Softshell turtles

(Trionychidae)

Class Reptilia

Order Testudines

Suborder Cryptodira

Family Trionychidae

Thumbnail description
Medium to very large aquatic turtles with a rounded, flattened carapace covered with leathery skin; fleshy skin on the head that covers the jaws; and streamlined forelimbs with three pronounced claws

Size
4–47 in (10–120 cm)

Number of genera, species
13 genera; 25 species

Habitat
Rivers, streams, lakes, marshes, and temporary ponds

Conservation status
Critically Endangered: 5 species; Endangered: 5 species; Vulnerable: 6 species

Distribution
North America, Africa, Asia, Indo-Australian archipelago

Evolution and systematics

These primitive cryptodires are most closely related to the Carettochelyidae of New Guinea and Australia. The oldest softshell fossil is from the late Jurassic. The recent revision of the phylogenetic arrangement proposed by Meylan has yet to gain universal acceptance and use, but it is generally supported by both morphological and molecular evidence. Two subfamilies are recognized: Trionychinae (without flexible flaps on plastron below hind limbs) and Cyclanorbinae (with flexible flaps on plastron below hind limbs).

Physical characteristics

Softshell turtles gain their name from the leathery layer of epidermis that covers the bony portion of the carapace and extends, in most species, to form a flexible disk overhanging the feet and tail. Flap-shelled species have a limited leathery carapace; however, they have developed retractable, flaplike hinges on the plastron that protect the limbs from below. When considering the entire leathery shell, the diversity of sizes found among softshell species covers an order of magnitude in carapace length. The smallest species attains a maximum size of 5 in (12 cm), whereas adults of the largest species may reach 47 in (120 cm) or more.

Most species possess a long retractile neck; this is especially well developed in the giant ambush-feeding species. However, the head of the Malayan softshell (Dogania subplana) is too large to be drawn completely within the shell. The long, tubelike proboscis found in most species allows them to breathe air from the water's surface without moving from the bottom. A thick layer of fleshy skin covers an incredibly sharp horny beak. The digits are strongly webbed with three claws present on each of the forelimbs. Most species are fairly uniform in coloration, allowing them to blend in with the substrate; however, several species in India and Southeast Asia have unique patterns on the carapace that may consist of broad stripes, vibrant spots, or elaborate designs. The plastron may exhibit two to nine callosities—thickened areas of epidermis overlying the plastral bones that develop as the turtle grows. Plastral callosities vary in number and shape among species and, when comparing adults, may be useful in taxonomic identification.

Distribution

Softshell turtles are found in lowland temperate regions east of the Rocky Mountains of North America, in sub-Saharan Africa and along the Nile River into Egypt, and across the Mediterranean Sea to southern Turkey and the Middle East. They also are present from eastern Pakistan, throughout India and southern Asia along the coast of China into southeastern Russia, and throughout the Indo-Australian archipelago to the southern shores of Irian Jaya, but there are no living species in Australia. The Malayan softshell (Dogania subplana) is the only species known to inhabit streams at high elevations. Trionychids formerly were distributed throughout Europe, South America, and Australia, as well as the three continents where they occur today. The Chinese softshells (Pelodiscus sinensis, Palea steindachneri) have been established by accidental or intentional introductions outside the known range. Extralimital populations of both species, farmed throughout Southeast Asia, have been documented in the Hawaiian Islands.

Habitat

These highly aquatic turtles may be found in all freshwater habitats. Most species prefer clean, well-oxygenated waterways with soft sand or mud bottoms. They commonly inhabit large rivers, streams, lakes, and ponds, but they also may occur in swamps, marshes, temporary ponds, and drainage ditches. A few species tolerate brackish water, but only the Asian giant softshell (Pelochelys cantorii) is a permanent resident of coastal waters.

Behavior

Softshell turtles may remain buried at the bottom of their aquatic habitat for most of the day; however, some species will bask on sandbars or steep riverbanks. Prolonged submergence is possible because they can absorb oxygen across the skin and lining of the throat. This is especially important for temperate species that hibernate for months beneath the ice. Conversely, the flap-shelled turtles may be adapted to long periods of drought. The protective shell enables these turtles to burrow deep into the mud and may prevent desiccation while estivating.

Feeding ecology and diet

Most species are primarily carnivorous; however, plant material may occasionally be consumed. The majority of softshell species are generalists; all forms of animal matter (live or as carrion) are taken opportunistically. A few species are highly specialized for ambushing fish and other free-swimming aquatic animals. Adaptations for this mode of prey acquisition are most highly developed in the Asian narrow-headed softshell (Chitra indica); these turtles have small, forward-set eyes and exceptionally long necks that can be thrust from the sandy bottom with explosive speed and deadly accuracy.

Reproductive biology

Mating generally occurs in the spring; however, some species may store sperm for several years. The spherical eggs are generally laid in sandy banks above the flood line along a river or lake. Clutch size ranges from as few as 3 eggs in some species to more than 100 in others. Some species lay several clutches in a single year. Most species that have been investigated exhibit genetic sex determination; however, heteromorphic sex chromosomes are not present.

Conservation status

Six species are listed as Vulnerable, five as Endangered, and five as Critically Endangered on the IUCN Red List. As with most turtle species endemic to Asia, the softshells of China, Southeast Asia, and India are heavily exploited for food and medicine. Most softshells are dependent upon high levels of dissolved oxygen in their aquatic environment; therefore, they are particularly vulnerable to the effects of habitat destruction and degradation. They are extremely sensitive to rotenone, a chemical that is intentionally added to streams to kill fish, and pesticides (e.g., DDT) in farmland runoff. Human consumption has taken a heavy toll on the populations of many Asian species. Many species are legally protected by local countries, but exploitation continues. Some species are used to meet the demands of food markets.

Significance to humans

Softshells are used for food throughout their range; however, Pelodiscus sinensis is intensively farmed throughout Asia to supply consumer demand in China.

Species accounts

List of Species

Indian flapshell turtle

Lissemys punctata

subfamily

Cyclanorbinae

taxonomy

Testudo punctata Lacépède, 1788, India. Two subspecies are recognized.

other common names

None known.

physical characteristics

This is a small turtle (maximum carapace length 11 in [27.5 cm]) with a relatively deep oval shell. This species is unique among softshells because the posterior margin of the bony carapace is ringed by peripheral bones. The evolutionary origins (primitive or derived) of this feature are unresolved; however, it provides additional protection for the hind limbs, which may be completely retracted when the plastral flaps are pulled tightly against the bony rim of the carapace. Seven plastral callosities develop on the hyoplastra and hypoplastra, xiphiplastra, epiplastra, and entoplastron.

distribution

The Indus and Ganges river drainages of Pakistan, India, Sri Lanka, southeastern Nepal, Bangladesh.

habitat

Shallow backwaters of rivers, ponds, and marshes with a soft muddy bottom, temporary ponds.

behavior

The retractable flaps over the limbs and the thick callosities on the plastron may be adapted to long periods of drought. The protective shell enables these turtles to burrow deep into the mud and may prevent desiccation. They have been observed in India to survive for 160 days while buried during estivation.

feeding ecology and diet

Like many flap-shelled species, these turtles tend to be omnivorous. Small aquatic animals and carrion make up a large portion

of their diet; however, aquatic vegetation is also a common food item.

reproductive biology

Courtship and mating have been observed in April and continue through the summer. The smaller male initiates mating by stroking the female's carapace with his chin. While facing the male, a receptive female extends her neck and they both bob their heads in a stereotypical pattern before settling to the bottom for copulation. The elaborate courtship observed for this species may be rare among softshells, but it is similar to that of many sexually dimorphic species where the larger female chooses her mate. The spherical, brittle-shelled eggs range from 0.9 to 1.3 in (24 to 33 mm) in diameter. Multiple clutches of two to 14 eggs are produced annually.

conservation status

Not threatened.

significance to humans

These turtles constitute the largest volume of any species in Indian food markets. They are consumed locally in Bangladesh, but also may be exported to Chinese markets.

Spiny softshell

Apalone spinifera

subfamily

Trionychinae

taxonomy

Trionyx spiniferus LeSueuer, 1827, New Harmony, Posey County, Indiana, on the Wabash River. Six subspecies are recognized.

other common names

English: Goose-neck turtle, leatherback turtle; French: Tortue luth, tortue-molle à épines; German: Lederschildkröte; Spanish: Tortuga-casco suave espinosa.

physical characteristics

This is a medium-sized softshell with spiny protuberances on the anterior rim of a drab olive carapace. A pattern of dark circles, which is especially distinct among the males, fades in the adult females. At least two pairs of callosities (hyo-hypoplastral and xiphiplastral) are present in adults.

distribution

Northern Mexico across most of the southern, central, and eastern United States to the Great Lakes region of southern Canada.

habitat

Slow-moving rivers, shallow streams, and large ponds and lakes.

behavior

This species may remain buried at the bottom of its aquatic habitat for most of the day; however, it occasionally basks on sandbars or steep riverbanks. The spiny softshell remains submerged for long periods by absorbing oxygen through the skin and lining of the throat while carbon dioxide diffuses across the skin. In northern regions, this species hibernates beneath the ice for several months each winter.

feeding ecology and diet

These turtles are predominantly carnivorous, ingesting all available aquatic animals; however, plant material including acorns and leaves is also consumed. In Iowa, crustaceans, fish, and insects were the most important food items, but plant matter was found in 61% of all turtles sampled.

reproductive biology

Mating occurs in the early spring and eggs are generally laid in June and July. The brittle, spherical eggs are approximately 1.1 in (28 mm) in diameter. At least one clutch of four to 32 eggs is produced annually. The eggs hatch in late summer and hatchlings emerge in the autumn to hibernate underwater.

conservation status

Not threatened. The widespread distribution and relative abundance of this species in its preferred habitats suggest that it is not currently at risk. Because these turtles are dependent upon high levels of dissolved oxygen in their aquatic environment, water pollution may be the most significant threat to softshell populations.

significance to humans

This species may be consumed locally; however, it is collected in large numbers for sale in Asian food markets in large North American cities and for export. Hatchlings are often available in the pet trade.

Chinese softshell turtle

Pelodiscus sinensis

subfamily

Trionychinae

taxonomy

Trionyx (Aspidonectes) sinensis Wiegmann, 1835, small island in the Tiger River near Macao. No subspecies are recognized.

other common names

Japanese: Suppon.

physical characteristics

This is a small to medium turtle (maximum leathery shell length 10 in [25 cm]) with an oval carapace. The shell is relatively smooth; however, the anterior portion of the carapace is studded with blunt knobs. Up to seven plastral callosities may develop on the hyoplastra and hypoplastra, xiphiplastra, and epiplastra. The tubelike proboscis has a horizontal ridge that projects from either side of the septum.

distribution

Widespread in the lowland areas of southern China, northern Vietnam, Hainan Island, Taiwan, and Japan.

habitat

Rivers, streams, lakes, marshes, and rice fields.

behavior

Although this species occasionally basks on the banks, it remains buried at the bottom of its aquatic habitat for most of the day. The Chinese softshell remains submerged for long periods by absorbing oxygen through the skin (33%) and lining of the throat (67%), while most carbon dioxide passively diffuses out across the skin. In Japan this species hibernates from October to April.

feeding ecology and diet

This species is primarily carnivorous, ingesting all available aquatic animals, especially fish and crustaceans; however, the seeds of marsh plants are also consumed.

reproductive biology

Mating has been observed from May to July in Japan. The male uses the claws of the forelimbs to clasp the front rim of the female's shell during copulation and may bite at her neck and limbs. Females begin nesting early in the spring and continue through the late summer. A shallow, boxlike nest is excavated and a clutch of 15–28 brittle, spherical eggs (average 0.8 in [20 mm] in diameter) is deposited. As many as four clutches are produced in a single year. Although incubation is generally completed in 60 days, hatching may occur after 40 to 80 days.

conservation status

Chinese softshell turtles are listed as Vulnerable on the IUCN Red List. Despite the heavy consumer pressure on this species, it is still relatively abundant in eastern China and northern Vietnam.

significance to humans

This species is consumed locally throughout its range; however, it is intensively farmed in Southeast Asia to supply the food and medicine market in China. Hatchlings are reared to subadulthood in two to three years before being shipped for processing into prepared soups, or more commonly to restaurants and markets where they are bought fresh and butchered alive.

Resources

Books

Das, Indraneil. Die Schildkröten des Indischen Subkontinents. Frankfurt, Germany: Chimaira, 2001.

Periodicals

Greenbaum, E., and J. L. Carr. "Sexual Differentiation in the Spiny Softshell Turtle (Apalone spinifera), a Turtle with Genetic Sex Determination." Journal of Experimental Zoology 290 (2001): 190–200.

Meylan, Peter A. "Phylogenetic Relationships of Soft-Shelled Turtles (Family: Trionychidae)." Bulletin of the American Museum of Natural History 186 (1987): 1–101.

Pritchard, Peter C. H. "Observations on Body Size, Sympatry, and Niche Divergence in Softshell Turtles (Trionychidae)." Chelonian Conservation and Biology 4 (2001): 5–27.

Patrick J. Baker, MS