Delivered By

SHITTU OlalereDept of Zoology

University of Ilorin, Nigeria

AMPHIBIANS – A TetrapodAMPHIBIANS – A Tetrapod

I. Movement Onto Land

• Amphibians are vertebrate transition to land; other organisms like plants, gastropods, and arthropods made transition earlier

• Since organisms made mostly of water, dangerous transition

• Also had to adapt to different oxygen content, density, temperature regulation, and habitat diversity

All sorts of “amphibians” in the fossil record

II. Evolution of Terrestrial Vertebrates• A. Devonian Origin

• Mild temperatures, and periods of flooding or droughts creating unstable sources of freshwater

• Fish with lungs were better able to survive• Early fish fossils that could crawl along mud

with “walking fins:” Eusthenopteron, Acanthostega, and Ichthyostega.

• Land adaptations include: skull, teeth, stronger pectoral and pelvic girdles, jointed limbs, stronger backbone, muscles to support body in air and elevate head, more protective rib cage, ear structure, and longer snout

• Many fossils have more than 5 digits

“Fish” Tetrapod

?

Limbs with digits

strengthened limb girdles

† Acanthostega

Early tetrapod trackway

1st Terrestrial tetrapods – Ichthyostegans, Labyrinthodonts

Ichthyostega

Labyrintodonts

1st terrestrial tetrapods

B. Carboniferous Radiation• Uniformly warm and wet• Tetrapods radiated in swampy, fern

areas eating insects, larvae, and invertebrates

• Temnospondyls form lineage from which modern amphibians are derived; they have 4 digits on forelimbs

• Became better adapted to aquatic life; bodies flattened, some like salamanders developed weaker limbs and stronger tails, and frogs developed webbing on hind limbs for better swimming

Internal choanae (nares)

III. Modern Amphibians (Class Amphibia)

• A. Diversity• Over 4200 species• Ears redesigned• Remain tied to water since eggs are laid

in water and larvae have gills• Thin skin loses moisture rapidly

restricting them to moist habitats• Ectothermic which also restricts habitat

and range

B. Caecilians: Order Gymnophiona

• 160 species live in tropical rainforests of South America, Africa, and Southeast Asia

• Elongate, limbless, and burrowing• Some larvae develop in folds of body

and in others develop in oviduct, eating it for nourishment

C. Salamanders: Order Caudata

Characteristics• 360 species living in northern temperate

areas • Most are small, under 15 cm, but Japanese

giant salamander is 1.5 m long• Limbs are at right angles of body, with fore

and hind limbs of equal length• Burrowing and some aquatic species have

lost limbs• Carnivorous, eating high fat and protein

foods so do not store much fat or glycogen

C. Salamanders: Order Caudata

• Aquatic:

• “Taste” chemicals in water

• Vibrations – Retain lateral line

system

• Terrestrial:

• Smell – Olfactory epithelium (volatile)

• Smell – Vomeronasal organ (Non-

volatile)

C. Salamanders: Order Caudata

• Sexually dimorphic (larger in males)

• Courtship / Identification (species / sex /

individual)

• Highly advanced in Plethodontidae

• Nasolabial Grooves = Non-ciliated

grooves; upper lip to nares

• Aids in collection / delivery of

chemical cues (capillary action)

• Sexually dimorphic (esp. during

reproductive season)

Amphibians

• Eel-like; maintain four reduced limbs

• Paedomorphic; lack gills (lungs present)

Morphology:

Habitat:

• Sluggish streams / rivers; swamps

Reproduction:• Internal fertilization; female nest guarding

• terrestrial egg-laying

• aestivate

Aquatic:

SE United States

Amphiumidae (Amphiums):

• Paedomorphic; feathery gills / caudal fins

Morphology:

Habitat:

• Lakes / streams

• Limestone caves (drastic reduction in numbers)

• dissection specimen

• nocturnal

Reproduction:

• Internal fertilization; ♂ / ♀ nest guarding

Proteidae (Waterdogs):

Aquatic:

Eastern United States / SE Europe

• Constriction at tail base = tail autotomy

• Very costly (tail = large fat reserve)

• ~ 10 – 12% tail regeneration (~ 2 years)

Anatomy:

• Prefers cool forests with litter / dead trees

• Tolerates logging better than most plethodons

Plethodontidae (example = Ensatina escholtzii):

Habitat:

• Mating begins (Nov.) and ends (March) with rains

• ♂ use hedonic glands / pheromones to attract ♀

Reproduction:

• Methods of pheromone administration:

• Slap ♀ nares with hedonic gland (video)

• Innoculate ♀ with sharp teeth

• Elaborately patterned courtship “dance”

• “Tail straddle-walking” (unique to plethodons)

• ♂ repeats dance (~ 1 – 5 hours)

• Pathway / movement unique to each species

Amphibians

Terrestrial:

North / Central / South America & Europe

Pheromone:

Chemical cue that affects thebehavior and / or physiology

of a conspecific

• ♂ lays down spermatophore; ♀ picks it up (video)

Reproduction:

• ♀ guards eggs ( predation / fungal infection)

• Direct development of eggs (~60 – 120 days)

Growth / Maturation:

• Sexual Maturation ~ 4 years

• Life Span ~ 10 years (best guess)

Amphibians

Terrestrial:

North / Central / South America & Europe

Plethodontidae (example = Ensatina escholtzii):

Sperm Cap

Gelatinous Base

• ♀ lay 8 – 12 eggs (~ 5 mm)

• Terrestrial – in logs; under logs; in burrows

• Cold, clear streams / seepages / waterfalls

• Very desiccation intolerant

• Low heat tolerance (susceptible to logging)

Rhyacotritonidae: (example = Rhyacotriton olympicus)

Amphibians

Terrestrial:

NW United States

• Full metamorphosis

• No operculum / opercular muscle ( hearing)

• Highly reduced lungs (cutaneous respiration)

Anatomy:

Habitat:

Growth / Maturation:

• Sexual Maturation ~ 4 years

• Life Span ~ 10 years (best guess)

Reproduction:

Anti-predator Display

• Internal fertilization

• Spermatophore deposition

• Tail-wagging display

• Aquatic egg-laying

• No nest guarding

Larval Anatomical Specializations:

Pond Larvae Stream Larvae

AmbystomatidaeSalamandridae

Dicamptodontidae RhyacotritonidaePlethodontidae

• High body profile

• broad tail fin extending up to head

• long, plume-like gills

• Only front legs at hatching

• Depressed body profile

• Narrow tail fin (not onto trunk)

• Short, thread-like / curly gills

• All four legs at hatching

Amphibians

• Prefer older growth forests during “newt” phase

• Flashes tail & brightly colored stomach

Warning Coloration:

Salamandridae (example = Taricha granulosa):

• Tarichatoxin (non-protein – VERY poisonous)

• Neurotoxin – blocks NA+ channels (paralysis)

• Small dose lethal to birds / mammals

Chemical Defense:

Habitat:

Red-spotted Garter Snake“The Arms Race”

• Large dose lethal to humans

Amphibians

Terrestrial:

North America / Europe / NW Africa / Asia

C. Salamanders: Order Caudata

• Hearing – limited high frequency (no ear

drum); primarily ground vibration

• Vision – acute; especially in

Plethodontidae

• Eel-like; lack hind limbs (lack pelvic girdle)

• Paedomorphic; gills present

Morphology:

Amphibians

Habitat:

• Swamps / lakes / marshes (slow-moving water)

• Nocturnal

• Vocalization

Sirenidae (sirens):

Aquatic:

SE United States / NE Mexico

Aquatic:

Central China / Eastern United States

Cryptobranchidae (Hellbenders):

• Dorsal-ventral compressed body (paedomorphic)• Multiple folds in skin (cutaneous respiration)

Morphology:

Habitat:

• Clear, cold mountain streams / lakes

• Severely reduced ranges (loss of habitat)

Reproduction:• External fertilization; female nest guarding

Reproduction:

• External fertilization; Males = “den masters”

• largest individuals

• live 50+ years

Salamanders

Breeding Behavior• Fertilize eggs internally• Terrestrial species deposit egg clusters

under logs or in soft earth; eggs hatch out to mini adults

• Most salamanders lay strings of eggs in water; larvae hatch with gills, and then may turn into terrestrial or aquatic adults

• Newts have “red eft” stage with a terrestrial juvenile, that then turns into aquatic, breeding adult

• Some newts stay entirely aquatic

Respiration• Thin skin easily exchanges gases; also use

mouth cavity• At various stages may also have gills, lungs,

both gills and lungs, or neither• If lungs are used, they are present from birth,

but only become functional following metamorphosis; hold nostrils above water to breathe

• Larvae hatch with gills, and lose them following metamorphosis, along with fin-like tail; if a lineage does not undergo metamorphosis, it retains these characteristics

• Those in terrestrial family Plethodontidae have no lungs and use only skin

Paedomorphosis• Preservation of larval features into

adulthood• Trend found in salamanders• Some never metamorphose, like the

mudpuppy• Others typically may not, but can change if

conditions change (typically in dry conditions)

• Another example is Ambystoma tigrinum, which stays in an axolotyl stage; can metamorphose if treated with thyroid hormone

• Some have partial paedomorphosis, retaining larval characteristics but switching to lungs

D. Frogs and Toads: Order Anura

Characteristics• 3450 species• Evolved during Jurassic Period, 150 mya• All have tailed larval stage but are tailless

as adults; none retain larval characteristics as adults

• 21 families• Family Ranidae, larger frogs of North

America• Family Hylidae, tree frogs• Family Bufonidae, toads

Habitat and Distribution

• 260 species of genus Rana found in temperate and tropical areas

• Frogs and toads are found in damp forested floors, although in tropical rainforests may live entire life in 1 tree

• Swamps, ponds, streams• Frogs and toads are declining

worldwide and becoming patchy in distribution; cause is unknown

Life Cycle

• Solitary except during breeding season• Spend breeding season swimming

around in water• During winter, burrow into mud to

hibernate, using energy from stored fat and glycogen

• Also accumulate glucose and glycerol in tissues to create an “antifreeze” to prevent tissues from forming ice crystals

• Easy prey; protects themselves by concealment, poison glands, and aggression

Integument and Coloration

• Thin, moist, and attached to body in several points

• Epidermis contains keratin; this is thicker in more terrestrial ones

• Inner dermal layer has mucous glands, which secrete waterproofing agents, and serous glands, which secrete poisons

• Dendrobatid frogs from South America secrete highly toxic poisons

Integument and Coloration

• Chromatophores are pigment cells that produce skin color

• Xanthophores are upper pigments with yellow, orange, and red

• Iridophores are middle layer with silvery light-reflecting pigment that gives iridescent quality

• Melanophores are deepest layer with brown or black melanin

• Green color is produced by interactions among all of these pigments

• Frogs can adjust color to camouflage themselves

Skin Pigmentation

Skeletal and Muscular Systems• Well developed endoskeleton of bone

and cartilage with changes to allow for jumping and swimming

• Front of skull is light weight and flattened

• Limbs have 3 joints: hip, knee, ankle• Foot has 5 rays on hind limb, and 4 on

front limb; digits jointed

Respiration• Use skin, mouth, and lungs with skin

being critical during hibernation• CO2 is mainly lost across skin whereas

O2 is mainly absorbed through lungs

• The movement of air into lungs is somewhat passive, depending on movement of throat

Vocalization• As air enters and leaves lungs, passes over vocal cords, on way to vocal sacs

• Both males and females have vocal cords but males have a more developed larynx

• Songs are unique and characteristic of the species

Circulation

• 6th aortic arch present in gills was converted into pulmonary artery to lungs

• Frog heart has 2 atria and 1 single ventricle

• Blood from body enters right atrium and blood from lungs enters left atrium; both deoxygenated and oxygenated blood mix in ventricle

• Valves do control blood though so mainly deoxygenated blood goes to lungs and oxygenated goes to body

Feeding and Digestion• Carnivorous

• Catch prey with tongue that is hinged at front of mouth

• Free end is glandular with sticky secretions that cause prey to adhere

• Teeth are used to hold prey, not to chew or bite

• Digestive tract produces enzymes to break down carbohydrates, proteins, and fats

• Tadpoles are herbivorous and have longer tracts to digest plant material

Nervous System and Senses

• a. Brain• Consists of 3 parts: forebrain (sense of

smell), midbrain (vision), and hindbrain (hearing and balance)

• Forebrain has cerebrum, but not really used. Olfactory lobe most important part

• Midbrain has optic lobe• Hindbrain has anterior cerebellum (not

used) and posterior medulla which controls auditory reflexes, respiration, swallowing, and circulatory system

b. Vision • Dominant sense in many amphibians (except caecilians)

• Eyelids keep eyes moist, protected, and free of dust; upper is fixed and lower has clear nictitating membrane

• Cornea and lens bend light to focus image on retina, with both rods and cones for color vision

• Iris changes to adjust to different light levels

• At rest, frog focuses on distant objects

c. Other Senses

• Pressure sensitive lateral line is only found in larvae and aquatic adults

• Ear is sensitive to airborne sounds; tympanic membrane passes vibrations to structure similar to cochlea

• Chemical receptors in skin, on tongue, and in nasal cavity

Reproduction and Development

• a. Egg• In spring, males migrate to same pond

or stream and call females• This is dependent on temperature,

humidity, and hormonal changes • Male clasps female in water and as she

releases eggs, male discharges sperm over them

• Eggs usually laid in masses• Eggs absorb water and swell;

development begins immediately• Tadpoles hatch in 6-9 days

Frog Life Cycle

b. Tadpoles• Tadpole has horny jaws for grazing

and a ventral adhesive disc for clinging to objects

• 3 pairs of external gills develop into internal gills covered by flap on right side fused to body wall and a spiracle on left side

• Water flows through mouth, over gills, and out spiracle

c. Metamorphosis

• Hind legs appear first; forelimbs are temporarily hidden by operculum

• Tail is reabsorbed• Intestine becomes shorter• Mouth transforms into adult version• Lungs develop and gills reabsorbed

Metamorphosis

Problems with Terrestrial Living

Solutions

Solutions

Senses

Olfaction and hearing

Hearing

Solutions

Senses

SolutionsRespiration

lungs and skin

Solutions

Circulation

Single pump / single circuit system

FishesCirculation

Tetrapods - primitive

Separate respiratory and systemic circuits

lungs

body

Circulation