Frogs, They Are A-Changin: a story of metamorphosis

Amphibians (frogs, salamanders, and caecilians) go through a process of development called metamorphosis.  They are the only vertebrates (they have a backbone) to complete true metamorphosis although some fish, such as zebrafish (Danio rerio) go through a partial metamorphosis too.  That said, many species of marine invertebrates (no backbone… or bones at all) go through metamorphosis too and of course there is the poster child for metamorphosis: the caterpillar to butterfly.  

Although all of these animals go through metamorphosis, their development is still not the same.  Metamorphosis just means that the larva baby that hatches from the egg looks absolutely entirely different from the adult.  And you thought puberty brought on a lot of changes for you, imagine being a little caterpillar that feasted on leaves a little too much and decided to take a little nap to sleep off the food coma and then woke up with wings!  Kinda would be awesome actually!  

Of course it isn’t that simple, and if I wanted to talk about the metamorphosis of all the organisms that develop that way I would be able to write a whole book… In fact there are whole books on it (1,2).  Many of these books use super scientific phrases like ontogenetic holometabolous metamorphosis (insect metamorphosis where the juvenile looks vastly different from the adult).  So here my goal is to focus on frog metamorphosis in a way that does not require you to become best friends with a dictionary. If you do want the more details in helping you make sense of it all, see the “Nitty Gritty” section below.

The Overviews

The Pre Game

In late winter to early spring you likely will hear the sound of frogs calling: the males trying to impress the females with how pretty they can sing.  After the male succeeds he will breed with a female (or several females) in a process called amplexus.  During amplexus the male will grab on to the female’s back and basically get a piggyback ride.  From this position, she will lay her eggs while he fertilizes them.

The Eggs

During mating, frogs will lay eggs.  Most commonly these eggs are laid in the water in clusters.  However, some frogs will lay eggs on plants overhanging water and many toads lay strings (instead of clusters) of eggs in the water.  Most (if not all) frog eggs start out as small little round specks surrounded by a jelly goo.  This jelly protects the eggs from predators, drying out, and many other things (3).  As the little round speck inside the egg starts developing, it will start taking the shape of a tadpole.  Eventually the tadpole will wiggle out of the egg jelly and swim off to begin it’s aquatic life.  For the frogs that lay their eggs on plants over water instead of in water, the tadpoles will have an initial thrill ride as they wiggle out of their egg jelly and nose dive down to the water below.

When tadpoles hatch out they have a round to oval body with a tail.  They have no arms and legs (front and rear limbs) yet.  Most tadpoles don’t even have lungs yet!  Tadpoles have gills that they breath through until around the time their front limbs pop out (4).

The Tadpole

During frog metamorphosis it looks like rear limbs develop first and then the front limbs appear after.  But actually the front limbs are developing behind the gill (under the skin) at the same time as the rear limbs are visibly developing.  As the lungs develop and the gills disappear the front limbs will pop out (since the gills they are growing under have now completely disappeared). The last thing to happen is the tadpole tail will disappear (5,6).

Overall, the length of time it takes from the egg being laid to the tadpole completing metamorphosis depends on the species of frog as well as some environmental factors.  Some species, like the wood frog (Lithobates sylvaticus) will only take between 40 and 60 days to complete metamorphosis.  Whereas other species like the green frog (Lithobates clamitans) can take up to 2 years to complete metamorphosis.  As you can see above, there is not an exact number of days it takes each species to complete metamorphosis.  This is largely because the environment can also affect the time it takes.  The temperature of the water is probably the biggest environmental factor on metamorphosis.  The colder the water, the longer metamorphosis will take.  Other environmental factors include, oxygen level in water and food availability.

The Unseen

There are so many changes occurring that aren’t visible too.  The gut changes shapes to go from eating plants to insects.  With this gut change, the mouth also changes from a little suctioning disk to a mouth like you would see on a frog.  This mouth change is actually visible but I thought it made sense to mention it with the gut bit.  Also, as previously mentioned, gills disappear and the lungs become the dominant respiratory organ.  Fun fact, frogs (as well as other amphibians) can breathe through their skin too!  That’s why their skin is thin and often covered in a protective mucus layer.  The last “unseen” change that happens throughout metamorphosis that I will mention is the hormone levels.  This is believed to be what actually drives all of the changes from being a tailed, limbless swimming tadpole to a tailless, four-limbed, hopping (and swimming) frog.  See the details below in how the hormones play a role in metamorphosis.

The Nitty Gritty

The tadpole or larval frog stage is divided into three categories, these are: premetamorphosis, prometamorphosis, and climax (5,6).  When a frog tadpole hatches from its egg there are low levels of hormones produced from the thyroid gland, this is considered the premetamorphic tadpole stage (5,6,7).  Shortly after hatching the limbs begin to develop in response to a rise in some of these thyroid hormones.  These thyroid hormones rise rapidly during this process of limb and toe development (7).  This is considered the prometamorphoic tadpole stage (5,6).  After the rear limbs and toes have visibly developed, the skin covering the mouth disintegrates and the skin covering the front limbs disintegrates and they become visible too (5,6). During this time the gills are also disappearing which allows for the skin over the limbs to disappear (5).  Lastly, the tail begins to reabsorb (5,6,7).  At this time, the anuran has reached the climax level of hormones produced by the thyroid gland, and this stage is conveniently called the climax tadpole stage (5,6).  After this climax, metamorphosis is complete and the thyroid hormones drop back down to low levels where they will remain throughout adulthood (5,6,7).

For the most part, premetamorphosis, prometamorphosis, and climax stages are categorized by the amount of thyroid hormones present.  When thyroid hormones are released, they connect with hormone receptors.  During metamorphosis, the receptors determine what the hormone will do to the tadpole/frog.  If the thyroid hormones connect with the first main type of hormone receptor the result is cell growth.  This leads to things like limb development.  If the thyroid hormones connect with the second main type of hormone receptors the result is cell death.  This leads to things like the tail disappearing, as well as the skin over the front limbs and around the mouth (7).

Citations

1. Ed. Gilbert, L.I. and E. Frieden (1981). Metamorphosis: A Problem in Developmental Biology, 2nd edition. pp 1-598. New York, NY: Plenum Press.
2. Ed. Shi, Y.B. (2013). Current Topics in Developmental Biology: Animal Metamorphosis, pp 1-456. San Diego, CA: Academic Press.
3. Duellman, W.E. and L. Trueb (1994). Life History: Eggs and Development. In: Biology of Amphibians. (ed. W.E. Duellman and L. Trueb). pp. 109-139. Baltimore, MD. John Hopkins University Press.
4. Neumeyer, C.H. (2018). Investigating Interspecific and Intraspecific Variation in Lung Development in Amphibians [Master’s Thesis, James Madison University] JMU Scholarly Commons.
5. Burggren, W.W. and J.J. Just (1992). Developmental Changes in Physiological Systems. In: Environmental Physiology of the Amphibians. (ed. M.E. Feder and W.W. Burggren). pp. 467-530. Chicago, IL. The University of Chicago Press.
6. Shi, Y.B. (2000). An Overview of Amphibian Metamorphosis. In: Amphibian Metamorphosis: From Morphology to Molecular Biology. (ed. Y.B. Shi). pp. 1-14. New York, NY. Wiley-Liss.
7. Brown, D.D. and L. Cai (2007). Amphibian metamorphosis. Developmental Biology. 306: 20-33.