What's the difference between a turtle and a tortoise?

Turtles and tortoises are both reptiles with shells, but they're not exactly the same. So how can you tell them apart? What's the difference between a turtle and a tortoise?

"All tortoises are turtles, but not all turtles are tortoises," Sydnee Fenn, a reptile keeper at Columbus Zoo and Aquarium, told Live Science. Generally, the reptiles that people call turtles spend a great deal of time in the water, whereas tortoises live on land, according to Genius Vets in San Diego, California.

Many of the differences between turtles and tortoises come from this habitat difference. For example, water-dwelling turtles' shells are usually flattened to help them swim in lakes, rivers or oceans. Tortoises do not swim, so their shells are often shaped like domes instead, Genius Vets notes, which has advantages on land.

"When tortoises are walking, they can flip onto their backs by accident, and the domed shape of their shells can help them flip back onto their feet," Fenn said. "Also, the dome shape allows more space inside, so some can retract all of their limbs into the shell."

Related: Why do turtles live so long?

There are exceptions to this rule, however. For instance, "the pancake tortoise from Africa does not have the domed, solid carapace of other tortoises," Brett Baldwin, curator of herpetology and ichthyology at the San Diego Zoo, told Live Science. "It has evolved a shell that is flat, less heavily calcified, and pliable, which allows it to scurry quickly into rock crevices and wedge itself in by inflating its pliable carapace."

Turtles that spend time in both the water and on land have webbed feet that can help them swim as well as maneuver on the ground if needed. Fully aquatic turtles, such as sea turtles, have flippers, according to Genius Vets. In contrast, tortoises usually have round feet and stumpy legs, the San Diego Zoo notes.

"Having shorter, stouter, more stable legs allows for easier mobility to carry the heavy shell in their terrestrial lifestyles," Baldwin said. For instance, Fenn noted that for desert tortoises, "wide feet help them walk across sand easier, just like camel feet."

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Unlike other tortoises that have domed shells, the pancake tortoise (Malacochersus tornieri) has a flat, thin and flexible shell.

The desert tortoise (Gopherus agassizii) has wide feet that helps it walk across the sand.

A hawksbill turtle (Eretmochelys imbricata) swims over the coral reef in Kimbe Bay, Papua New Guinea. Notice its flattened shell, which helps it more easily swim through the water than the domed shell of a tortoise would.

Generally speaking, non-tortoise turtles are omnivorous, and tortoises are herbivorous, Fenn said. Because non-tortoise turtles can swim, they can move fast enough to catch prey in the water, thus broadening their diet, she said. In contrast, tortoises are famously slow on land, so they usually stick to plants.

That said, "tortoises can opportunistically eat meat," Fenn said. "They're not going to actively hunt like a tiger would, but if there's meat," such as insects or carrion, "they might eat it." In addition, some turtles, such as the green sea turtle (Chelonia mydas), are herbivorous, according to the National Oceanic and Atmospheric Administration.

Tortoises are found on every continent except Australia and Antarctica, generally in warm to hot environments, such as deserts and jungles, according to the International Fund for Animal Welfare. Turtles live on every continent except Antarctica, and sea turtles spend their lives in oceans spanning the globe, Genius Vets notes. Turtles' aquatic nature helps them survive even when air and ground temperatures get cold, Fenn said.

"They can go to deeper waters where temperature might stay warmer, especially in the winter," she explained. "Some turtles are able to actually breathe underwater through their cloacas, which makes it easier to hide under frozen water. All in all, the fact that they can live in cooler climates opens up where they can be found in the world."