15.2: Amphibian Characteristics

Learning Objectives

By the end of this section, you will be able to do the following:

• Describe the important difference between the life cycle of amphibians and the life cycles of other vertebrates
• Describe the general characteristics of amphibians

 

Amphibians are vertebrate tetrapods (“four limbs”), and include frogs, salamanders, and caecilians. As tetrapods, most amphibians are characterized by four well-developed limbs. In some species of salamanders, hindlimbs are reduced or absent, but all caecilians are (secondarily) limbless. Amphibians evolved during the Devonian period and were the earliest terrestrial tetrapods. They represent an evolutionary transition from water to land that occurred over many millions of years. Thus, the Amphibia are the only living true vertebrates that have made a transition from water to land in both their ontogeny (life development) and phylogeny (evolution). They have not changed much in morphology over the past 350 million years!

 

Reproduction

The term “amphibian” loosely translates from the Greek as “dual life,” which is a reference to the metamorphosis that many frogs and salamanders undergo and the unique mix of aquatic and terrestrial phases that are required in their life cycle. In fact, they cannot stray far from water because their reproduction is intimately tied to aqueous environments. The larval stage, which is generally aquatic, possess a tail, gills, and a lateral line system. As they metamorphose into adults, which are generally terrestrial, the juveniles may trade their tails for limbs, their gills for lungs, and their lateral line system for eardrums. There are some species that are strictly aquatic and rarely metamorphose, such as the axolotl. Adult axolotls retain their gills and flattened tail associated with their aquatic lifestyle, but metamorphosis can be artificially induced by administering thyroid hormone and/or iodine.

 

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Respiration

An important characteristic of extant amphibians is a moist, permeable skin that is achieved via mucus glands. Most water is taken in across the skin rather than by drinking. The skin is also one of three respiratory surfaces used by amphibians. This is critical, because the mode of respiration/breathing in amphibians is not as efficient as that of most other terrestrial vertebrates.

Most terrestrial vertebrates, including mammals, utilize a system called negative pressure ventilation to inflate the lungs. During inhalation, contraction of muscles around the chest causes the chest wall to expand. In mammals, the primary muscle used for breathing is the diaphragm, which is aided by smaller muscles in the ribcage. This expansion causes the chest cavity and the lungs to increase in volume, which causes pressure in the lungs to decreases (according to Boyle’s Law). This decrease of pressure in the thoracic cavity relative to the environment makes the cavity less than the atmosphere (Figure 39.16a). Because of this drop in pressure, air rushes into the respiratory passages. Upon exhalation, the muscles around the chest relax, which causes the lungs recoil to force the air out of the lungs, returning the chest wall back to its original position (Figure 39.16b). This increases the pressure within the thoracic cavity relative to the environment, and air rushes out of the lungs (the movement of air out of the lungs is a passive event: no muscles are contracting to expel the air).

Figure 39.16 The lungs, chest wall, and diaphragm are all involved in respiration, both (a) inhalation and (b) expiration. (credit: modification of work by Mariana Ruiz Villareal)

 

Adult amphibians are lacking or have a reduced diaphragm, so breathing via lungs is forced. They utilize what is known as positive pressure ventilation, where air is forced into the lungs from another chamber. In amphibians, the buccal (mouth) cavity is used to pump air into the lungs. Fresh air is taken into the buccal cavity through the nares (nostrils). The fresh air that was collected in the buccal cavity is then pushed by positive pressure into the lungs by closing the nares and elevating the throat. Old air is removed from the lungs in a reverse pattern. This system is less efficient than the negative pressure system explained above because old and fresh air mix in the buccal cavity. As amphibians can respire through their skin as well, this is able to compensate for the inefficiencies of positive pressure ventilation.

Figure 39.167 Diagram of two stroke positive pressure ventilation in frogs. (credit: Mokele)

 

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Diet and Nutrition

All extant adult amphibians are carnivorous, and some terrestrial amphibians have a sticky tongue used to capture prey. Amphibians also have multiple small teeth at the edge of the jaws. In salamanders and caecilians, teeth are present in both jaws, sometimes in multiple rows. In frogs and toads, teeth are seen only in the upper jaw. Additional teeth, called vomerine teeth, may be found in the roof of the mouth. Amphibian teeth are pedicellate, which means that the root and crown are calcified, separated by a zone of noncalcified tissue.

 

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Sensory Systems

Amphibians have image-forming eyes and color vision. Ears are best developed in frogs and toads, which vocalize to communicate. Frogs use separate regions of the inner ear for detecting higher and lower sounds: the papilla amphibiorum, which is sensitive to frequencies below 10,000 hertz and unique to amphibians, and the papilla basilaris, which is sensitive to higher frequencies, including mating calls, transmitted from the eardrum through the stapes bone. Amphibians also have an extra bone in the ear, the operculum, which transmits low-frequency vibrations from the forelimbs and shoulders to the inner ear, and may be used for the detection of seismic signals.

 

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This page titled 15.2: Amphibian Characteristics is a derivative of Biology 2e by OpenStax that is licensed under a CC BY 4.0 license.