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The content and activities in this topic will work towards building an appreciation for the evolutionary and ecological significance of amphibians.

From Fish to Tetrapods

Tetrapods are vertebrate animals with four limbs or feet, a term that is derived from the Greek root words tetra meaning “four” and pod meaning “foot.” Scientists believe the evolutionary adaptation of four limbs likely began in an aquatic environment among a group called the lobe-finned fishes. However, the purpose of the first limbs is unknown. They may have been an adaptation to stalk prey or to prop the body up above the water in shallow areas for aerial respiration.


<p><strong>Fig. 5.2.</strong> (<strong>A</strong>) Red-eyed tree frog (<em>Agalychnis callidryas</em>), an example of an amphibian , Playa Jaco, Costa Rica</p><br />
<p><strong>Fig. 5.2.</strong>&nbsp;(<strong>B</strong>) Arrau River turtles (<em>Podocnemis expansa</em>), examples of reptiles, Ecuador</p><br />

<p><strong>Fig. 5.2.</strong>&nbsp;(<strong>C</strong>) European bee-eaters (<em>Merops apiaster</em>), examples of birds, Ariège, France</p><br />
<p><strong>Fig. 5.2.</strong>&nbsp;(<strong>D</strong>) Black rhinoceroses (<em>Diceros bicornis</em>), examples of mammals, Etosha National Park, Namibia</p><br />


The tetrapod group includes four classes of vertebrates. The first are the amphibians (Class Amphibia), a group of tetrapods with an aquatic developmental stage and a terrestrial adult stage (Fig. 5.2 A). The reptiles (Class Reptilia) are scaled tetrapods that use lungs to breathe (Fig. 5.2 B). The birds (Class Aves) are tetrapods in which the two front limbs are wings (Fig. 5.2 C). The mammals (Class Mammalia) are warm-blooded and possess hair, among other characteristics (Fig. 5.2 D). It should be noted that scientists now consider birds to be a type of reptile (Fig. 5.3). Even though all tetrapods share a common ancestry, there is great diversity in their morphology and physiology, thus the study of tetrapods has been divided into several disciplines. The study of amphibians and reptiles is known as herpetology. The study of birds is called ornithology. The study of mammals is known as mammalogy.


<p><strong>Fig. 5.3.</strong> Phylogenetic tree of all vertebrate animals</p><br />

Organisms within the subphylum Vertebrata, which includes the tetrapods, are an amazingly diverse group, currently living in almost every habitat on Earth. Vertebrates possess an internal skeleton, or endoskeleton, with a backbone made of vertebrae and cartilage. Within the subphylum Vertebrata there are seven classes of organisms and approximately 52,000 species (Fig. 5.3). However, prior to 360 million years ago, the only vertebrates that existed were the fishes, including the jawless fish (Class Agnatha), the sharks and rays (Class Chondrichthyes), and the bony fishes (Class Osteichthyes). Ancient fishes were a very diverse group, but they had been constrained to marine habitats. At that time plants, molluscs, and arthropods dominated terrestrial habitats, though they were primarily centered on coastlines and areas with an abundance of water. Around 390 to 360 million years ago, a significant evolutionary step took place that initiated a radiation of species into terrestrial habitats. The fins of some lobe-finned fishes evolved into the limbs of tetrapods. Figure 5.4 shows two examples of extant lobe-fined fishes.


<p><strong>Fig. 5.4.</strong> (<strong>A</strong>) West Indian ocean coelacanth (<em>Latimeria chalumnae</em>)</p><br />
<p><strong>Fig. 5.4.</strong>&nbsp;(<strong>B</strong>) South American lungfish (<em>Lepidosiren paradoxa</em>)</p><br />


Adaptive radiation is a process in which organisms diversify rapidly into many new forms. Adaptive radiations particularly occur when new environmental resources arise, providing new habitats and food sources. When tetrapods first evolved, the land represented a new environment with an abundance of plant and insect food resources that were not being exploited by the relatively large land animals. However, life on land presented a number of challenges for aquatic organisms. In the ocean, an organism’s weight is supported by the water, and some organisms have additional adaptations that help them remain buoyant such as swim bladders in fish. The water’s resistance against the body can make movement and other behaviors like grasping food easier. Air is much less dense than water, so organisms that live on land had to develop structural systems to support their weight and allow for movement. For example, the evolution of jointed limbs allowed tetrapods to walk by transmitting muscle energy against their endo-skeleton to the ground. Another challenge on land is the threat of desiccation, or drying out, from water loss. Land-based organisms developed much thicker skin compared with fish as a way to prevent water loss. Other sensory organs also had to work in air rather than water. The lateral line and electric organs of bony fish no longer worked, so ears, nasal passages, and focusing eyes developed. A common misconception is that lungs developed in early tetrapods. However, lungs that were used for respiration had already developed in some bony fishes from their swim bladders. The lungfish (Fig. 5.5), for example, has lungs and is currently found in Africa, South America, and Australia. Tetrapods did develop pumping mechanisms to bring air in and out of the body cavity.


<p><strong>Fig. 5.5.</strong> (<strong>A</strong>) Armored birchir (<em>Polypterus delhezi</em>) and other birchirs and ropefish are ray-finned fishes with lungs</p><br />
<p><strong>Fig. 5.5.</strong>&nbsp;(<strong>B</strong>) Lung inside a dissected spotted African lungfish (<em>Protopterus dolloi</em>)</p><br />

Marine Tetrapod

Tetrapods evolved from fish ancestors and diversified on dry land or in freshwater habitats. Some of these terrestrial reptiles (including birds) and mammals then independently evolved adaptations that allowed them to survive in ocean conditions. There are no extant marine amphibians. Examples of marine tetrapods include sea turtles, shorebirds, penguins, whales, and seals. Most marine tetrapods retain some connection to dry land. Sea turtles and penguins must return to shore to build nests and lay eggs just as their non-marine ancestors did. All marine tetrapods have to surface for air to breathe. Figure 5.6 shows some examples of marine tetrapods.


<p><strong>Fig. 5.6.</strong> (<strong>A</strong>) Marine iguana (<em>Amblyrhynchus cristatus</em>), Española Island, Galápagos Islands</p><br />
<p><strong>Fig. 5.6.</strong>&nbsp;(<strong>B</strong>) Green sea turtle (<em>Chelonia mydas</em>), Keauhou, Island of Hawai‘i</p><br />

<p><strong>Fig. 5.6.</strong>&nbsp;(<strong>C</strong>) Emperor penguin (<em>Aptenodytes forsteri</em>), Gould Bay, Antarctica</p><br />
<p><strong>Fig. 5.6.</strong>&nbsp;(<strong>D</strong>) Polar bear (<em>Ursus maritimus</em>), Svalbard, Norway</p><br />

What is an Amphibian?

Amphibians are a group of tetrapod vertebrate animals with moist, scaleless skin, all sharing a common evolutionary ancestry. Examples of amphibians include frogs, toads, salamanders, and newts (Fig. 5.7). Amphibians are a diverse group of animals that includes almost 7,000 species and that are adapted to live in a wide variety of environments. All amphibians are aquatic animals to some degree. Most amphibian species live in freshwater or moist terrestrial environments but not marine environments. Although there are no marine amphibians alive today, some species of frogs can tolerant brackish water. Some species, like desert toads, have adapted to arid conditions but still require aquatic habitats to breed.

<p><strong>Fig. 5.7.</strong> (<strong>A</strong>) Purple caecilian (<em>Gymnopis multiplicata</em>), El Zota, Costa Rica</p><br />
<p><strong>Fig. 5.7.</strong>&nbsp;(<strong>B</strong>) Central newt (<em>Notophthalmus viridescens louisianensis</em>) in eft stage, Leon County, Florida</p><br />
<p><strong>Fig. 5.7.</strong>&nbsp;(<strong>C</strong>) Blue poison dart frog (<em>Dendrobates azureus</em>)</p><br />

Evidence of Common Ancestry and Diversity

<p><strong>Fig. 5.8.</strong> Phylogenetic tree illustrating the evolutionary relationships among the three major amphibian groups</p><br />

Amphibians were the earliest tetrapods to evolve. Modern amphibians can be classified into three major groups: legless caecilians, salamanders, and frogs. The first amphibians evolved from lobe-finned fishes approximately 390 to 360 million years ago (Fig. 5.8). Modern caecilians, salamanders, and frogs all evolved from these basal amphibian ancestors.


Caecilians (pronounced “see-seal-ee-un”) are long, slender, legless amphibians superficially resembling earthworms or snakes (Fig. 5.9). Most caecilians burrow through the soil, but some species thrive in freshwater ecosystems. There are approximately 170 extant species, all occurring in the tropical regions of Central and South America, Southeast Asia and equatorial Africa. They vary in length from 7 centimeters (cm) to 1.5 meters (m).

<p><strong>Fig. 5.9.</strong> (<strong>A</strong>) Purple caecilian (<em>Gymnopis multiplicata</em>)</p><br />
<p><strong>Fig. 5.9.</strong>&nbsp;(<strong>B</strong>) Yellow-striped caecilian (<em>Ichthyophis kohtaoensis</em>)</p><br />

<p><strong>Fig. 5.9.</strong>&nbsp;(<strong>C</strong>) Mexican caecilian (<em>Dermophis mexicanus</em>)</p><br />
<p><strong>Fig. 5.9.</strong> (<strong>D</strong>) Head of an Asian caecilian (<em>Ichthyophis</em> sp.)</p><br />

Salamanders are amphibians with elongated, slender bodies and long tails (Fig. 5.10 A). There are approximately 550 extant species of salamanders. One group of salamanders, commonly called the giant salamanders, is the largest of all the amphibians (Fig. 5.10 B). The largest species is the Chinese giant salamander (Andrias davidianus), which can reach up to 1.8 m in length. One familiar group of salamanders is the newt family. Newts are generally smaller than 20 cm but are famously known for their toxic skin secretions and bright colors (Fig. 5.10 C). Sirens are a group of salamanders that lack hind limbs. They have reduced forelimbs giving them an eel-like appearance (Fig. 5.10 D). The sirens are common in slow moving waters of North America.

<p><strong>Fig. 5.10.</strong> (<strong>A</strong>) Ensatina salamander (<em>Ensatina eschscholtzii</em>)</p><br />
<p><strong>Fig. 5.10.</strong>&nbsp;(<strong>B</strong>) Hellbender (<em>Cryptobranchus alleganiensis</em>)</p><br />

<p><strong>Fig. 5.10.</strong>&nbsp;(<strong>C</strong>) Eastern newt (<em>Notophthalmus viridensis</em>) in “red eft” developmental stage</p><br />
<p><strong>Fig. 5.10.</strong> (<strong>D</strong>) Western lesser siren (<em>Siren intermedia nettingi</em>)</p><br />

Frogs are arguably the most successful group of amphibians, with approximately 5,000 known species alive today. Frogs are squat, tailless amphibians with short forelimbs and muscular hind limbs (Fig. 5.11). Several families of frogs with leathery skin and short legs are commonly called toads (Fig. 5.11 C). However, this is simply a colloquial term. Frogs have adapted to a wide range of habitats and can sometimes be found in very dense populations.

<p><strong>Fig. 5.11.</strong> (<strong>A</strong>) Italian tree frog (<em>Hyla intermedia</em>)</p><br />
<p><strong>Fig. 5.11.</strong>&nbsp;(<strong>B</strong>) Iberian midwife toad (<em>Alytes cisternasii</em>)</p><br />

<p><strong>Fig. 5.11.</strong>&nbsp;(<strong>C</strong>) Cane toad (<em>Rhinella marina</em>)</p><br />
<p><strong>Fig. 5.11.</strong>&nbsp;(<strong>D</strong>) Phantasmal poison frog (<em>Epipedobates tricolor</em>)</p><br />

Structure and Function

<p><strong>Fig. 5.12.</strong> (<strong>A</strong>) Skeleton of an angler or monkfish (<em>Lophius piscatorius</em>), a bony fish, Museum of Toulouse</p><br />
<p><strong>Fig. 5.12.</strong>&nbsp;(<strong>B</strong>) Skeleton of a salamander</p><br />
<p><strong>Fig. 5.12.</strong>&nbsp;(<strong>C</strong>) Skeleton of a bullfrog</p><br />


<p><strong>Fig. 5.13.</strong> Frog anatomy. Organs labeled by number: (1) Right atrium, (2) Liver, (3) Aorta, (4) Egg mass, (5) Colon, (6) Left atrium, (7) Ventricle, (8) Stomach, (9) Left lung, (10) Spleen, (11) Small intestine, and (12) Cloaca</p><br />

Amphibians have some major changes in body structures compared with their most recent ancestors, the fish. One of the most striking is the change in skeletal structure with muscular limbs. All tetrapods have hard bony skeletons (Fig. 5.12). Sturdy skeletons are necessary to support the body weight of animals out of water. Fishes have musclar heart organs with two chambers that pump out and take in blood. Amphibians have a three-chambered heart (Fig. 5.13).


Activity: Identifying Amphibians

Use a simple dichotomous key to correctly identify amphibian major groups.


Amphibians and reptiles are sometimes called “cold-blooded” animals. The term cold-blooded does not mean that their body is always cold, rather that is ectothermic. Ectothermic means that organisms maintain their body temperature by absorbing heat from their environment. By contrast, endothermic organisms maintain a relatively constant temperature in their bodies. Birds and mammals are examples of endothermic or “warm-blooded” animals. Maintaining an appropriate body temperature is important for metabolism, locomotion, and many cellular processes. Ectothermic organisms will often move to new locations to increase or decrease their temperature if necessary. Some amphibians and reptiles will find a location in the sun and absorb sunlight to increase their temperature, this process is known as basking. Amphibians in general have relatively moist skin, meaning that evaporative cooling occurs quickly in the air.  Amphibians maintain their temperature by moving locations or changing their postures.


All animals require oxygen to survive. Vertebrate animals have respiratory systems, or collections of organs that allow the exchange of oyxgen (O2) and carbon dioxide (CO2) gases to and from the surrounding environment. Most fish species use gills to extract dissolved oxygen from water. The first true lungs evolved in a group of fishes called lungfishes (Fig 5.5). Lungs are balloon-like internal organs that can be filled with gases for respiration. Most amphibians have lungs in their adult life stages. However, the largest group of salamanders (family Plethodontidae) is the lungless salamanders. The species in this group “lost” their lungs secondarily over the course of evolution, although the cause for this remains unclear. Biologists use the term secondary loss to describe traits that have reverted back to an ancestral condition or appear to be more similar to earlier ancestral traits than recent traits. For example, “lungless” plethodontid salamanders lack lungs just like most lungless fishes, even though they evolved from ancestors with lungs (e.g. lobe-finned fishes). Similarly, snakes evolved from lizards with legs. It can be said that snakes lost their legs secondarily through evolution. To read more about lungfishes, see the introduction to lobe-finned fishes and this article, From Water to Land.


Another adaptation in amphibians is their skin. Amphibians generally have soft moist skin without scales. There are some exceptions. Toads have dry and warty skin. Skin acts as a preventive barrier against changes in the external environment and can prevent cuts and microbial infections. The skin of amphibians is unique because it can be used for gas exchange and respiration. In fact, some species have lost lung organs over time. In addition to respiration, the skin has two types of glands present in juveniles and adults: mucous and poison glands. Mucous glands secrete a protective coating over the skin. Some amphibian species have evolved poisonous secretions that serve as protection against predators. Poisonous amphibians are typically brightly colored (Fig. 5.14).

<p><strong>Fig. 5.14.</strong> (<strong>A</strong>) Red-bellied newt (<em>Taricha rivularis</em>), northern California</p><br />
<p><strong>Fig. 5.14.</strong>&nbsp;(<strong>B</strong>) The dyeing dart frog (<em>Dendrobates tinctorius</em>) is one of over 170 species in the “poison dart frog” family native to Central and South America.</p><br />

Energy Acquisition

Almost all amphibians are predatory. Juvenile frogs, which eat mainly bacteria and algae, are the exception. Amphibians have several interesting adaptations that they use to capture prey. Most amphibians capture prey by biting and grasping (Fig. 5.15). Teeth structure is diverse depending on the prey type and serves to hold the prey or to break it down. The tongue of most amphibians is used to swallow and help guide the food to the esophagus. Frogs and most salamanders use their sticky projectile tongues to capture prey. Their diet mainly consists of insects but some larger species feed on birds, lizards, and even small mammals. Some salamanders and frogs also use a form of suction feeding in which they capture prey by opening their mouth and mouth cavity at the same time, sucking the prey into the mouth.

<p><strong>Fig. 5.15.</strong> (<strong>A</strong>) Allegheny Mountain dusky salamander (<em>Desmognathus ochrophaeus</em>), eating an earthworm, Allegany County, New York</p><br />
<p><strong>Fig. 5.15.</strong>&nbsp;(<strong>B</strong>) Frog eating another frog, Siem Reap, Cambodia</p><br />

Growth, Development & Reproduction

Almost all amphibians produce offspring through sexual reproduction. Recall that sexual reproduction is the joining of male and female gametes (sperm and eggs, respectively). For more information about sextual reproduction, please read the Growth, Development, and Reproduction topic in Aquatic Plants and Algae.


<p><strong>Fig. 5.16.</strong> The male wood frog (<em>Lithobates sylvaticus</em>) clasps a female and deposits sperm as the female lays eggs.</p><br />

In caecilians and salamanders, fertilization of the female’s eggs occurs internally. A male will deposit sperm into the female cloaca. The cloaca is a posterior opening that serves as an opening for the reproductive, intestinal, and urinary tracts in amphibians, reptiles, and birds. In frogs, fertilization occurs externally. The male clasps onto the female and deposits sperm as the female lays eggs (Fig. 5.16).


Amphibian eggs are highly vulnerable to water loss, and they are typically laid in aquatic habitats or very moist sites. Some frogs that live in drier habitats form bubble nests to keep their eggs moist following fertilization (Fig. 5.17).

<p><strong>Fig. 5.17.</strong>&nbsp;(<strong>A</strong>) The túngara frogs (<em>Engystomops pustulosus</em>) of Central America generate floating foam or bubble nests around their egg masses.</p><br />
<p><strong>Fig. 5.17.</strong>&nbsp;(<strong>B</strong>) Many amphibian species such as the northern red-legged frog (<em>Rana aurora</em>) attach egg masses to submerged freshwater plants in marshes, streams and temporary vernal pools.</p><br />
<p><strong>Fig. 5.17.</strong> (<strong>C</strong>) All poison dart frog species carry newly-hatched tadpoles on their backs. Adults deposit the tadpoles into small streams and water pools for further development.</p><br />


The word amphibian is derived from the Greek word amphibios meaning “double life.” This term highlights the metamorphosis that occurs in the lifecycle of amphibians. Metamorphosis is the transition from an embryonic larva to a juvenile or adult form. The larvae of amphibians are free-living embryos that are aquatic and have pharyngeal slits and external gills. Like their fish ancestors, they have lateral line sensory systems. They also have a cartilaginous skeleton, muscular body, and tail to move in the water. In general, the caecilian and salamander larvae are anatomically similar to the adults (Fig. 5.18 A and Fig. 5.18 B). The larval frog, called a tadpole, is much more elongated than the squat adult (Fig. 5.18 C). Tadpoles typically have rounded bodies with long flat tails. Young frogs will remain in this tadpole stage for 1 to 15 months depending on the species and local environmental conditions. As the tadpole undergoes metamorphosis, the hind limbs grow first and the forelimbs later. Lungs develop at the time of leg development, and the tadpole tail shortens (Fig. 5.19). The metamorphosis generally takes about 24 hours to complete.

<p><strong>Fig. 5.18.</strong> (<strong>A</strong>) Spotted salamander (<em>Ambystoma maculatum</em>) larva</p><br />
<p><strong>Fig. 5.18.</strong>&nbsp;(<strong>B</strong>) Spotted salamander (<em>Ambystoma maculatum</em>) adult</p><br />

<p><strong>Fig. 5.18.</strong> (<strong>C</strong>) Wood frog (<em>Lithobates sylvaticus</em>) tadpole larva</p><br />
<p><strong>Fig. 5.18.</strong>&nbsp;(<strong>D</strong>) Wood frog (<em>Lithobates sylvaticus</em>) undergoing metamorphosis</p><br />
<p><strong>Fig. 5.18.</strong>&nbsp;(<strong>E</strong>) Wood frog (<em>Lithobates sylvaticus</em>) adult</p><br />



<p><span style="font-size: 13.008px;"><strong>Fig. 5.19.</strong> Metamorphosis of a mimic poison frog (<em>Ranitomeya imitator</em>) from larval tadpole to adult</span></p><br />

Because amphibians require a moist habitat to reproduce, the breeding season commonly occurs during the rainy season in many parts of the world. Certain species of amphibians, particularly frogs, are adapted to live in ephemeral pools and streams. Ephemeral pools and streams are freshwater bodies only periodically covered in water. Ephemeral, or vernal, pools often dry out during the summer months. Reproduction is challenging because mating, fertilization, and egg hatching must occur while there is water present. Since larval amphibians also rely on water, they must completely metamorphose before the water dries up.


A very important behavior in many animals is courtship. Courtship involves all the reproductive activities that take place prior to mating. During courtship, communication is critical. Communication is signaling from one mate to another and can occur via visual, auditory, smell, and touch queues. During reproduction, the first step is to bring mates together. Some species have auditory calls that can travel long distances. Other species will release pheromones, which are chemicals secreted that can be smelled by mates. In other species, there may be learned environmental queues that trigger meeting in the same location. For example, a heavy rain may signal to adults in an ephemeral pond that it is time to breed. After the adult amphibians are in the same location, communication is then used to assess each other for suitability as mates.


<p><span style="font-size: 13.008px;"><strong>Fig. 5.20.</strong> Mating Sierra newts (<em>Taricha sierrae</em>), northern California</span></p><br />

Courtship behavior varies greatly among amphibian species. There is very little known about courtship behavior in caecilians. Most salamanders perform a ritualized courtship in which touch and chemosensory signals are alternately sent between males and female in a specific order (Fig. 5.20). Vocalizations are very common in many frog species. Male frogs will typically vocalize to announce their location to a potential mate and also to establish their territory to competitors. Interestingly, no two frog species produce the same call.


Further Investigations: Amphibians

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Exploring Our Fluid Earth, a product of the Curriculum Research & Development Group (CRDG), College of Education. University of Hawaii, 2011. This document may be freely reproduced and distributed for non-profit educational purposes.