CLASS AMPHIBIA
Amphibia refers to "double life", or life in water and on land
Includes the salamanders, frogs, toads, and caecilians with approximately
3,900 spp.
Characterized by:
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Being tetrapods (4 limbs) that facilitate moving about on land - these
limbs evolved from the pectoral and pelvic fins
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Skin is thin, soft, glandular and magid (lack scales except in the caecilians)
- skin of caecilians with scales similar to those of fish
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Ectothermic
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Both gill and lung breathers - usually gills in the larval stage, replaced
by lungs in the adult; cutaneous respiration in many
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Three-chambered heart with two atria and one ventricle
Amphibian reproduction variable
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fertilization may be internal or external
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egg-layers (oviparous) but may have modifications associated with development
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egg anamniotic - doesn’t have a shell but covered with a series of gelatinous
layers
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hatchling in aquatic larval form which breath by gills
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most larvae herbivorous, some omnivorous to carnivorous
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larval stage may last from 10 days to 20+ years
Amphibians are
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unable to regulate body temperature
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skin types limit distribution to warm, moist climates (i.e. tropics, subtropics,
and temperate zones)
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most US species in the south; not found in dry areas
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generally limited to freshwater lakes, streams, ponds - none are true marine
forms
Amphibians are represented by three subclasses
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Subclass Labrinthodontia - looked like a salamander
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represented by genus Ichthyostega (fish with feet or legs)
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identified by specific labronthodont tooth
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Subclass Lepospondyli
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named for vertebral type with three fused parts
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Subclass Lissamphibia - contains all living specimens
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where living amphibians evolved is not clear - modern (living) amphibians
appeared in Triassic
Three primary orders of Amphibia within the Subclass Lissamphibia
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Caudata (Urodela) - Salamanders
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Anura (Salientia) - Frogs and toads
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Apoda (Gymnophiona) - Caecilians
Order Caudata - Salamanders
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"bearing a tail"
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~340 species worldwide, 9 Nearctic families, 6 in US - most holarctic (N.
hemisphere)
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have four limbs usually of equal size, a tail, and elongated body
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skin is smooth and glandular with mucus and poison glands
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found primarily under leaf litter, in soils, or may be fully aquatic
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most have internal fertilization by way of a spematophore (packet of sperm
deposited by the male which is picked up by the female following courtship;
gelatinous coating dissolves to release sperm
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all predaceous - none herbivorous: feed on insects, worms, etc.
In southern US reproduction occurs primarily in the winter
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come out of burrows, mate, and return to burrows
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eggs develop in ponds
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increased rainfall seems to stimulate breeding
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most species breed annually although some breen only every two years
Sexual dimorphism limited - little differences between males and females
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cloacal glands enlarge during reproductive activity to contribute mucus
layers to spermatophore
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male skin slightly rougher and caudal fins become longer
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hedonic glands in male
90% of all salamanders utilize spermatophores for internal fertilization
Egg and larval development may follow one of five general pathways
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Both egg and larval stages aquatic
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Necturidae, Amphiumidae, most Ambystomatidae permanently aquatic
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Eggs terrestrial, larvae aquatic
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Ambystoma opacum (marbled salamander) lay eggs in depressions which
fill with rainfall; eggs hatch as aquatic larvae
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eggs terrestrial, larvae terrestrial
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Plethodontid Desmognathus aeneus - larval form does not feed before
developing into adult
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eggs terrestrial with direct development
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Plethodontids which develop from egg directly through larval stage to adult
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eggs retained in the oviduct and fully develop, followed by live birth
Reproductive strategies based on site of oviposition
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in open static water (lentic systems) - sirens and salamandrids
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in running water (lotic systems) - some plethodontids
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eggs deposited under rocks, debris
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terrestrial sites - Plethodontids
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deposited away from water under logs, rocks, leaf litter
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eggs of salamanders typical for amphibians
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mesolecithal (~50% yolk) surrounded by vitelline membrane and gelatinous
capsule
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sperm typically have acrosome, headpiece, and tail
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acrosome contains some golgi and important for fertilization
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headpiece contains the nuclear material
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tail consists of flagella for locomotion and mitochondria in midpiece
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sperm delivered by spermatophore - stalk supporting sperm cap 2-10mm above
substrate
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stalk secreted by cloacal glands; sperm cap by pelvic glands
Family Cryptobranchidae - hellbenders
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"hidden gill"
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contain the largest living salamanders
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distributed in eastern US, Japan, one species in China to 9’
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Cryptobranchis alleganiensis found in Appalachians of Kentucky/Tennessee
Family Ambystomatidae - mole salamanders
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restricted to US and Canada
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represented by marbled, tiger, and small-mouthed salamanders
Family Amphiumidae - congo eels
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only three species in the world
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named for the number of toes
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Amphiuma tridactylum in Walker County
Family Plethodontidae - lungless salamanders
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breath entirely through the skin and floor of the mouth
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many New World species which may breed on land
Family Necturidae (Proteidae) - mudpuppies
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represented by five species in the US all in the genus Necturus
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permanently aquatic salamanders which retain functional gills as adults
Family Salamandridae - newts
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US and European distribution
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rough-skinned with no costal grooves
Family Sirenidae - sirens
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3 species restricted to the eastern US - lesser, greater, and dwarf sirens
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Siren intermedia (lesser siren) in Walker Co.)
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permanently aquatic with front legs and no back legs, external gills
Order Anura - frogs and toads
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~3,400 species
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have four limbs usually of unequal size - hind limbs are elongated and
modified for jumping
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head and trunk fused, tail in larval form lost as adult
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more widespread than the salamanders
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vocalizations are restricted primarily to the male for courtship, establishing
territories - may be resonated by a vocal pouch
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most have external fertilization where eggs and sperm are shed directly
into the water - general reproduction patterns follow that of salamanders
Generalized reproductive pattern
males vocalize to attract females
amplexus (clasping of the female by the male for mating) brings the
cloacas of the male and female together
external fertilization of eggs by sperm; both deposited in the water
tadpoles develop - length of development depends on the species
metamorphosis
adults predaceous - insectivorous/carnivorous
Two primary reproductive positions:
Axillary - male grasps the female just behind the forelimbs
Inguinal - male grasps the female around the waist just in front of
the hind limbs
remain in position until eggs and sperm are released for fertilization
Family Ascaphidae - tailed frog
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represented only by Ascaphus truei in mountainous Pacific NW
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male retains its tail as an intromittent organ as an adaptation for mating
in fast-flowing streams - copulation may last for 24-30 hours, sperm viable
for up to 2 years
Family Pelobatidae - spadefoot toads
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eye with an elliptical pupil; expanded horny digger on hind feet
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primarily live underground and come out at night
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breed in temporary pools and young may develop from egg and larval stage
in 12 days
Family Leptodactylidae - narrow-toed toads
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SW US and Rio Grande River valley
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most species tropical laying eggs on land
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frog-like in appearance with prominent tubercles on underside of toes near
joints
Family Bufonidae - true toads
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~350 species worldwide - all 17 US species in the genus Bufo
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skins very warty with partid glands behind the eyes - secretes bad tasting
poisonous substance as a protection against predators
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shorter legs designed for hopping rather than leaping
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typically burrow at night
Family Hylidae - tree frogs
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~700 species worldwide; Acris, Hyla, and Pseudacris
in Walker Co.
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Hyla are the true tree frogs with large adhesive toe pads and very
little webbing between the toes - arboreal
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Pseudacris mostly ground dwellers with almost webless toes and very
small or no toe pads
Family Ranidae - true frogs
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small to large frogs with long legs and slim waist
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large, distinct eardrum
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broadly webbed hind feet and usually with a prominent ridge down each side
of the back
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aquatic to terrestrial with aquatic larvae
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represented by the genus Rana
Family Myrohylidae - narrow-mouthed toads
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small, chunky anurans with a smooth skin, a small head with a pointed snout,
and short limbs
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arboreal, terrestrial, or burrowing
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terrestrial larvae may be nonfeeding
Primary skeletal modifications in Anurans
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a general reduction in skull and girdle elements
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pelvic girdle attackes to a single sacral vertebra
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single cervical vertebra (atlas) articulates with two occipital condyles
- no second differentiated axis
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no differentiation in the thoracic and lumbar vertebrae anterior to the
urostyle followed by a single sacral vertebra
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ribs if present never articulate with the sternum
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distal limb bones fused - tibiofibula and radio-ulna
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increased joints in pelvic limbs to increase jumping ability
Order Apoda (Gymnophiona) - Caecilians
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name means “without legs” - a worm-like animal
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subterranean diggers
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~160 species restricted to tropics and subtropics
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eyes may be covered by skin or even bone
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tentacle of head may serve as a chemosensory organ to detect underground
prey
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vivipary common - young 30-60% of female’s body size when born
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initial growth of fetus supported by yolk sac, embryos feed on uterine
milk secreted from the epithelial wall of oviduct
Generalized amphibian integument
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earliest amphibians probably covered with scales - still apparent in some
cacaelians
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epidermal layers highly glandular although glands actually located in dermis
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mucus secretions for reduction of water loss
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venom glands - poisonous or toxic substances
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hedonic glands - male head or chin - pheromonally stimulates female to
clasp and reproduce
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cornified layers shed in pieces
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males usually lacking claws, some with cornified extensions similar in
appearance
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colors variable, produced by lipophores (lipid-soluble, yellow-red), melanophores
(melanin, tawny-black), guanophores (guanine crystals, clear-white), ameboid
(change shape and location to produce color variation)