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
- palearctic
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
- freshwater
- Aquatic Biomes
- lakes and ponds
- rivers and streams
- Other Habitat Features
- 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
- 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
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
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
- seasonal breeding
- gonochoric/gonochoristic/dioecious (sexes separate)
- sexual
- 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
-
provisioning
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
- diurnal
- motile
- hibernation
- 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
- Perception Channels
- visual
- tactile
- vibrations
- 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
- Animal Foods
- birds
- mammals
- amphibians
- fish
- 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
).
- 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.
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
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.
- Nearctic
-
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
- native range
-
the area in which the animal is naturally found, the region in which it is endemic.
- Palearctic
-
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
- introduced
-
referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.
- temperate
-
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
- freshwater
-
mainly lives in water that is not salty.
- marsh
-
marshes are wetland areas often dominated by grasses and reeds.
- swamp
-
a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.
- bog
-
a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.
- riparian
-
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
- indeterminate growth
-
Animals with indeterminate growth continue to grow throughout their lives.
- polygynous
-
having more than one female as a mate at one time
- iteroparous
-
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
- seasonal breeding
-
breeding is confined to a particular season
- sexual
-
reproduction that includes combining the genetic contribution of two individuals, a male and a female
- oviparous
-
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
- natatorial
-
specialized for swimming
- diurnal
-
- active during the day, 2. lasting for one day.
- motile
-
having the capacity to move from one place to another.
- hibernation
-
the state that some animals enter during winter in which normal physiological processes are significantly reduced, thus lowering the animal's energy requirements. The act or condition of passing winter in a torpid or resting state, typically involving the abandonment of homoiothermy in mammals.
- solitary
-
lives alone
- tactile
-
uses touch to communicate
- visual
-
uses sight to communicate
- tactile
-
uses touch to communicate
- vibrations
-
movements of a hard surface that are produced by animals as signals to others
- chemical
-
uses smells or other chemicals to communicate
- carrion
-
flesh of dead animals.
- pet trade
-
the business of buying and selling animals for people to keep in their homes as pets.
- food
-
A substance that provides both nutrients and energy to a living thing.
- carnivore
-
an animal that mainly eats meat
- insectivore
-
An animal that eats mainly insects or spiders.
- ectothermic
-
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
- bilateral symmetry
-
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
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