Apalone mutica

Geographic Range

Smooth softshell turtles are native to temperate areas of North America, ranging throughout the central and south-central United States. Their range extends from Pennsylvania to New Mexico and south to the Florida panhandle. This species is thought to have been extirpated from Pennsylvania. Smooth softshell turtles are believed to have occurred in North America since the Cretaceous Period, although this is not yet supported by fossil evidence. Smooth softshell turtles have also been introduced to France.

Two subspecies of smooth softshell turtles have been identified. Midland smooth softshells, Apalone mutica mutica , are found throughout the central United States, while Gulf coast smooth softshells, Apalone mutica calvata , range from Louisiana to the panhandle of Florida.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • introduced introduced

Habitat

Smooth softshell turtles are typically found along major riverine systems such as the Ohio, Missouri, and Mississippi rivers. They prefer large rivers and streams with medium to fast currents . Within river systems, smooth softshell turtles had highest populations in the open side channels and main channels of the Mississippi and Ohio Rivers in southern Illinois. Despite their preference for riverine systems, smooth softshell turtles have also been found in lakes, bogs, ponds, and drainage ditches. They generally prefer areas of sandy or mucky bottoms with little aquatic vegetation, and they avoid aquatic systems with a rocky substrate.

Smooth softshell turtles have microhabitat preferences within the Kansas River in Kansas. Males were observed more frequently than subadult and adult females in shallow depths along the river. Hatchlings also preferred shallow areas along the river, most likely to avoid larger aquatic predators.

Of the subspecies of smooth softshell turtles, Gulf coast smooth softshells ( Apalone mutica calvata ), have been found only in riverine and stream systems, while midland smooth softshells, ( Apalone mutica mutica ), have also been observed inhabiting lakes, bogs, drainage ditches, and ponds.

• Habitat Regions
• temperate temperate
• freshwater freshwater

• Aquatic Biomes
• lakes and ponds
• rivers and streams

• Wetlands
• marsh marsh
• swamp swamp
• bog bog

• Other Habitat Features
• riparian riparian

Physical Description

Smooth softshell turtles are medium to large freshwater turtles. Females have a back shell that is 16.5 to 35.6 cm long. Females are larger than males, who have a shell that is 11.5 to 26.6 cm long. Like other softshell turtles, smooth softshell turtles have a carapace that is covered by skin instead of the hard scutes commonly observed in other turtle species. The carapace is ovoid and lacks spines on the front edge. The coloring of the carapace ranges from olive to orange. Females typically have a tan or brown carapace, while males have a brown or gray carapace. Both sexes have dark markings (spots, streaks, or blotches) on their carapace, although females typically have a blotchier pattern. The plastron is light (white or gray) with no markings, and the underlying bones are visible. Dorsal coloration of an individual's head, limbs, and tail are similar to that of its carapace. A cream or orange line bordered in black extends from the back of each eye to the neck. Juvenile smooth softshell turtles do not differ in coloration from adults, but juveniles of subspecies are used to tell them apart in the field. Hatchlings have a brown or olive carapace with many markings on the carapace.

Smooth softshell turtles have a tubular snout with round nostrils that are usually positioned inferior, and they lack a septal ridge. Male smooth softshell turtles have thicker tails than females, a trait commonly observed in turtle species. In males, the anal vent is located near the tip of the tail, while in females, the anal vent is usually located near the edge of or under the carapace. As with other species of turtles, female smooth softshells have longer hind claws than males, which have longer foreclaws than females.

Smooth softshell turtles are the most aquatic of all North American softshells. Their aquatic nature is made possible by a variety of behavioral and morphological adaptations. The skin covering the shell causes a high rate of water exchange. Water exchange in freshwater is typically 6.3 mL/100g wet BM/hour. The lining of the cloaca and pharynx enables uptake of oxygen from the water. Furthermore, the surface of their shell and skin is cutaneous, increasing the permeability of gases and water. For instance, smooth softshell turtles lose about 64% of respiration-produced carbon dioxide through their skin. Their long neck and snout contribute to the ability to remain submerged for extensive periods of time.

Smooth softshell turtles are mostly easily confused with spiny softshell turtles ( Apalone spinifera ). The two species, however, can be distinguished by examining the carapace. Spiny softshells have spines along the front edge of their carapace, and the carapace usually has a sandpaper texture. Smooth softshells lack these characteristics. Spiny softshells, as with most softshell turtles, also have a septal ridge in the nose. However, smooth softshells lack a septal ridge. Distinguishing features of other softshell species include heavily marked limbs and spines on the carapace.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• female larger
• sexes colored or patterned differently

Development

Embyonic development of smooth softshell turtles is categorized into three stages: division of cells and tissue development, organogenesis, and the growth of the embryo before the hatching stage. The timing of these three stages is related to three equal corresponding periods during incubation. Sex determination is not temperature dependent, and relatively equal proportions of hatchlings are male and female.

With time, the round shape of a hatchling smooth softshell turtle's carapace changes to an oval shape through allometric growth. There is no change, however, in plastron shape as the hatchling grows, making for little change in the straight carapace length/plastron length (SCL/PL) ratio. Once the plastron has reached a minimum length of 60 mm, individuals can be sexed using physical characteristics described above. Unlike many other species of turtles, smooth softshell turtles do not form growth annuli on their shells, making them difficult to age in the field.

Growth of smooth softsell turtles typically occurs from May to September, and growth rates are usually highest from June to August. Males grow on average 1.95 mm/month when between lengths of 61 and 65 mm. At a plastron length (PL) of 66 to 75 mm, males grow at a rate of 2.1 to 2.5 mm/month. This rate decreases to 0.09 mm/month when males reach a PL of 111 to 115 mm. Females exhibit a faster growth rate than males. In females with a PL of 61 to 70 mm, growth rate is 2.8 mm/month. This decreases to 0.7 mm/month at a PL of 151 to 160 mm. Most species of turtles experience indeterminate growth, although this has not verified for this species.

• Development - Life Cycle
• indeterminate growth indeterminate growth

Reproduction

Breeding activity of smooth softshell turtles occurs from April to June and possibly into September. Males seek out females by approaching other adults. If the approached individual is male, the response to the investigation is usually passive but can be aggressive on occasion. Non-receptive females are often aggressive; during the breeding season many males are observed with wounds inflicted by females. Receptive females, on the other hand, are passive to the advances of a male. Many males may be present near a receptive female, and all may attempt to mate with her. When there is only one male present, the female may chase the male.

Copulation almost always occurs in the water, and a sexually receptive female is mounted from behind. Deep water is usually needed to successfully mount a female and a male must swim in place for upwards of 20 min to keep his position. During this time, the male needs to keep its vents aligned.

• Mating System
• polygynous polygynous

Smooth softshell turtles nest between late May and July. In more northern locations such as Minnesota and Wisconsin, nesting typically takes place from June to early July. Any disturbance can cause early abandonment of the nest by wary females. Females dig a nest cavity 15 to 30 cm deep with their hind feet. Nests are typically located on a sand bar with little vegetation, although some nests have been found in dense vegetation. Nests are generally located within 18 m of the water and only rarely are more than 30 m away from water. Nests are usually 0.5 to 6.1 m above the water. Nest density can be quite high, and in some cases there are connecting chambers between nests. Oviposition usually occurs in the early morning. Eggs are laid in two layers in the nesting chamber. Females cover their eggs with sand using their hind feet. A female then usually burrows a tunnel and places herself on the opposite end of the nest. This tunnel can be up to 4 m in length.

Female smooth softshell turtles lay only one clutch per year. Average clutch size is 15 to 25 eggs. The minimum observed clutch size is 1 egg while the maximum clutch size observed is 33 eggs. Clutches laid later in the season are usually smaller in size than early season clutches. The size of a clutch is proportional to the straight carapace length (SCL) of the female. Eggs are a spheroidal shape and resemble ping-pong balls. In a typical clutch, 75% of eggs are likely to survive. Predation and flooding can have large effects on egg survival.

Eggs generally hatch in 8 to 12 weeks. Hatching frequency is highest in August to September. Hatchlings use their front claws to break through the egg, relying on their claws more than their caruncle (egg tooth), which is less used in comparison to other species. Hatchlings generally emerge from the nest around sunset. At the moment of hatching, hatchlings are completely independent. Recently hatched smooth softshell turtles average 4 cm in straight carapace length and have a body mass of 3.0 to 7.5 g (mean 5.4 g). Caruncles drop off in a week, and the umbilical scar is usually 2 mm in diameter.

Male smooth softshell turtles become sexually mature during their fourth year with mean plastron length (PL) ranging from 80 to 85 mm. Females become sexually mature during their ninth year with mean PL ranging from 140 to 150 mm. Mature individuals attempt to mate immediately upon emergence from hibernation. The volume of mature male testes varies with season, with testes reaching a maximum diameter immediately before hibernation. In April and May, the vas deferens of males become swollen, 2 mm in diameter. The vas deferens is no longer swollen by June but become swollen with sperm once more by October. Vitellogenesis in females begins in July in Kansas.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• oviparous oviparous

Smooth softshell turtles provide prenatal care for their offspring. Females produce high levels of non-polar lipids that provide energy for their growing embryos. They produce energy at a level much higher than what is necessary to keep the embryos alive, which is known as parental investment in embryogenesis. At birth, hatchlings have high concentrations of lipids. These act as a food source until they are mature enough to begin feeding. This allocation of energy into high storage reserves for hatchlings represents parental investment in care. This provisioning of lipids enables hatchlings to survive in areas that have low resource availability.

• Parental Investment
• pre-hatching/birth
  • provisioning
    • female

Lifespan/Longevity

Unlike many other species of turtles, smooth softshell turtles do not form growth annuli on their shells, which makes them very difficult to age in the field. The lifespan of this species is unrecorded. Individuals in captivity have lived over 11 years, and they are believed to be capable of living 20 years. Florida softshell turtles ( Apalone ferox ) and spiny softshell turtles ( Apalone spinifera ) can live up to 25 years in captivity.

Behavior

Smooth softshell turtles are the most aquatic of all North American softshells. Behavioral and morphological adaptations permit their aquatic nature. Smooth softshell turtles are able to remain submerged for extensive periods of time, which is made possible in part by their long neck and snout. They often bury themselves in the substrate of a body of water deep enough so their snout just barely reaches the surface. To achieve this position, a smooth softshell turtle pushes itself head-first into the sediment; it pulls itself with its frontlimbs while pushing with its hindlimbs. When its body is positioned tilting downward toward the front, it stirs up the sediment. The falling material covers the turtle so only its head is visible.

Smooth solfshell turtles hibernate by burying themselves in substrate underwater. They emerge from winter hibernation in March or early April in Kansas and in May in northern areas such as Minnesota and Wisconsin. Activity of smooth softshell turtles is typically observed from May through September. In more southern regions such as New Mexico, members of this species are usually active from April to October. After emerging from hibernation, smooth softshell turtles are often observed basking on sand bars or in shallow water between 0700 and 1700 hours. Sandy and muddy bars within a few meters of the water's edge are preferred basking sites, although logs and rocks near the water may also be used. When basking, smooth softshell turtles extend their neck and tuck their limbs into their shell.

Smooth softshell turtles are wary and abandon their basking place if danger is perceived. They are highly agile, both in water and on land, which permits quick escape from predators as well as feeding on fast-moving prey. Members of this species are generally solitary.

• Key Behaviors
• natatorial natatorial
• diurnal diurnal
• motile motile
• hibernation hibernation
• solitary solitary

Home Range

Communication and Perception

Smooth softshell turtles primarily interact through visual and tactile cues. When seeking out mates, males physically investigate females. Although little information was found regarding perception and communication by this species, a close relative, spiny softshell turtles ( Apalone spinifera ), perceives its environment using chemical, visual, and tactile cues, vibrations and communicates using tactile cues.

• Communication Channels
• tactile tactile

• Perception Channels
• visual visual
• tactile tactile
• vibrations vibrations
• chemical chemical

Food Habits

Smooth softshell turtles are carnivorous, eating a variety of organisms including fish, amphibians (adults and larvae), arthropods, spiders ( Araneae ), snails ( Campeloma ), mollusks, isopods ( Isopoda ), millipedes ( Diplopoda ), and worms ( Annelida ). Although A. mutica is a dietary generalist, it can be classified as an insectivore. Arthropods typically consumed by smooth softshell turtles are aquatic and larval forms of Coleoptera , Diptera , Calliphoridae , Chironomidae , Cyclorrhapha , Empidae , Muscidae , Ephemeroptera , Tipulidae ( Tipulo bicornis ), Hemiptera , Hymenoptera , Homoptera , Isoptera , Lepidoptera , Ichneumonidae , Odonata , Neuroptera , Orthoptera , Plecoptera , and Trichoptera . Fish species consumed by smooth softshell turtles include Castostomus commersoni , Cyprinella whipplei , Cyprinella spilopterus , Lepomis macrochirus , Morone chrysops , Hypentilium nigricans , Perca flavescens , and Salvelinus fontinalis . Although primarily carnivorous, smooth softshell turtles occasionally eat vegetation such as algae, potatoes, seeds, stems, mulberry ( Morus ), fruits, and hard nuts.

Smooth softshell turtles hunt on both land and in water. They are ambush predators; while concealed in substrate, they use their long neck to grab passing prey. They have also been observed pharyngeal gulping, in which they suck in nearby small prey organisms. They also use their nose to seek food in sediment and vegetation. Females generally utilize deeper water to obtain food, while males tend to forage in shallow water near the shore. Although foraging area differs between sexes, food size does not vary with size or sex.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore

• Animal Foods
• birds
• mammals
• amphibians
• fish
• carrion carrion
• insects
• terrestrial non-insect arthropods
• mollusks
• terrestrial worms
• aquatic crustaceans

• Plant Foods
• leaves
• roots and tubers
• wood, bark, or stems
• seeds, grains, and nuts
• fruit
• algae

Predation

Due to their high agility on land and water and these avoidance strategies, adults have few natural predators. Humans and alligators are the main predator of adults. Predators of hatchlings include fish, other turtles ( common snapping turtles , alligator snapping turtles , possibly adult Apalone ), water snakes , shoreline birds, bald eagles , and other mammals. Nest predation is usually from raccoons , skunks ( Memphitis and Spilogale ), crows , fire ants ( Solenopsis invicta ), fly larvae ( Sarcophagidae ), dogs , red foxes , moles ( eastern moles , and other small mammals.

Smooth softshell turtles are wary and abandon their basking place if danger is perceived. They are highly agile, both in water and on land, which permits quick escape from predators. They also use their strong diving ability to flee predators and conceal themselves in mud. When caught by predators, a smooth softshell turtle pulls itself into its shell.

Ecosystem Roles

Smooth softshell turtles are aquatic predators and fall prey to a variety of organisms. Eggs of this speices are parasitized by fly larvae ( Sarcophagidae ).

Economic Importance for Humans: Positive

In some regions, adults and eggs are gathered for food. Additionally, this species is occasionally a part of the pet trade.

• Positive Impacts
• pet trade pet trade
• food food

Economic Importance for Humans: Negative

Although smooth softshell turtles may prey upon game fish, their impact on fish populations is believed to be insignificant. There are no known adverse effects of smooth softshell turtles on humans, although they may bite when handled.

• Negative Impacts
• injures humans
  • bites or stings

Conservation Status

Although smooth softshell turtles are listed as a speices of least concern by the IUCN, little data is available to make a confident assessment. Currently, this species is widespread and locally common, suitable habitat is present, and harvest rates do not contribute to decline. Smooth softshell turtles are listed as a species of special concern in the state of Minnesota. Their slow maturation rate increases time necessary to restore populations.

Their ability to absorb oxygen from the water has made smooth softshell turtles susceptible to water pollution. Other negative impacts include habitat degradation, harvesting for food, changes in hydrological regimes (construction of dams and locks), and an increase in human disturbances at nesting sites. Human disturbance includes boating near nest sites, with waves resulting from boat use causing erosion and egg exposure. This species is also caught as bycatch (capture of non-target species) in the commercial fishing industry.

Efforts to conserve smooth softshell turtle should include the protection of waterways and surrounding terrestrial land that are known to support large populations of this species. Upstream sites should also be protected. Protected areas ideally should include basking and nesting habitats with high densities of prey, hibernation areas, and be of ample size to support home ranges.

Other Comments

Apalone mutica was previously known as Trionyx muticus .

Additional Links

Encyclopedia of Life

BioKIDS Critter Catalog

Contributors

Jeana Albers (author), Minnesota State University, Mankato, Robert Sorensen (editor), Minnesota State University, Mankato, Gail McCormick (editor), Animal Diversity Web Staff, Catherine Kent (editor), Special Projects.

References

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