Zoology Chapter 17

Chapter 17 Study Questions

1. How did the characteristic differences between aquatic and terrestrial environments

influence the early evolution of tetrapods?

There were four major environmental differences that needed to be overcome as tetrapods evolved for aquatic to terrestrial living; (1) oxygen content (2) fluid density (3) temperature regulation and (4) habitat diversity.

1. Oxygen is at least 20x more abundant in air than in water. Additionally, oxygen diffuses

more rapidly through air than through water. Once structures like lungs were developed

terrestrial animals could obtain oxygen much more rapidly and easily than in the aquatic

environment.

2. Fluid density or buoyancy: Air is 1000x less buoyant and 50x less viscous than water. This

means airs provides very little support compared to water. For this reason the movement

from water to land required the development of strong limbs and to re-‐develop their

skeletons to give greater support.

3. Temperature Regulation: Water temperature fluctuate much less and much slower than air.

Because air temperatures can change faster and more extremely than water; movement to

land required an ability to both sustain temperature changes and decrease the effective

change in air temperature.

4. Habitat Diversity: Terrestrial animals required behavioral and physiological strategies to

protect themselves from thermal extremes.

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2. Describe the different modes of respiration used by amphibians. What paradox do the amphiumas and terrestrial plethodontids present regarding the association of lungs with life on land?

Firstly, two important characteristic evolved in aquatic habitats gave amphibian ancestors the ability to move toward terrestrial habitats. These characteristics included two structures that connected to the pharynx. An air filled cavity, which functioned as a swim bladder and paired internal nares which functioned in chemoreception. On land, this combination of structures would be used to draw oxygen-‐rich air through the nares and into the air-‐filled cavity, whose surface would permit some respiratory gas exchange with body fluids. The internal nares, air-‐filled cavity, and paired limbs provided a tetrapod ancestor for movement to land. These


characteristics illustrates an important evolutionary principle in which a structure that has evolved by natural selection for an initial utility or role is recruited for a new role. The air-‐filled cavity acts as a swim bladder in the aquatic setting and as lungs in the terrestrial setting. The association of lungs with life on land is generally quite strong. This relationship breaks down to a degree when looking at salamanders. The amphiumas live their entire life in the water but lose their gills before adulthood and then breath primarily by lungs, raising their nostrils above the water to get air. In contrast to the amphiumas all the species of the large family Plethodontidae have no lungs yet many of these species are entirely terrestrial. These species depend on well developed cutaneous respiration and supplemented by gas exchange in vascularized membranes of the mouth. In summary amphiumas lives its entire life in water and has no gills (but lungs) and the other ( Plethodontidae ) live entirely on land as an adult and has no lungs and no gills.

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3. Evolution of the tetrapod limb was one on of the most important advances in vertebrate history. Describe the inferred sequence of its evolution.

Tetrapods evolved limbs in an ancestral aquatic habitat during the Devonian period prior to their evolutionary movement onto land. Although fish fins at first appear very different from the jointed limbs of tetrapods, an examination of the bony elements of the paired fins of the lobed-‐finned fishes shows that they broadly resemble the homologous structures of amphibian limbs. The fossil record of Eusthenopteron (a Devonian lobe-‐fin), Tiktaalik , Acanthostega, and Ichthyostega show an evolutionary development of tetrapods in water and the tetrapods only later invaded land.

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5. Give the literal meaning of the name Gymnophiona. What animals are in this amphibian order? Describe their appearance and habitats.

Gymnophiona means naked + of a snake or "naked snake"

This order contains approx. 173 species of elongated, limbless, burrowing creatures commonly called caecilians. The possess a long, slender body, many vertebrae, long ribs, no limbs and a terminal anus. Eyes are small and most species are totally blind as adults. They occur in tropical forests in South America, Africa, India, and Southeast Asia.

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6. Give the literal meaning of the order names Urodela and Anura? What major features distinguish members of these two orders from each other?

Urodela: tail + evident = evident tail ( tailed amphibians such as salamanders )

Anura: without + tail = without tail ( not tailed amphibians such as toads and frogs )

Besides the difference of having a tail -‐vs-‐ no tail, salamanders usu. have four limbs ser at right angles to the trunk and are equal size whereas Anura obviously have a different adaptation. Salamanders are typically much smaller than Anura. Both groups have special variations in reproductive methods.

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7. Describe the breeding behavior of a typical woodland salamander.

After judging the female's receptivity by the presence of her chin on his tail base, the male deposits a spermatophore on the groud, and then moves forward a few paces. The white mass of the sperm atop a gelatinous base is visible at the level of the female's forelimb. The male moves ahead, the female following until the spermatophore is at the level of her vent. The female recovers the sperm mass in her vent, while the male arches his tail, tilting the female upward presumably facilitating recovery of the sperm mass. The female vent is a common opening for anus and reproductive system. The female later uses sperm stored in her body to fertilize eggs internally before laying them. Terrestrial species (woodland) deposit eggs in small, grapelike clusters under logs or in excavations in soft moist earth. Many terrestrial species guard their eggs. Terrestrial species undergo direct development. They bypass the larval stage and hatch as miniature versions of their parents.

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8. How is paedomorphosis important to evolutionary diversification of salamanders?

Whereas most salamanders complete their development by metamorphosis to the adult body form, some species reach sexual maturity while retaining their gills, aquatic lifestyle, and other larval characteristics. This condition illustrates paedomorphosis ("child form"), defined as evolution of an adult form that resembles an ancestral juvenile. Some characteristics of an ancestral adult morphology are consequently eliminated. An example of this is mud puppies who never continue to metamorphose. Some species reach sexual maturity with larval morphology continue to metamorphose only when conditions require it necessary to survive. Paedomorphosis is also an important means of evolutionary diversification in salamanders that


lack an aquatic larval stage. An example of this is the evolved adaptations for climbing of the Bolitoglossa rufescens webbed foot. This highly webbed foot was the result of paedomorphic evolution in which development of digits is greatly reduced; making the foot an adhesive surface for attaching to smooth surfaces. This is like evolution replayed with a different outcome.

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9. Briefly describe the reproductive behavior of frogs. In what important ways do frogs and salamanders differ in their reproduction.

Frogs breed, feed, and grow only during warm seasons (ectothermic). With warming spring temperatures and rainfall, males call vociferously to attract females. After a brief courtship, females enter the water and are clasped by the males in a process called amplexus, during which eggs are fertilized externally (only after leaving the female's body). As the female lays eggs, the male discharges seminal fluid containing sperm over the eggs to fertilize them. Frogs and salamanders differ firstly in that in frogs the males find the females but in salamanders the female approaches the male. Also, as described in question #7 above the salamander usu. has internal fertilization due to the sperm uptake of the female. Frog obviously only have external fertilization with the male present.

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Zoology Chapter 17