tadpoles eject water through the nostrils and through the spiracles.
During metamorphosis, the gills are broken down, the lungs develop rapidly into the main organ of respiration.
Tadpoles Also Breathe Through Their Skin
In addition to lungs and gills, tadpoles also breathe by taking in oxygen through the skin of their large tail fins.
The skin is made up of thin membranous tissue that is highly permeable to water and contains a large network of capillaries and other blood vessels close to the surface.
The thin membranous skin allows oxygen from the air to be absorbed into the bloodstream via diffusion. At the same time, carbon dioxide from the bloodstream passes through the skin and membranes and is diffused into the water.
This process of “skin breathing” is known as cutaneous respiration.
Stage of Development
Organs for Gas Exchange
|Eary Tadpole||External gills, Skin, (and Lungs in some species)|
|Mid Tadpole||Internal gills, Skin, (and Lungs in some species)|
|Late Tadpole||Internal gills, Skin (and Lungs in some species)|
|Froglet||Lungs, Skin, and membranes in the mouth and throat|
|Adult frog||Lungs, Skin, and membranes in the mouth and throat|
The Frog Without Lungs
The endangered Bornean Flat-Headed frog found in Indonesia is the only currently known species of frog that does not have any lungs at all! It breathes entirely via its skin, just like many lungless salamanders.
This aquatic frog lives in cold, fast-moving rivers that provide a rich supply of oxygen and most likely lower the frog’s metabolic rate. So to the frog, oxygen is in high supply and low demand.
Also, the flattened body and head (which gives this frog its name) give its skin a large surface area for absorbing oxygen from the surrounding water.
Since Bornean Flat-Headed frogs are lungless, their tadpoles do not have any lungs and breathe entirely through their gills and skin for the entire larval period.
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