I learned a lot about amphibians while obtaining my certificate in Master Herpetology. We talked about how different amphibian species reproduced and the types of eggs they deposited.
It amazed me how variable amphibians can be when it comes to their eggs and offspring!
All three classes of amphibians (caecilians, salamanders, and frogs) produce young through eggs containing yolk. The reproductive and birthing process, clutch size, egg size and shape, and type of development differ among species.
This table compares the different types of amphibians and their eggs:
|Order||Clutch Size||Egg Size (mm)||Development Type||Main Environment||Parental Care|
Fun Fact: Most amphibian species are oviparous! (This means they lay eggs)
We will discuss each in more detail later. There are many various ways amphibians reproduce and it may vary per species.
This article will compare the differing types of eggs and egg development among amphibian species.
We will mainly focus on frogs and toads, but many of the concepts are similar for caecilians and salamanders.
We will talk about clutch sizes, egg sizes, and location in the environment where eggs may be deposited.
The Majority of Frogs Are Oviparous
The majority of frog species are oviparous egg layers.
Many frogs and toads migrate from a terrestrial living habitat to an aquatic breeding habitat where eggs may be laid.
I was excited to find a large batch of super healthy Wood Frog eggs last spring:
Wood Frogs lay eggs that are very typical looking compared to other frog specie’s eggs.
Although there are many other species, frog eggs are often laid in clusters as depicted below:
These eggs were very healthy and in well oxygenated water:
They may be individually attached to plants, laid in long strings, or as a thin film on the surface of the water.
There was a lot of vegetation to anchor down the eggs and hold them in place:
Although most frogs lay eggs in clusters, toads lay eggs in strings:
The size and shape of the egg clutch may depend on the oxygen status in the water at the breeding site.
Water that is not very well oxygenated typically renders smaller sized eggs that may not clump together.
Other issues like location, water purity, and presence of other contaminants may kill the eggs
Not to far form the site where I took the photos of the healthy Wood Frog eggs above, there was another few clusters of wood frog eggs in a ditch.
These frog eggs were not in good shape at all and were clearly dying:
Unfortunately, these eggs did not make it to the larval stage:
Many eggs laid in fast-moving bodies of water, such as streams, are larger in size. Direct-developing eggs are also larger than larval eggs.
They will be in the egg longer using the nutrients in the yolk so they may emerge as a froglet instead of a tadpole.
Some frog species deposit their eggs in small depressions of water large enough for just their clutch to develop.
Others may deposit their eggs in tree holes or on leaves above where water collects.
Some build foam nests on top of water, or mixed with water on land that will get washed into a waterway after hatching.
Frogs show a low percentage of parental care, which is providing care for the developing egg or newly hatched tadpole or froglet.
The majority of parental care is exhibited as egg attendance and nest guarding in both aquatic and terrestrial nests.
In such cases, either males and/or females may guard eggs and provide some degree of care.
Fun Fact: The Surinam Toad will grow skin over fertilized eggs on the female’s back to carry around the eggs until they emerge from her back as froglets!
Some species carry eggs on their bodies to complete development. Carrying eggs can be used to transport them elsewhere, while others carry them in their mouths.
Most Salamanders Are Oviparous
Most salamander species are oviparous meaning they lay eggs. Very few are viviparous.
Location of egg deposition can impact the size of the egg. The size of the egg directly affects the time it takes the embryo to develop.
Eggs laid in still-standing ponds tend to be very small and develop quickly, leading to quick hatching.
Eggs laid in streams of fast aquatic environments are larger and tend to be clumped together, attached to vegetation, or hidden behind rocks.
Eggs deposited terrestrially are the largest salamander egg size and take the longest time to develop.
Many salamander eggs laid terrestrially are laid next to a source of water so they may hatch into the aquatic larval stage.
Some deposition sites may be temporary or seasonal ponds or pools, while some are rushing waters.
If parental care such as nest guarding is seen in salamander species it is more likely in a terrestrial nest.
75% of Caecilians Are Viviparous
When I was gaining my certificate in Master Herpetology I really had no idea what a caecilian was or why it was called an amphibian.
Some people have nicknamed these species the ‘blind snakes’ because they’re long, do not have protruding eyes, and do not have limbs.
About 25% of caecilians lay eggs and live underground in moist environments. However, the vast majority of species are viviparous (75%).
Oviparous caecilians deposit their eggs terrestrially and once they hatch they move through the ground or mud to water to finish development.
Some hatch after experiencing direct development, this is skipping the aquatic larval stage.
Certain species of caecilians exhibit parental care for their eggs by wrapping themselves around them for protection.
Afterwards, they may hatch and consume the skin off the mother for nutrients. This is called dermophagy.
What is an Amphibian Egg?
Fun Fact: Amphibian eggs are not amniotic eggs like reptiles or birds!
Amphibians do not lay or hatch from amniotic eggs.
This is, they do not have a hard shell encasing a fluid-filled sac called amnion that supplies the embryo with nutrients.
Instead, amphibians get their nutrients solely from the yolk.
The yolk is surrounded by the yolk sac, which helps protect it. Around the yolk sac are jelly layers used for respiration and gas exchange. The size of the egg depends on the size of the yolk.
The yolk supplies nutrients that it received from the mother to the developing embryo for a period of time.
The nutrients will eventually be depleted and this is typically the time of hatching.
Hatching refers to emerging from an egg into a developing stage while birth refers to being born from the mother in a developing stage, known as viviparous.
Viviparous means giving birth to live young that developed inside the mother. Humans are viviparous!
Wilkinson M, Sherratt E, Starace F, Gower DJ (2013) A New Species of Skin-Feeding Caecilian and the First Report of Reproductive Mode in Microcaecilia (Amphibia: Gymnophiona: Siphonopidae). PLoS ONE 8(3): e57756. doi:10.1371/journal.pone.0057756
Red-Backed Salamander Photo by Matt Smokosa. 2014 Seney National Wildlife Refuge Photo Contest Entry. Red-backed Salamander Guarding Eggs, from USFWSmidwest.
Amphibia – the amphibians: Wildlife journal junior – wildlife journal junior. New Hampshire PBS. (n.d.). Retrieved from https://nhpbs.org/wild/Amphibia.asp
Inside Ecology. (2018, November). Caecilians – unusual reproductive ecology. Inside Ecology. Retrieved from https://insideecology.com/2018/12/27/caecilians-unusual-reproductive-ecology/
Pough, F. H. (2018). Herpetology (4th ed.). Sinauer Associates, Inc., Publishers.
Surinam toad. San Diego Zoo Wildlife Alliance Animals and Plants. (n.d.). Retrieved from https://animals.sandiegozoo.org/animals/surinam-toad#:~:text=An%20amazing%20toad%3A%20The%20Surinam,narrow%20tubes%20on%20its%20snout.