Emydidae

Class Reptilia Order Testudines Suborder Cryptodira Family Emydidae

Thumbnail description

Small- to medium-sized turtles; carapace may be depressed, domed, or strongly keeled; plastron may or may not be hinged; double articulation found between the fifth and sixth cervical vertebrae

Size

Number of genera, species

12 genera; 35 species

Habitat

Freshwater rivers, streams, lakes, and ponds; other species are semiaquatic to fully terrestrial; still other species frequent estuaries and coastal waters

Conservation status

Endangered: 6 species; Vulnerable: 7 species; Lower Risk/Near Threatened: 14 species

Distribution

Temperate and tropical regions of North and South America, Europe, western Asia, and northern Africa

Distribution

Temperate and tropical regions of North and South America, Europe, western Asia, and northern Africa

Evolution and systematics

The oldest fossils are known from the Upper Cretaceous and Paleocene of North America. These modern cryptodires are most closely related to the Geoemydidae of South America, Europe, Africa, and Asia and the Testudinidae, which also are found in North America.

Morphological and molecular evidence suggests a close relationship among western pond turtles (Actinemys marmorata), European pond turtles (Emys orbicularis), and Blanding's turtles (Emydoidea blandingii). Some researchers consider them to be members of a single genus (Emys), while others recognize them individually as the sole representatives of monotypic genera. Two subfamilies are recognized: the Emydinae (palatine excluded from the triturating surface) and the Deirochelyinae (humeropectoral sulcus excluded from the entoplastron).

Physical characteristics

There are typically eight pleurals, five vertebrals, and 24 marginals on the carapace and 12 scutes on the plastron. The seam between the posterior marginal scutes and the last vertebral overlap the pygal bone. A double articulation is found between the fifth and sixth cervical vertebrae. Most species have at least some webbing between the toes, and some species have a hinged plastron.

Bog turtles (Glyptemys muhlenbergii) attain a maximum size of 5 in (12 cm), whereas adult Gray's sliders (Trachemys venusta grayi) may reach 24 in (60 cm) or more. Males are generally smaller than females in the aquatic emydids; however, among semiaquatic and terrestrial species this may be reversed.

Distribution

These turtles are found in the lowland temperate regions of North America, North Africa, southern Turkey, the Middle East, and throughout Europe to southern Russia. They were formerly more widespread in Europe, but the Scandinavian populations were extirpated during the Pleistocene.

Habitat

This extremely diverse family is found in many habitats. They occur in abundance in most permanent freshwater rivers, streams, lakes, and ponds. One species is only found in estuaries and coastal waters, and a few species are semi-aquatic to fully terrestrial.

Behavior

Whether fully aquatic or terrestrial, most emydids have a well-developed basking habit. Some species are active year-round; others are seasonally inactive (dry season or winter). Males of many species exhibit elaborate courtship displays.

Among the temperate northern species, hibemacula are generally located in well-oxygenated areas; however, painted turtles (Chrysemys picta) and Blanding's turtles are tolerant of extremely hypoxic, or low oxygen, conditions. At least two aquatic species, the chicken turtle (Deirochelys reticularia) and the western pond turtle, are known to hibernate terrestrially. The eastern box turtle (Terrapene Carolina) burrows beneath leaf litter and hibernates in shallow soil where it may experience subfreezing temperatures.

Feeding ecology and diet

This family includes species that are strictly herbivorous to those that are strictly carnivorous. Hatchlings of many species are highly carnivorous, but switch to a more omnivorous diet as they mature. Some species have diverse, generalized diets and others are highly specialized. In map turtles (genus Graptemys), the females may develop huge heads with broad palates that enable them to crush large mollusks. Chicken turtles and Blanding's turtles have independently evolved a long neck with a well-developed hyoid apparatus, an elaborate bony structure that rapidly expands the throat to suck in prey items. This feeding adaptation is frequently found in piscivorous (fish-eating) turtle species.

Reproductive biology

In sexually dimorphic aquatic species, the female is larger than the male. The size difference is most extreme among species of the genus Graptemys. Mating generally occurs in the spring; however, some species may store sperm from an earlier mating for several years. The male is brightly colored and may possess long thin claws on the forelimbs that are vigorously waved before the female during courtship. A unique pattern of head bobs also may be exchanged before

Hinged plastron of a box turtle (Terrapene sp.)—the turtle is closed up inside its shell. (Illustration by Gillian Harris)

the female allows the male to mount. This elaborate courtship suggests that females choose their mate. The elongate eggs, which may be flexible or brittle shelled, are generally laid in nests dug in the soil away from the water (sometimes more than 0.6 mi [1 km] away). Most species that have been investigated exhibit temperature-related sex determination.

Conservation status

Seven species are listed as Vulnerable and six as Endangered on the IUCN Red List; 14 others are listed as Lower Risk/Near Threatened. Human activities (e.g., pollution, habitat destruction, road mortality, and collecting for the pet

A red-bellied turtle (Pseudemys nelsoni) climbs aboard an alligator (Alligator mississippiensis) in Everglades National Park, Florida, USA. (Photo by J. H. Robinson/Photo Researchers, Inc. Reproduced by permission.)
The European pond turtle (Emys orbicularis) is native to Spain, France, northern Italy, southern Germany, Poland, Turkey, Iran, Russia, Morocco, and Algeria. (Photo by Henri Janssen. Reproduced by permission.)

trade) are responsible for declines in most species. No species demonstrates the destructive effect that human exploitation may have on a turtle population better than the diamondback terrapin (Malaclemys terrapin), which once faced extinction throughout its range due to overcollection for human consumption. This turtle recovered as it fell out of favor with the wealthy.

A river cooter hatchling (Pseudemys concinna). (Photo by Animals Animals ©Mella Panzella. Reprodced by permission.)

Significance to humans

Many species are prominent in the international pet trade; the red-eared slider (Trachemys scripta elegans) has been the world's pet turtle for several decades. The hatchlings are exported by the tens of thousands from ranching operations in Louisiana. This turtle has established breeding populations throughout the world and is considered an invasive pest because it may harm native species. A few species are consumed by humans locally.

Gilliam -Harkis ©¿ooz

1. Painted turtle (Chrysemys picta); 2. European pond turtle (Emys orbicularis); 3. Pond slider (Trachemys scripta); 4. Female diamondback terrapin (Malaclemys terrapin); 5. Eastern box turtle (Terrapene carolina); 6. Female spotted turtle (Clemmys guttata). (Illustration by Gillian Harris)

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