What is an amphibian?
Common knowledge dictates that an amphibian is an animal that is:
- And egg-laying
What about turtles, then?
Most people know that turtles are classified as semi-aquatic, cold-blooded, egg-laying reptiles.
Wait a minute… Aren’t turtles amphibians, then?!
It’s not quite so simple.
Scientifically speaking, amphibians are further back on the evolutionary tree, being more closely related to fish than to reptiles, dinosaurs, birds, or mammals.
They have vastly different methods of reproduction and growth, as well as some critical physical differences.
Keep reading to learn more about the fine line between amphibians and other semi-aquatic animals, like reptiles.
Table of Contents
Reason #1: Turtles Have Scales
Most reptiles are covered in tough, keratinous plates called scales.
The main exception to this rule is some pet snakes and lizards that were selectively bred to exhibit a scaleless gene.
Still, these animals would probably not survive in the wild because they do not have the “armor” they evolved to live with.
Turtles, being reptiles, are no exception to the rule. Their vulnerable heads, legs, and tails are covered in the same types of scales you’d associate with snakes and lizards.
As is the case with most reptiles, a turtle’s scales’ primary function is protection – especially for body parts exposed outside of the bony, protective shell.
In addition to serving a primarily defensive role, many turtle scales are modified to serve other purposes.
Even the large scutes on turtles’ shells are, technically, modified scales.
Scales on a turtle can look very different, depending on the species lifestyle and native habitat.
Desert tortoises have evolved scales that help them to retain water. Most terrestrial turtles and tortoises have larger, harder, and more protective scales since they usually can’t pull themselves into their shells.
They also retain shed skin on their scutes.
This build-up of old skin offers an ever-increasing degree of protection. Experts even use the layers of these accumulations to estimate an animal’s age!
Meanwhile, aquatic and sea turtles’ scales are smaller and smooth to streamline their body for underwater swimming.
A turtle’s beak is also made from a pair of specialized scales.
Soft-shelled turtles are named for the absence of scales on their leathery shells.
Amphibians don’t have scales because they would block oxygen and water absorption.
Frogs, salamanders, and their relatives evolved to survive with permeable skin.
Unlike terrestrial tortoises and turtles, all amphibians absolutely must have access to absorb water or moisture through their skin, or they would dehydrate and perish.
Reason #2: Turtles Mate…
… and if you’ve ever seen one of the comical viral videos, you know they’re vocally enthusiastic about their reproduction, as well!
Like most other reptiles, birds, and mammals, turtles engage in a physical mating ritual.
The females’ eggs are then fertilized internally, and a few weeks or months later, she lays them. They’ll incubate for a few more months, then hatch.
Most amphibians, on the other hand, fertilize their eggs externally. First, the female lays her eggs; then, the male fertilizes them.
Amphibians do engage in a reproductive behavior known as amplexus, which looks a lot like mating.
The male grabs onto the female’s abdomen or armpits with his front legs. This behavior stimulates egg-laying in the female and ensures that the male is present to deposit his sperm as soon as she lays them.
Some amphibians reproduce via internal fertilization, but they still do not mate in the traditional sense.
In almost all salamanders and newts, the male deposits a capsule of sperm cells which the female collects with her cloaca.
She stores it in a special compartment, known as a spermatheca, for fertilizing her eggs when she ovulates – which could be months later!
Only caecilians have a penis-like structure for depositing sperm directly inside of the female.
No turtle species is capable of externally fertilizing their eggs.
Unlike amphibian eggs, turtle eggs have a solid, protective shell that blocks most outside contaminants – including sperm. All male turtles have a penis.
Reason #3: Turtles Must Lay Their Eggs on Dry Land
Continuing our turtle and amphibian reproduction discussion, even though they all lay eggs, turtle eggs are vastly different from amphibian eggs.
Turtles lay amniotic eggs that have a semi-soft, leathery shell.
The shell protects the embryo, to some degree, from dehydration, dirt, and predators.
It’s still semi-permeable to allow for oxygen exchange, but their permeability leaves the egg at risk for drowning if the nest floods.
Even sea turtles, which spend most of their lives in the ocean, need to haul themselves onto dry land to lay their eggs so they won’t drown.
Amniotic eggs feature specific membranes and fluids designed to facilitate the embryo’s growth and survival with a safe and stable fluid environment, waste removal, gas exchange, and nutrients from the yolk.
Amphibian eggs are quite different.
They have no shell, membranes, or amniotic fluids, so they must be laid in an environment that is safe for the embryo to be exposed to and grow in.
This typically takes place in slow-moving bodies of water, amongst thick aquatic vegetation.
After they’re laid and fertilized, amphibian eggs will swell from absorbing the surrounding water and moisture, so an optimum environment is essential.
Frog and amphibian eggs do have a gelatinous coating that offers mild protection from pathogens and tiny predators. Without a shell, the coating and the eggs are very prone to dehydrating outside of water.
The ability to lay amniotic eggs that are safe to grow and develop outside of, and away from, water is one of the most significant factors that allowed for the evolution of terrestrial animals.
Biologically speaking, this is the most significant factor that divides amphibians and fish, known as anamniotes, from amniotes, or creatures that develop their young within a traditional or modified amniotic egg.
Yes, even human fetuses are “incubated” in an internal, modified egg!
Reason #4: Turtles Don’t Go Through Metamorphosis
Amphibian metamorphosis is one of the hallmark lessons that young children learn about in science class.
Many classes even “adopt” amphibian eggs or larvae to observe the fantastic process.
Most amphibians emerge from their gelatinous egg cluster as a tadpole or larvae.
Some species remain in the egg throughout their larval stage, but they still undergo metamorphosis inside their egg.
Amphibian larvae typically have gills and get their oxygen from their aquatic environment, like fish.
The process of changing from a larva to an adult varies drastically based on the animals’ environment and lifestyle, but can entail:
- Absorption of the tail or tail fins and gill pouches, exposing the front legs that have developed underneath.
- Growth of rear legs, lower jaw, and lungs.
- Reforming of the spiral-shaped herbivorous digestive tract to a short, carnivorous digestive tract.
Turtles hatch from their shell with the help of an egg tooth, which falls off shortly after they make their appearance.
Hatchling turtles emerge as adorable, tiny versions of their grown-up parents.
Not much changes as they grow and mature, besides their size.
Baby amphibians could easily be mistaken for an entirely different species. They have different means of oxygen exchange and locomotion, different diets, and different appendages than their adult counterparts.
Baby turtles are miniature versions of adults with slightly different nutrition requirements for growth.
Reason #5: Turtles Don’t Have Permeable Skin
Not only do amphibians not have scales, but their skin is also actually very thin, fragile, and absorbent.
This is an adaptation to help them survive their semi-aquatic lifestyle.
Oddly enough, most amphibians can not only breathe through their skin, but they also drink through their skin!
Unfortunately, this leaves them extremely susceptible to dehydration in dry environments.
Have you ever noticed that there aren’t many frogs in the desert, but there are tons of them in swamps? Now you know why!
The skin difference is also a great example of why turtles and tortoises are more common than amphibians in dry landscapes.
The thick skin of desert tortoises is partially designed to prevent water loss.
Like most living species, there is some degree of skin permeability, even in turtles.
While most species won’t typically lose large volumes of fluid through their skin, some species can absorb water through the soft skin around their cloaca and on the underside of their neck.
Many amphibians excrete a poisonous, foul-tasting, or smelly substance through their permeable skin as a means of self-defense.
Their secretions also have antimicrobial properties to ward off fungal and bacterial infections.
Turtles CAN’T excrete anything from their skin. Instead, they use their shells to protect themselves from predators.
One thing that turtles and amphibians do have in common, which many people don’t realize, is that they both shed their old skin.
Amphibians tend to shed their skin in one whole piece, like a snake. In contrast, turtles constantly shed the skin on their appendages and even on their shell in large flakes.
Many terrestrial species allow the old, dead skin to build up on their shell scutes for additional strength and protection.
While a turtle’s skin (and shell) are made to provide a physical barrier to the threats of the outside world, the soft skin of amphibians evolved to offer chemical protections and survival functions, instead.