Turtles and terrapins are charismatic animals that are found in many different parts of the world. The term ‘turtle’ can be used to describe all species within the order of reptiles known as Testudines, including tortoises and terrapins. Within Testudines, there is a suborder, Cryptodira, which branches into four superfamilies: Chelonioidea, Testudinoidea, Kinosternoidea, and Trionychoidea. ‘Turtle’ is often used to refer to members of Chelonioidea, the sea turtles.
Testudinoidea, the superfamily that comprises tortoises and pond turtles, thus includes terrapins. Terrapins are effectively turtles, due to belonging to Testudines, however, they are distinct from sea turtles and display a number of differences in distribution, appearance, and reproductive behavior.
Distribution, Habitat, and Diet
Both turtles and terrapins have a wide distribution across the world, however, they rarely overlap; turtles exist in saltwater environments, whereas terrapins reside in brackish or freshwater. Marine turtles can be found throughout the world’s oceans with the exception of the polar regions, and terrapins are distributed across North, Central, and South America, Central Europe, Africa, and Asia.
Turtles prefer relatively shallow continental shelf waters, and although they occasionally venture out into deeper parts of the ocean, they tend to be found around the coast. They require sandy beaches for reproduction, although only the females leave the water – males spend the entirety of their lives in the sea after hatching.
In contrast, terrapins occupy reedy marshes, often preferring areas with heavy vegetation and a muddy substrate. In areas where these animals live in brackish water, they still require a source of freshwater for drinking.
The diet of sea turtles varies depending on the species: some species, such as the green turtle, are herbivores as adults, consuming algae, seagrasses, and seaweed, whereas other species such as the loggerhead are carnivorous, feeding on crabs, conchs, and whelks. Terrapins tend to be omnivorous, seeking a variety of plant material as well as invertebrates such as small clams.
Appearance and Size
Due to their differences in lifestyle and habitat, there are various anatomical contrasts between turtles and terrapins. For example, the sea turtle has flipper-shaped limbs, with the forelimbs being significantly larger than the hindlimbs – this reflects the aquatic lifestyle of these animals and that they spend much of their time swimming. The terrapin, on the other hand, has stout limbs with either four or five claws on each, depending on the species. These claws allow the terrapin to climb; however, they also have webbing between the toes to aid in swimming.
Similarly, while the terrapin has prominent scutes on its shell, in the turtle, these are flattened to make the shell streamlined. In combination with the low profile of the turtle’s shell, this adaptation allows the turtle to move underwater with ease. While all terrapins display very tough shells, some species of turtle show a reduced form of this: for example, the leatherback turtle has a softer shell which grants it greater flexibility of movement.
Terrapins have the ability to retract their head, which, being a member of Cryptodira, they do along a sagittal plane. Sea turtles, on the other hand, cannot retract their heads. Both animals have horned beaks that they use to cut through food items – there are no grinding surfaces, so the beak cannot be used for chewing.
Although there can be great variation in the size of both Chelonioidea and Testudinoidea, terrapins tend to be smaller than turtles. The largest extant sea turtle species is the leatherback sea turtle (Dermochelys coriacea), which can grow to impressive lengths of 210 cm (6 ft 11 in), while the painted terrapin (Batagur borneoensis) and the Malaysian giant turtle (Orlitia borneensis) are two of the largest terrapin species, capable of reaching around 90 cm (2 ft 11 in) in length. In many terrapin species, females are larger than males, whereas in marine turtles, males and females do not show this sexual dimorphism, and are the same size.
Reproduction
Marine turtle courtship may vary depending on the species, however, common behaviors observed include circling, biting, chasing, escorting, and mounting. In loggerhead turtles (Caretta caretta), males have also been observed to nuzzle the female and stroke her face and neck with their forelimbs. In contrast, terrapins have a ‘poorly developed courtship behavior system’, which typically involves little more than the male nudging the female before mating with her.
Following mating, female turtles return to the beach on which they themselves hatched. They crawl up the sand to locate a spot above the high-water mark, where they dig a nest using their back flippers and lay their eggs. The deposition of the eggs may take up to three hours.
Terrapins do things a little differently from their marine counterparts. They still nest terrestrially, seeking out areas with loose, sandy soil and open canopy cover, although they have also been seen to nest in sand dunes. Nesting near marshes provides a habitat for the juveniles after hatching.
The females dig out the nest with their hindlimbs, and unlike marine turtles, their oviposition lasts just a few minutes. This difference in the time taken to lay the eggs is most likely due to the number laid. Sea turtles may lay between 100 – 150 eggs at a time, whereas terrapin species lay just 10 – 15 at one time.
In some species of Chelonioidea, the temperature at which the eggs are incubated affects the sex of the embryos. For example, loggerhead turtle eggs incubated at warmer temperatures produce females, while those incubated at cooler temperatures produce males. The same trend is seen in members of Testudinoidea, such as the diamondback terrapin.
Threats and Natural Predators
Both terrapins and turtles are vulnerable to predators in their natural environment. Terrapins are especially vulnerable to larger mammals such as foxes, coyotes, and wolves in North America, whereas populations in Asia may be predated on by big cats such as leopards. Common predators of adult turtles include tiger sharks and killer whales, while juveniles and eggs are vulnerable to fish, seabirds, and raccoons.
Both superfamilies face numerous threats. Terrapins are placed at risk by the pet trade, habitat destruction, tangling in fishing gear, and road mortalities, while turtles are threatened by poaching, coastal development, climate change, and pollution.
Sea turtles are considered a sentinel species, important indicators of ecosystem health. They also play a role in maintaining ecosystem health: they function as transport for epibionts, as well as enabling the transfer of nutrients between locations and trophic levels. Although they do not have such an important role in the ecosystem, terrapins are still crucial to food webs and exercise effects on both their predators and prey.
