Transcript Amphibians The First Terrestrial Vertebrates

Alexander True, Ryan Adams, Michael
Sinfield-Albets Toler
General Characteristics
Amphibians are present on all continents except for Antarctica.
3,000 modern day species.
• This is a remnant of a once diverse group.
Belong to three orders:
• Caudata (salamanders), Anura (frogs and toads), Gymnophiona (caecilians)
Evolutionary Perspective
Amphibians are animals that either move back and forth between water and
land or live one of their stages of life in water and another on land.
Amphibians are tetrapods “Four-foot”
• The presence of four muscular limbs and feet with toes and fingers
Phylogenetic Relationships
Fossil record provides evidence of many extinct taxa and no one knows what
animal was the first stegocephalian.
Taxonomists agree that amphibians are monophyletic and closely related to
reptiles, birds, and mammals.
Order Caudata
This order consists of salamanders.
Means “tail-bearing”
Characteristics:
• Posses a tail
• Have two pairs of unspecialized legs
• External gills
Habitat:
• Live in moist forest-floor litter
• Spend most of life in water
Reproduction:
•
Internal fertilization
•
Produce aquatic larvae
•
Larvae are similar to adults but smaller
Order Gymnophiona
This order consists of caecilians.
Means “naked like a snake”
Characteristics:
• Worm-like
• Appear segmented because of skin folds that overlie separations between muscle bundles.
• Skin over eyes (most likely blind)
Habitat:
• Burrow into the ground
Reproduction:
• fertilization is internal
• Can have aquatic larvae or embryotic larvae that develop on land
• Young are miniature adults
Feeding:
• Eat soil and worms when they burrow
Anura
This order consists of frogs and toads.
Means “without tail”
About 4,000 species
Characteristics:
•
Long muscular hind limbs
•
Webbed feet
Habitat:
•
Generally moist environments (can really be found almost anywhere)
Reproduction:
•
External fertilization
•
Undergoes metamorphosis from tadpole to adult
Anura Feeding
They mostly eat small insects and
some plants.
They utilize a sticky tongue, located
on their lower jaw, that is extended
from their mouth as a sort of sticky
lasso to catch their prey.
Most frogs sit and wait for their
prey. Their skin colors and textures
perfectly camouflages them in their
surroundings.
External Structure and Locomotion
The skin of amphibians functions in defense, gas exchange, temperature
regulation, and absorption and storage of water.
• Skin is highly glandular, these glands keep the skin moist to prevent drying.
• The glands also produce a sticky secretion that helps them cling to mates during sexual
activities.
• The glands produce toxic chemicals that discourage predators.
The bone structure is set up in a way that makes terrestrial locomotion possible.
• They have joints at the shoulder, hip, elbow, knee, wrist, and ankle allow freedom of
movement and better contact with the substrate.
• Each type of Amphibian has a distinct form of movement Salamanders move like a fish swims
through water, a Caecilian has accordion movement like a snake, and Frogs have strong hind
limbs specialized for jumping.
Locomotion Continued
The terrestrial skeleton is modified to provide support against gravity and it
must be strong enough to support the relatively powerful muscle that propel
terrestrial vertebrates across land.
The amphibian skull is flattened, smaller, and has fewer bony elements than
the skull of fishes.
• This lightens the skull so it can be supported out of water.
Changes in jaw structure and musculature allow terrestrial vertebrates to
crush pray held in the mouth.
Locomotion Continued Continued
Salamanders move in a way that is reminiscent of the undulatory waves that
pass along the body of a fish.
• Terrestrial salamanders also move by a pattern of muscle contractions that throw the
body into a curve to advance the stride of a limb.
Caecilians have an accordion-like movement where adjacent body parts
push or pull forward at the same time.
The long hind-limbs and pelvic girdle of anurans are modified for jumping.
• The dorsal bone of the pelvis (ilium) extends anteriorly and securely attaches to the
vertebral column, and the urostyle extends posteriorly and attaches to the pelvis.
Nutrition and Digestion System
Adult amphibians are carnivores that feed on a wide variety of invertebrates.
• However, some anurans are more diverse. For example, some bullfrogs on small
mammals, birds, and other anurans.
Main factor that determines what amphibians eat is prey size and
availability.
Most larvae are herbivorous and feed on algae and other plant matter.
Most amphibians locate prey via sight and simply wait for prey to pass by.
• Olfaction plays an important role in prey detection by aquatic salamanders and
caecilians.
Nutrition and Digestive System
Continued
Most salamanders use just their jaws to capture prey but anurans and
plethodontid salamanders use their tongues and jaws.
A true tongue is first seen in amphibians.
The anuran tongue attaches at the anterior margin of the jaw and folds back
over the floor of the mouth.
• Mucous and buccal glands on the tip of the tongue exude sticky secretions.
When prey comes within range, an anuran flicks out its tongue and uses it as
a sort of sticky lasso to capture its prey.
Circulation
The amphibian circulatory system shows remarkable adaptations for a life
divided between aquatic and terrestrial habitats.
The pulmonary and systemic circuits are separated.
The atrium is partially divided in urodeles and completely divided in anuras.
There is no ventricle in the septum.
A spiral valve in the conus arteriosus or ventral aorta helps direct blood flow
into the pulmonary and systemic circuits.
Gas Exchange
Because the skin of amphibians is moist and richly supplied with capillary
beds, the skin can act as a respiratory organ.
• Gas exchange across the skin is called cutaneous respiration and can occur in water and
on land.
• In salamanders, 30 to 90% of gas exchange occurs across the skin.
Gas exchange also occurs across the moist surfaces of the mouth or pharynx
which is called buccopharyngeal respiration.
• This accounts for 1 to 70% of total gas exchange.
Most amphibians, except plethedontid salamanders, posses lungs.
The lunges of salamanders are simple sacs and the lungs of anurans are
subdivided which increases surface area for gas exchange.
• The buccal pump mechanism allows pulmonary ventilation by creating positive
pressure to force air into the lungs with the use of the muscles in the mouth and
pharynx.
Temperature Regulation
Amphibians are cotothermic
• They depend on external heat sources to maintain body temperature.
They are ectothermic because they are poorly insulated aquatic animals; so,
regardless of how much metabolic heat it produces, it will lose heat as
quickly as it is produced because of powerful heat-absorbing properties of
the water.
Amphibian Nervous and Sensory
Functions
The amphibian brain consists of three embryological subdivisions:
1. The forebrain contains olfactory center and regions that regulate color change and
visceral functions.
2. The midbrain contains a region called the optic factum that assimilates sensory
information and initiates motor responses, and also processes visual sensory
information.
3. The hindbrain functions in motor coordination and in regulating heart rate and the
mechanics of respiration.
Sensory receptors are widely distributed across the skin that respond to
heat, cold and pain.
Amphibian Nervous and Sensory
Functions Continued
Amphibians have singly or small grouping of lateral line systems that
respond to low frequency vibrations in water.
They have chemoreceptors in the nasal epithelium and the lining of the
mouth, on the tongue, and over the skin.
Olfaction is used in mate recognition as well as in detecting noxious
chemicals and in locating food.
Vision is one of the most important senses in amphibians because they are
primarily sight feeders.
• Eyes are located at the front of the head, providing binocular vision and depth
perception.
• The lower eyelid is moveable and it cleanse and protects the eye.
Amphibian Excretion and
Osmoregulation
The kidneys of amphibians lie on either side of the dorsal aorta on the dorsal
wall of the body cavity. A duct leads to the cloaca, and a storage structure,
the urinary bladder, is a ventral outgrowth of the cloaca.
Nitrogenous waste product comes in the forms of either ammonia or urea.
• Freshwater amphibians excrete ammonia.
• Land-based amphibians excrete urea that is produced from ammonia in the liver.
One of the biggest problems amphibians face is osmoregulation.
• Facing the same osmoregulatory problems as freshwater fishes they must rid the body
of excess water and conserve essential ions.
Amphibians must conserve water.
• Many amphibians reduce evaporative water loss by reducing the amount of body
surface exposed to air.
Amphibian Reproduction,
Development and Metamorphosis
Amphibians are dioecious with ovaries and testes located near the dorsal
body wall.
Fertilization is usually external (caecilians are an exception), and because the
developing eggs lack any resistant covering, development is tied to moist
habitats, usually water.
• Some anurans have terrestrial nests that are kept moist by being enveloped in foam or
by being located near the water and subjected to flooding.
Only about 10% of all salamanders have external fertilization while all others
produce spermatopohores, and fertilization is internal.
• 75% have internal development.
Metamorphosis is a series of abrupt structural, physiological, and behavioral
changes that transform a larva into an adult.
A variety of environmental conditions, including crowding and food
availability, influence the time required for metamorphosis.
Vocabulary
Cutaneous Respiration – Gas exchange across thin, moist surfaces of the
skin. Also called cutaneous exchange of integumentary exchange.
Amplexus – The positioning of a male amphibian dorsal to a female
amphibian, his forelimbs around her waist. During amplexus, the male
releases sperm as the female releases eggs.
Buccal Pump – The mechanism by which lung ventilation occurs in
amphibians. Muscles of the moth and pharynx create positive pressure to
force air into the lungs.
Nicitating Membrane – The thin, transparent lower eyelid of amphibians and
reptiles.
Tetrapods – A taxonomic designation that refers to extant amphibians,
reptiles, birds and mammals and their closest common ancestor.