Frogs go through various metamorphic changes from their formative stages until adulthood, many of which concern their breathing methods. Although frogs have lungs when they are mature, tadpoles are fully aquatic when they transform from fertilized frog eggs, requiring more creative tools for breathing.
Tadpoles have external gills from 0 to 6 weeks of metamorphosis, and then develop internal gills from 6 to 16 weeks while they are fully aquatic. After an average of 14 to 16 weeks, tadpoles begin to lose their gills and develop lungs to breathe on land.
Tadpoles and frogs also breathe through their skin by absorbing oxygen from the surrounding water. Tadpoles may access oxygen above the water’s surface through a technique called “bubble-sucking.”
Although tadpoles use gills for breathing in their early stages, they lose their gills at the froglet stage and can no longer breathe underwater without using their skin.
Join us as we discuss how tadpoles develop gills, how they breathe, and how tadpoles lose their gills to breathe on land as they transition into the froglet phase.
Young Tadpoles Have External Gills
Tadpoles that are 0 to 6 weeks old have external gills and are fully aquatic. Tadpoles can also intake oxygen and expel carbon dioxide through their skin.
Once fertilized, frog eggs transform into tadpoles, otherwise known as polliwogs, they emerge and begin cautiously exploring their freshwater surroundings. Tadpoles are fully aquatic during their early phases, and they have physical features to match this lifestyle.
As a result, tadpoles have gills that they use to breathe underwater, and their gills are their primary respiratory organ during this metamorphic stage. However, tadpoles’ gills develop as they experience metamorphic changes in their journey toward becoming mature and semi-aquatic frogs.
External gills are visible, often occurring in pairs, and Anuran tadpoles typically develop external gills when they are born. External gills also act as mucosal surfaces in tadpoles, as these creatures have not yet developed skin and other organs that are used as mucosal surfaces in mature frogs.
Mid-Tadpoles Have Internal Gills
Tadpoles that are 6 to 16 weeks old have internal gills and are fully aquatic. Tadpoles can also intake oxygen and expel carbon dioxide through their skin.
Tadpoles generally develop internal gills once they reach 6 weeks of age (on average). However, it should be noted that some frog species may retain their external gills for numerous reasons.
The transition from external gills to internal gills is one of the first metamorphic changes experienced by tadpoles during the early stages, as well as the development of hind legs.
The internal gills are formed when the external gills become concealed by an additional layer of skin, formally referred to as an operculum. This layer of skin creates an opercular chamber around the now internal gills, and the gills are vented by spiracles.
Depending on the frog species, the spiracles used to vent the gills may be formed in the following ways:
- There may be two spiracles on both sides of the tadpole’s body
- There may be a single spiracle on the left side of the body
- There may be a single spiracle near the vent on the underside
Numerous mitochondria-rich cells (MR cells) are scattered on the lateral and ventral epithelia of the internal gill tufts and arches. These cells help the overall transition of ions, which assists the breathing process for tadpoles.
Tadpoles Breathe Through Their Gills & Skin
Tadpoles breathe through their gills and skin for the first 20 to 25 weeks of their lives and begin to develop lungs to breathe on land after this period. Tadpoles can also breathe through their skin and may attempt “bubble sucking” at the water’s surface during this time.
External and internal gills are the primary respiratory method for tadpoles, but the process is far more complex compared to the gills used by marine life. Tadpoles generally live in freshwater bodies with relatively low oxygen levels to evade predators.
However, their gills often aid the breathing process without providing sufficient oxygen for survival. As a result, tadpoles need to use a combination of methods to ensure that enough oxygen is absorbed into their bodies.
For species that only have gills during their early phases, breathing through the skin is another common method. Since anuran species have permeable skin, oxygen can be absorbed from the surrounding water via gas exchange. Carbon dioxide is expelled back through the skin, creating a secondary respiratory method.
Tadpoles May Attempt “Bubble-Sucking”
With over 7,500 known frog species, there are some exceptions to the general rules we have discussed so far in this article. Researchers have discovered that some tadpole species breathe air from above the water’s surface without breaking the water’s surface tension (Schwenk et al, 2020).
Since tadpoles are so small, they often charge toward the water’s surface and get forced back down by the tension, leaving them helplessly searching for more oxygen.
Tadpoles attach their mouths to the base of the water’s surface, opening their jaws wide while drawing a bubble of air into their mouths. After they close their mouths, a portion of the fresh air bubble is forced down into their bodies.
Since they are so tiny, they usually cannot absorb the entire air bubble, and the excess is expelled, after which it floats back up to the water’s surface. This entire process takes around three-tenths of a second, and tadpoles typically only use this method until they are large enough to break through the water’s surface and ‘breach-breathe’.
When Do Tadpoles Lose Their Gills?
Tadpoles only need their gills to breathe during their fully aquatic phases, but they require different respiratory systems for breathing on land (lungs).
As a result, tadpoles slowly lose their gills as they transition into the froglet phase, although some froglets may still retain their gills for some time until they reach maturity.
Over time, the internal gills used by tadpoles will be replaced by functional lungs, which are necessary for breathing out of water. Many froglets have lungs and gills for a time, as they cannot remain on land for lengthy timeframes and will still be aquatic for the most part.
|Stage||Gill Development||Time Frame||Habitat|
|Early Tadpole||External gills||3 Days||Aquatic|
|Mid-Tadpole||Internal gills||6 weeks||Aquatic|
|Late Tadpole||Lungs being to develop||14 to 16 weeks||Semi-aquatic|
|Froglet||Lungs developing||6 to 9 weeks||Semi-aquatic|
|Adult Frog||Fully developed lungs||–||Species-dependent|
Their gills will only disappear completely once the frogs have become froglets, ready to spend some of their time on land. In many cases, frogs may only return to water bodies during mating seasons and will be able to use their skin to breathe underwater for the rest of their lives (CTNF).
While tadpoles are equipped with functional gills, they often have to use more creative respiratory approaches to ensure they receive sufficient oxygen. Like we saw, some frog species form underdeveloped lungs as tadpoles, which they can use to absorb oxygen bubbles from above the water’s surface, while others use their gills and permeable skin in combination.
Schwenk Kurt, and Phillips Jackson R., Circumventing surface tension: tadpoles suck bubbles to breathe airProc. R. Soc. B.2872019270420192704, 2020 https://doi.org/10.1098/rspb.2019.2704