WEEK 6, BIOLOGY 03051 (VERTEBRATE ZOOLOGY): AMPHIBIANS 2/22/00
Text (7th ed.): Ch. 28 (Ch. 8, Figs. 8-10, 8-20, and 8-22; Ch. 14, pp. 331-335 helpful too)
Dr. W. Crone (303 FTZ, 629-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://frog.simplenet.com/froggy/ (The Froggy Page, from the silly to the scientific)
possible HVCC a/v resources: VT 3374,"Eyewitness: Amphibian" (to be seen in recitation)
VT 1184,"Life on Earth; 13: Amphibians" (emphasis on frogs)
Two major considerations to live on land: lungs and limbs. Lungs are modified swim bladders, with plenty of blood flow to take up oxygen, and increasing inpocketing to generate more surface area. Limbs must be able to support the weight of an organism against gravity and to allow movement against a hard surface.
review:
Forelimb bones, from proximal to distal: humerus, radius, ulna, carpals, metacarpals, phalanges.
Hindlimb bones, from proximal to distal: femur, tibia, fibula, tarsals, metatarsals, phalanges.
A series of fossils from the Devonian Period (410 to 360 million years ago)indicates hints at both of these changes (lungs and limbs).
Eusthenopteron: a lobe-fin fish with discernable humerus/radius/ulna, but with fin rays distally
Acanthostega: carpals and phalanges at the distal end of weak limbs, but with skull evidence for gills
Ichthyostega: still with gill bones and caudal fin rays, but stronger limbs and more effective shoulder, pelvis
Tetrapod limbs: How did limbs develop from the fins of lobe-finned fishes, given this fossil evidence? Recent research indicates that Hox (homeobox) genes control limb development (and much, much more), and that prolongation of Hox gene expression can account for the difference between the tetrapod limb and fish fin.3 Tetrapod skeletons have consistent (homologous) bones seen in both fore- and hindlimbs, as listed above. We also see consistent limb muscle groups. Primitively, these are groups of dorsal elevator muscles and ventral depressor muscles. These blocks of hypaxial muscle subdivide after innervation: hence muscle groups, e.g., quadriceps, hamstrings, etc., with a common nerve innervation, and that are separated by connective tissue sheets (intermuscular septa).1
Amphibian axial skeleton supports against gravity as well. The hip bone of ilium, ischium, and pubis is clearly evident, and its connection to sacral vertebrae creates a pelvis. The unused caudal (tail) vertebrae in a frog are fused into a urostyle.
Class Amphibia ("on both sides life"): tetrapods that live in both water and land, or live one stage in water and another on land. Those alive today:
Order Caudata ("tailed") (Urodela): salamanders and newts. 360 species; tail present throughout life. Moist ground dwellers, with aquatic gilled larvae (and some gilled adults, e.g., Necturus, the mud puppy).
Order Gymnophiona ("naked snake"): caecilians. 160 species; tropical, wormlike burrowers.
Order Anura ("without tail"): the frogs and toads. 3500 species. Adults tailess, with long, muscular hind legs that end in webbed feet. Gilled, aquatic, larval tadpoles have tails.
Most amphibians are carnivores, with large mouths and small homodont (similar-looking) teeth. Many salamanders use their jaws to catch prey, but frogs also have a tongue for flicking.
The amphibian heart is different than the fish heart, in that there are now a consistent set of lungs and so a two-pass circulation. There are venae cavae (anterior and posterior) feeding into the right atrium. This deoxygenated blood will drain from the right atrium into a common ventricle. The ventricle then pumps the blood into a conus (truncus) arteriosus, in which a spiral valve will tend to direct this deoxygenated blood into a"pulmocutaneous" artery. With the moist skin of amphibians, oxygen can be taken up through the skin so cutaneous arteries in addition to pulmonary arteries help to oxygenate blood. The now-oxygenated blood drains back into the left atrium, to be pumped by the ventricle to systemic arteries to the body and carotid arteries to the head.
Frog lungs work via positive pressure,"pushing" air into their lungs, in addition to ventilating their vascularized mouth cavity.2 As a result, the mylohyoid muscle at the floor of the mouth is a major respiratory muscle (what is the major mammalian respiratory muscle?).
Amphibians depend on the surroundings for temperature regulation, and affect their temperature by changes in behavior (e.g., burrow in the mud on hot/cold days). These"cold-blooded" animals are therefore ectothermic. Disadvantage: cannot remain active in cold weather. Advantage: less food requirement if not maintaining internal body temperature.
Kidneys are dorsal (as always!), and drain into a cloaca. Amphibians in water share the same osmoregulatory concerns as do freshwater fish, and so there excrete a lot of dilute urine. In contrast, on land, amphibians have to conserve water by different means (burrowing, rehydration through the skin). Amphibian skin lacks scales, etc., but does have many glands and epidermal thickenings. The specialized coloring seen in many amphibians is the result of chromatophores.
Life cycle: amphibians are male and female. Courtship usually includes sound production, via a larynx and vocal sacs. The female usually lays eggs in moist areas or water, as the clasping male fertilizes them externally during amplexus. The egg does not have a protective shell. An aquatic tadpole with a tail and gills swims until thyroid hormone control leads to metamorphosis into an adult. Some salamanders retain many larval features as adults, e.g., the gills seen in the mud puppy (Necturus).
ORIGIN OF THE AMNIOTES: Given their dependence on moist habitats for their skin and their reproduction, it is not surprising that amphibians are not currently dominant land organisms. During the Carboniferous Period (360 to 286 million years ago), amphibian-like tetrapod fossils have been found with features more in line with modern day reptiles. Class Reptilia ("creep"): the reptiles. In contrast to the amphibians, the reptiles are able to live away from water. Two major advances to allow this lifestyle:
1) skin: Thick, dry skin with keratinized epidermal scales. Keratin is a protective epidermal protein. The outer epidermal layer is occasionally shed, except in crocodiles/turtles. Chromatophores are present in the skin, but not to the dramatic extent as amphibians.
2) eggs: The amniotic egg: an amniotic egg has a hard shell and extraembryonic membranes. The yolk or nutrient of the egg is enclosed in a yolk sac, and other membranes form an inner amnion and outer chorion. The amnion is protective around the embryo and the chorion becomes hightly vascular and aids in gas exchange. Waste products are stored as nontoxic uric acid in the allantois, a ventral outgrowth from the GI tract that will fill up space between the amnion and the chorion, supplying many blood vessels to help oxygenate the embryo. But an eggshell needs pores for this gas exchange, which can lead to water loss. Also, fertilization must occur internally before such a protective egg can be laid.
1 M Cartmill et al., Human Structure (Harvard U. Press, Cambridge, MA, 1987), pp. 209-214.
2 CP Hickman Jr et al., Biology of Animals, 7th ed., (WCB McGraw-Hill, Boston, 1998) pp. 202-203.
3 FH Pough et al., Vertebrate Life, 4th ed. (Prentice Hall, Upper Saddle River, NJ, 1996), pp. 283-284, 290-291.
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