All the 250 or so species of Tortoises, Terrapins and Turtles are reptiles. Scientists refer to all of them as Chelonians, because they all belong to the taxonomic order known as Chelonia, derived from the Greek word for tortoise. They are all cold blooded (ectothermic), have scales and lay eggs. So what are the differences and how can we tell them apart?
The animals can be differentiated from there habitat – where they like to live, where they call home. The Tortoise is terrestrial, living on land. The Terrapin is aquatic, living in fresh or brackish water. While the Turtle is also aquatic it makes its home in seawater.
What the species eats can also be used as a general way of distinguishing the majority of the different species. A Tortoise is a vegetarian; a Terrapin is omnivorous, while a Turtle is carnivorous.
There are also major physical differences between the species, a Tortoise, if it can retract its head, will do so directly back into its shell. The carapace (the upper shell) of the Tortoise is domed. The Terrapin and Turtle both have a more flattened carapace to allow them to be more streamlined in their aquatic environment. A Terrapin retracts its head sideways, while a turtle cannot retract its head. Tortoises’ feet have claws, to allow it to move easier on land, terrapins have webbed feet, but still retain claws, while Turtles feet have become flippers.
Of course the world is a very big place and the species of Chelonians are very diverse, from the largest Leatherback Turtle (Dermochelys coriacea) which can reach 8 feet (2.4 m) in length, to the diminutive Bog Terrapin (Clemmys muhlenburgii) 4.5 inches (11.4 cm) for example, there will probably always be exceptions to the rule somewhere but these facts described above will mean, with any luck, you will be able to the difference between any Tortoise, Terrapin or Turtle you come across!
The original question is not as clear cut as it might appear however. While in British English the points above hold true, different parts of the world using different types of English use the terms in very different ways! For example in American English the word Turtle is used as a general term for all the aquatic species, while Tortoise is used for land dwelling animals. In Australian English Turtle is used for both marine and freshwater species, while Tortoise is used for species that live on the land. This can make using identification books or websites from these countries a bit confusing! So when you’re identifying species from these areas of the world or from resources produced in these areas its always best to keep these facts in mind.