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I've noticed that both are quite often called the other (turtles being called tortoises and tortoises being called turtles). Just in case there is some confusion, could someone clarify what makes a tortoise a tortoise, and what makes a turtle a turtle? What makes them different?

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Simple Answer


Turtles are an aquatic, or semi-aquatic, species, while tortoises reside only on land.

Difference between turtles and tortoises

Image sources: 1, 2, 3.


In-Depth Answer


Summary

Technically, tortoises are a kind of turtle. Turtles reside in the order of testudines, while tortoises reside in their order of testudinidae. To make it even more confusing, the word terrapin is often used to reference all types of turtles, though technically it should only reference turtles living in brackish waters. The proper term for all types of turtles is chelonian.

The differences

Diet

Terrestrial and aquatic chelonians have differing diets. Aquatic chelonians have evolved to be omnivorous, taking advantage of their swimming speed to capture live prey, while also eating vegetation. Meanwhile terrestrial chelonians have evolved to survive solely on vegetation.

You can tell the difference in the diets, by the shape of the chelonian's beak. Omnivorous chelonians will have a straight-edged beak, to slice through meat. Herbivorous chelonians will have a more serrated beak, in order to tear off pieces of vegetation.

Eyes

Terrestrial chelonians have evolved to look down at objects in front of them. As such, their eyes will be facing forward and downward. Meanwhile, aquatic chelonians have evolved to lay submerged in water, and will have eyes facing more outwards. Some species of turtles, mainly soft-shelled, will have their eyes more prominently on the top of their heads, to allow themselves to breath and view above the water without leaving it.

Note: Eye placement is not a sole indicator of a turtle or a reptile, as box turtles are primarily terrestrial and have similar eye placement.

Difference between turtle eyes.

Image sources: 1, 2, 3.

Shell

In general, terrestrial chelonians will have more domed shells to prevent predator's jawws from being able to bite down on them, while aquatic chelonians will have more flat, streamlined, shells for swimming. There are a couple exceptions to this for both sides.

Limbs

The limbs are probably the key point, if you're trying to identify whether or not the chelonian you're looking at is a tortoise. First, you're going to want to see if the feet are webbed. As you can imagine, webbed feet will mean that it is an aquatic chelonian. Terrestrial chelonians will have shorter, stockier limbs, in order to hold their bodies off the ground.

More importantly, what you'll notice, is that tortoise limbs have larger, more prominent scales. Several species of tortoises will also have scales on the back of their legs referred to as spurs. This is mainly because tortoises aren't quite able to bring their entire legs into their shells as the other chelonians can. So they need to be tougher in order to keep safe.

Tortoise legs

Image source

Lifespan

Tortoises have a much longer lifespan than other chelonians. Living as long as, if not longer, than humans. They will generally live for about 120-200 years in captivity, while turtles will only live for about 20-30 years.

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