Figure 13.12
An ocean sunfish, the most massive bony fish in the world, up to 11 ft in length and 5,070 lb in weight!
Why Fish are Important
Now that you have some understanding of the general features of fish, you might think of some ways in which fish are important. Of course, there is their use as food (Figure below). In fact, people all over the world either catch fish in the wild or farm them in much the same way as cattle or chickens (aquaculture). Fish are also exploited for recreation, through angling and fish-keeping, and are commonly exhibited in public aquaria.
Figure 13.13
Workers harvest catfish from the Delta Pride Catfish farms in Mississippi.
Fish also have an important role in many cultures and art through the ages, ranging as widely as deities and religious symbols to subjects of books and popular movies(Figure below). For example, such deities that take the form of a fish are Ikee-Roa of the Polynesians, Dagon of various ancient Semitic peoples, and Matsya of the Dravidas of India. Fish have been used figuratively in many different ways, for example the ichthys used by early Christians to identify themselves and the fish as a symbol of fertility among Bengalis.
Figure 13.14
Fish play an important role in many cultures, including art, through the ages. Here is a still life of fish, eels, and fishing nets, by Johannes Fabritius.
In literature, legends of half-human, half-fish mermaids are featured in stories of Hans Christian Anderson and fish feature prominently in The Old Man and the Sea. Fish and other fanciful fish also play a major role in such movies as Splash, Jaws, Shark Tale, and Finding Nemo.
Lesson Summary
The general traits of fish help adapt them for living in an aquatic environment, mostly for swimming, and also for extracting oxygen.
Fish are typically ectothermic, although some show warm-blooded adaptations.
Jawless fish, the Agnatha, have some other common features.
Fish with jaws comprise both the cartilaginous fish and the bony fish.
The cartilaginous fishes, or Chondrichthyes, include the sharks, rays, skates and chimaera.
The bony fish, or Osteichthyes, form the largest group of vertebrates in existence today, and have certain traits in common.
Fish are important economically, recreationally and culturally.
Review Questions
What are the general traits of fish?
What are some exceptions to the general traits of fish?
Mudskippers are an example of a fish species that must absorb oxygen across the skin, instead of via gills, since they spend much of their time out of water. Describe an environmental situation in which air breathing would be of great use to a fish species.
What are the characteristics of jawless fish?
What is one structure only found in cartilaginous fishes and what is its function?
What are some reasons why it would be an advantage for fish to be endothermic?
List some ways that fish are important.
Further Reading / Supplemental Links
Unabridged Dictionary, Second Edition. Random House, New York, 1998.
http://kids.nationalgeographic.com/Animals
http://www.fws.gov/educators/students.html
http://www.igfa.org/kidshome.asp
http://www.pbs.org/emptyoceans/educators/activities/fish-youre-eating.html
http://en.wikipedia.org
Vocabulary
agnatha
A superclass of jawless fish, belonging to the phylum Chordata, subphylum Vertebrata.
aquaculture
The raising of aquatic plants and animals, especially seaweed, shellfish and other fish, in environments either natural or with controlled freshwater or marine conditions.
barbels
A thin fleshy structure on the external part of the head, such as the jaw, mouth or nostrils, of certain fishes.
chondrichthyes
The group of cartilaginous fishes, containing sharks, rays, skates and chimaeras.
ectothermic
Cold-blooded
osteichthyes
Contains all the bony fish, divided into the ray-finned and lobe finned fish.
ovoviviparous reproduction
The eggs develop inside the mother’s body after internal fertilization, and depend on the yolk for most of the nutrition; the young are born alive.
placoid
Plate-like, as in the scales of sharks.
Points to Consider
Juvenile bichirs, a type of fish, have external gills, a very primitive feature that they hold in common with larval amphibians. Think about how the external gills could be a transition between internal gills and lungs?
Lungfish and bichirs have paired lungs similar to those of tetrapods and must rise to the water’s surface to gulp fresh air through the mouth and pass spent air out through the gills. Discuss how lungfish could be similar to and different from tetrapods in the way they breathe?
The structure, the pineal body, located in the brain, performs many different functions including detecting light, maintaining circadian rhythms and controlling color changes. What structures could perform similar functions in amphibians, as a result of living on land?
Lesson 13.3: Amphibians
Lesson Objectives
Describe amphibian traits.
List the features of salamanders.
Compare and contrast frogs and toads with other amphibians.
Describe the roles of amphibians.
Check Your Understanding
What are some adaptations that amphibians, like fish, have for living in the water?
What are the characteristics that amphibians share with all other vertebrates?
Introduction
What group of animals begins its life in the water, but then spends most of its life on land? You were right, if you guessed amphibians. Amphibians are a group of vertebrates that has adaptations for both aquatic and terrestrial lifestyles. Evolutionarily, their ancestors made the transition from the sea to land. They comprise approximately 6,000 species of various body types, physiology, and habitats, ranging from tropical to subarctic regions.
Characteristics of Amphibian
Amphibians are ectothermic vertebrates, belonging to the class Amphibia and consist of three orders: Urodela, containing the salamanders and newts; Anura, consisting of frogs and toads; and Apoda, containing the caecilians. The larvae are typically aquatic and breathe using gills. The adults are typically semiterrestrial and breathe both through moist skin and by lungs.
For the purposes of reproduction, most amphibians are bound to fresh water. Although there are no true seawater amphibians, a few tolerate brackish (slightly salty) water. Some species do not need any water whatsoever, and several species have also adapted to arid and semi-arid environments, but most still need water to lay their eggs.
In general, the life cycle of amphibians begins with a shell-less egg stage, usually laid the previous winter in a pond. A larval stage follows in which the organism is legless, fully aquatic and breathes with exterior gills. After hatching, the larvae start to transform gradually (metamorphosis) into the adult’s appearance, including loss of gills, growth of four legs, and the ability to live in a terrestrial environment.
Adaptations for living in a terrestrial environment include replacement of gills with another respiratory organ, such as lungs; a development of glandular (containing cells, a group of cells, or an organ producing a secretion) skin to avoid dehydration, and the development of eyelids and adaptation to vision outside the water. An eardrum also develops that separates the external ear from the middle ear and, in frogs and toads, the tail disappears.
Salamanders
This is a group of approximately 500 species of amphibians, typically characterized by slender bodies, short legs, and long tails, and most closely related to the caecilians, little known legless amphibians (Figure below). Having moist skin, salamanders (Figure below) rely on habitats i
n or near water or under some protection on moist ground, often in a swamp. Some species are aquatic throughout life, some are aquatic intermittently and some are entirely terrestrial as adults.
Figure 13.15
The marbled salamander, , shows the typical salamander body plan: slender body, short legs, long tail and moist skin.
Figure 13.16
A species of African caecilian, , a legless amphibian, most closely related to the salamanders.
Respiration varies among the different species of salamanders; in those that retain lungs, respiration occurs through the gills as water passes over the gill slits. Some terrestrial species have lungs that are used in respiration in a similar way as in mammals. Other terrestrial salamanders lack both lungs and gills and exchange gases through their skin. This is known as valarian respiration, in which the capillary beds are spread throughout the epidermis.
Hunting prey is another unique aspect of salamanders. Muscles surrounding the hyoid bone contract to create pressure and “shoot” the hyoid bone out of the mouth along with the tongue. The tip of the tongue has mucus which creates a sticky end to which the prey is attached and captured. Muscles in the pelvic region are then used to bring the tongue and hyoid back to their original positions. Another trait, unique among vertebrates, is the ability to regenerate lost limbs, as well as other body parts, in a process known as ecdysis.
Salamanders are found in most moist or arid habitats in the northern hemisphere. They are generally small, but some can reach 30 cm (a foot) or more, as in the mudpuppy of North America. In Japan and China, the giant salamander reaches 1.8 m (6 ft) and weighs up to 30 kg (66 lb) (Figure below).
Figure 13.17
The Pacific giant salamander can reach up to 6 ft in length and 66 lb in weight.
The order Urodela, containing the salamanders and newts, is divided into three suborders. These consist of the giant salamanders (including the hellbender and Asiatic salamanders), advanced salamanders (including lungless salamanders, mudpuppies, and newts), and sirens.
Frogs and Toads
Frogs and toads (Figure below) are amphibians in the order Anura. A distinction is often made between frogs and toads on the basis of their appearance, caused by the convergent adaptation among so-called toads to dry environments (leathery skin for better water retention and brown coloration for camouflage), but this distinction has no taxonomic basis. One family, Bufonidae, is exclusively given the common name “toad,” but many species from other families are also called “toads.”
Figure 13.18
A species of toad, showing typical characteristics of leathery and warty skin, and brown coloration.
Frogs are distributed from the tropics to subarctic regions, but most species are found in tropical rainforests. Consisting of more than 5,000 species (about 88% of amphibian species are frogs), they are among the most diverse groups of vertebrates. Frogs range in size from 10 mm (less than ½ in) in species in Brazil and Cuba to the 300 mm (1 ft) goliath frog of Cameroon.
Adult frogs are characterized by long hind legs, a short body, webbed digits, protruding eyes and no tail. They also have a three-chambered heart, which they share with all tetrapods except birds and mammals. Most frogs have a semi-aquatic lifestyle, but move easily on land by jumping or climbing. They typically lay their eggs in puddles, ponds or lakes, and their larvae, or tadpoles, have gills and develop in water.
The reliance of frogs on an aquatic environment for the egg and tadpole stages gives rise to a variety of mating behaviors that include the calls used by the males of most species to attract females to the bodies of water chosen for breeding. Frogs are most noticeable by these calls, which can occur during the day or night.
Frogs are usually well suited to jumping with long hind legs and elongated ankle bones. They have a short vertebral column, with no more than ten free vertebrae, followed by a fused tailbone. Skin hangs loosely on the body because of the lack of loose connective tissue(tissue that surrounds, supports, or connects organs, other tissues, etc.). Skin texture varies, either smooth, warty or folded.
Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaque. Frogs have a tympanum, involved in hearing, located on each side of the head, and in some species, it is covered by skin.
Adult frogs are carnivorous and eat mostly arthropods, annelids and gastropods. Adults have a ridge of very small cone teeth, called maxillary teeth, around the upper edge of the jaw and they have what are called vomerine teeth on the roof of the mouth. Since they don’t have teeth on their lower jaw, frogs usually swallow their food whole, and use the teeth they do have to hold the prey in place. Toads do not have any teeth, and so they must swallow their prey whole.
Roles of Amphibians
Frogs are raised commercially as a food source (frog legs are a delicacy in China, France, the Philippines, northern Greece and the American south, especially Louisiana). They are used in cloning research and other branches of embryology, because they lack egg shells, and therefore facilitate observations of early development. The African clawed frog or platanna (Xenopus laevis) is used as a model organism (a species that is extensively studied to understand certain biological phenomena) in developmental biology, because it is easy to raise in captivity and has a large and easily manipulated embryo. Many Xenopus genes have been identified, isolated, and cloned as a result.
Many environmental scientists believe that amphibians, including frogs, are excellent biological indicators of broader ecosystem health because of their intermediate position in food webs, their permeable skins, and typically biphasic life (aquatic larvae and terrestrial adults).
Amphibians also figure prominently in folklore, fairy tales and popular culture. Numerous legends have developed over the centuries around the salamander (its name originates from the Persian, for “fire” and “within") , many related to fire. This connection likely originates from the tendency of many salamanders to dwell inside rotting logs. When placed into the fire, salamanders would escape from the logs, lending to the belief that the salamander was created from flames.
Associations of the salamander with fire appear in the Talmud (a collection of Jewish law and tradition) and the Hadith (a traditional account of things said or done by Muhammad or his companions), as well as in the writings of Conrad Lycosthenes ( a sixteenth century humanist, encyclopedist), Benvenuto Cellini (a sixteenth century Italian goldsmith, painter, sculptor, musician, and soldier), science fiction authors Ray Bradbury and David Weber, Paracelsus (a fifteenth century alchemist, physician, and astrologer) and Leonardo da Vinci.
In other representations in popular culture, salamanders are known as minor snake demons according to some folklore; they, and frogs, appear as some characters in video games; salamanders appear in anime series, and they were even the focus of a dance craze (the Salamander Homp) in the early 1980’s. Frogs tend to be portrayed as benign, ugly, and clumsy, but with hidden talents. Examples include Michigan J. Frog, The Frog Prince, and Kermit the Frog.
The Moche people of ancient Peru worshiped animals and often depicted frogs and toads in their art. The toad also appears as symbol and in story in Vietnamese culture.
Lesson Summary
Amphibians have adaptations for both aquatic (gills), and terrestrial (lungs and moist skin) lifestyles.
Most amphibians are bound to water for reproduction.
Development includes a shell-less egg, larval stage and adult.
Salamanders have some unique features, including the use of the hyoid bone in hunting prey, and the process of ecdysis.
Adult frogs and toads have features for living in the water (such as webbed digits) and for living on the land (such as long hind legs for jumping).
Frogs are well known for their mating calls, which are used to attract females to aquatic breeding grounds.
Amphibians play a role economically as a food source; are used in various types of biological research, can serve as indica
tors of ecosystem health, and figure prominently in folklore and popular culture.
Review Questions
Describe the general traits of amphibians.
Describe the life cycle of amphibians.
What are some adaptations of amphibians for living in a terrestrial environment?
A frog’s skin must remain moist at all times in order for oxygen to pass through the skin and into the blood. Why does this fact make frogs susceptible to many toxins in the environment?
The permeability of a frog’s skin can result in water loss. What adaptations would benefit a frog by counteracting this water loss?
Name how one feature of frog development lends itself to research applications.
Amphibians have a number of adaptations which make it easy for them to avoid predation. Describe some of these.
Further Reading / Supplemental Links
Unabridged Dictionary, Second Edition. Random House, New York, 1998.
http://en.wikipedia.org/wiki
http://kids.nationalgeographic.com/Animals
http://amphibiaweb.org
http://helpafrog.org
http://www.epa.gov/gmpo/education/photo/amphibians.html
Vocabulary
convergent adaptation
The appearance of similar traits in groups of animals that are evolutionarily unrelated to each other.
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