13. Fishes, Amphibians, and Reptiles - Life Sciences in Maine
Life Sciences in Maine
Chapters
13. Fishes, Amphibians, and Reptiles
Vertebrates  have a backbone. But there are many organisms that have backbones,  including fish, amphibians, reptiles, birds, and mammals! So, how do we  distinguish between them? We put them in categories.
But  sometimes it is difficult to put organisms into specific categories.  Observe the above image. What kind of organism is it? A snake? A lizard?  A newt? It looks like a snake because it does not have legs. But it is  also has the head of a lizard. What other information do you need to  know before you can classify the above organism? What questions do you  need to ask?
You may ask: Does it have ears? What other  organisms have similar hearts or lungs? Are the organism's genes more  like those of lizards or to those of snakes? Do they live in water, on  land, or both?
These are the type of questions scientists ask  when they encounter an organism not easily classified. Consider the  differences amongst organisms who are closely related as you read about  vertebrate creatures. And then see if you can identify the above  organism.
Introduction to Vertebrates
Describe the general features of chordates.
List the three groups of chordates and their characteristics.
List the general features of vertebrates.
Describe the classification of vertebrates.
Check Your Understanding
notochord
Chordates
Did you know that fish, amphibians, reptiles, birds and mammals are all  related? They are all chordates. Chordates are a group of animals that  includes vertebrates, as well as several closely related invertebrates.  All chordates (phylum Chordata) have a notochord, a hollow nerve cord along the back.
They also have:
Pharyngeal slits, which help to filter out food particles.
An endostyle, which has small hairs and is used to gather food particles and move them along the digestive tract.
A post-anal tail, which is present during the lifetimes of some chordates and during the development of others.
The chordate phylum is broken down into three subphyla:
Urochordata (represented by tunicates): Urochordates have a notochord and nerve cord only during the larval stage (Figure below ).  The urochordates consist of 3,000 species of tunicates, sessile marine  animals with sack-like bodies and tubes for water movement.
Cephalochordata  (represented by lancelets): Cephalochordates have a notochord and nerve  cord but no vertebrae, or bones in the backbone (Figure below ). Cephalochordates consist of 30 species of lancelets (burrowing marine animals).
Vertebrata  (the vertebrates): Humans fall in this category. In all vertebrates  except for hagfish, the notochord is smaller and surrounded by  vertebrae. Vertebrates all have backbones or spinal columns. About  58,000 species have been described, including many familiar groups of  large land animals.
The origin of chordates is  currently unknown. The first clearly identifiable chordates appear in  the Cambrian Period (about 542 - 488 million years ago) as lancelet-like  specimens.
Tunicate colonies of
(subphylum Cephalochordates), perhaps the oldest known ancestor of  modern vertebrates, resembled a living chordate, known as a lancelet,  and perhaps swam much like an eel. What other modern-day organisms does a  pikaia look like?
What are Vertebrates?
Vertebrates, in the subphylum Vertebrata, are chordates with a backbone. Vertebrates have a braincase, or cranium,  and an internal skeleton (except for lampreys). You can tell the  difference between verebrates and other chordates by looking at their  head. Vertebrates have cephalization. Cephalization means an organism's  nervous tissue is found toward one end of the organism. In other words,  this is like having eyes in your head. Why do you think this type of  body design is an advantage?
Typical vertebrate traits include:
A backbone or spinal column.
Cranium.
Defined head with pronounced cephalization.
Sensory organs, especially eyes.
Living vertebrates range in size from a carp species (Figure below ), as little as 0.3 inches, to the blue whale, as large as 110 feet (Figure below ).
A species of carp.
An  image of the blue whale (a mammal), the largest living vertebrate,  reaching up to 110 feet long. Shown below it is the smallest whale  species, Hector's dolphin (about 5 feet in length), and beside it a  human.
Classification of Vertebrates
Vertebrates can be divided into two major groups: those with or without  jaws. There are more than 100 species of jawless vertebrate. There are  more than 50,000 species of vertebrate with jaws.
The jawed  vertebrates include species of fish with cartilage, the strong, flexible  tissue found in human ears, bony fish, and four-limbed animals. These  animals are known as tetrapods. Some tetrapods include amphibians,  reptiles, birds, and mammals (Table below ).
Species of the Main Groups of Tetrapods
Type of Tetrapod
Chordates are characterized by a notochord.
There are three main groups of chordates, including tunicates, lancelets and vertebrates.
Vertebrates are distinguished by having a backbone or spinal column.
Vertebrates are classified into two major groups: those without jaws and those with jaws.
Review Questions
1. What is the main feature that characterizes the chordates?
2. What are the main features of vertebrates?
Apply Concepts
3. Which two structures that all chordates possess sometime during  their life cycle (during development or otherwise) are used for food  gathering, and how are these structures used?
4. Why do you  think cephalization is not necessary in urochordates and  cephalochordates? Explain how this is illustrated in tunicates.
Critical Thinking
5. The first clearly-identifiable chordates are lancelet-like (small,  burrowing marine animals with a lancet shape) specimens. Propose one way  that these first chordates could have evolved into a swimming-like  animal.
Further Reading / Supplemental Links
How do you think a notochord could help fish adapt to swimming?
How do you think cephalization could be an advantage in movement and feeding in fish?
Fishes
List the general traits of fish.
Describe the features of jawless fish.
List the general features of the cartilaginous fish.
Describe the features of bony fish and the significance of this superclass.
List some of the reasons why fish are important.
Check Your Understanding
What are the unique characteristics of vertebrates?
What are the two main groups of vertebrates?
Vocabulary
placoid
Characteristics of Fish
What exactly is a fish? You probably think the answer is obvious. You  may say that a fish is an animal that swims in the ocean or a lake. Fish  are aquatic vertebrates, which became a dominant form of sea life and  eventually evolved into land vertebrates.
They have a number of  characteristic traits and are classified into two major groups: jawless  fish and jawed fish. Jawed fish are further divided into those with  bones and those with just cartilage. Fish, in general, are important to  humans in many ways. Can you think of some of these ways?
Some characteristics of fish include:
They are ectothermic,  meaning their temperature depends on the temperature of their  environment. This is unlike humans, whose temperature is controlled  inside of the body.
They are covered with scales.
They have two sets of paired fins and several unpaired fins.
Fish also have a streamlined body that allows them to swim rapidly (Figure below ).
The humphead or Napoleon wrasse shows some of the general traits of fish, including scales, fins and a streamlined body.
How do Fish Breathe?
In order to absorb oxygen from the water, fish use gills (Figure below ). Gills take dissolved oxygen from water as the water flows over the surface of the gill.
Gills help a fish breathe.
How Do Fish Reproduce?
Fish reproduce sexually. They lay eggs that can be fertilized either  inside or outside the body. In most fish, the eggs develop outside the  mother's body. In the majority of these species, fertilization takes  place outside the mother's body.  The male and female fish release their  gametes into the surrounding water, where fertilization occurs. Female  fish release very high numbers of eggs to increase the chances of  fertilization.
How Big Are Fish?
Fish range in size from the 51-foot whale shark (Figure below ) to the stout infantfish, which is about 0.5 inches.
One of two male whale sharks at the Georgia Aquarium. Whale sharks are the largest cartilaginous fish.
Exceptions to Common Fish Traits
There are exceptions to many of these fish traits. For example, tuna,  swordfish, and some species of shark show some warm-blooded adaptations,  and are able to raise their body temperature significantly above that  of the water around them.
Some species of fish have a slower, more maneuverable, swimming style, like eels and rays (Figure below ).  Body shape and the arrangement of fins are highly variable, and the  surface of the skin may be naked, as in moray eels, or covered with  scales. Scales can be of a variety of different types.
One of the cartilaginous fish, a stingray, shows very flexible pectoral fins connected to the head.
Although  most fish live in aquatic habitats, such as ocean, lakes, and rivers,  there are some that spend a lot of time out of water. Mudskippers, for  example, feed and interact with each other on mudflats for up to several  days at a time and only go underwater when digging burrows (Figure below ). They breathe by absorbing oxygen across the skin, similar to how frogs breathe.
See http://www.youtube.com/watch?v=ZUsARF-CBcI&#38;feature=channel for further information (5:35).
A mudskipper, shown on the mudflats, where it spends time feeding and interacting with other individuals.
Agnatha: Jawless Fishes
Jawless fish, part of the class Agnatha, belong to the phylum Chordata,  subphylum Vertebrata. There are two living groups of jawless fish, with  about 100 species in total: lampreys and hagfish (Figure below ). Although hagfish belong to the subphylum Vertebrata, they do not technically have vertebrae.
A hagfish
In  addition to the absence of jaws, fish in this class are characterized  by the absence of paired fins or an identifiable stomach.  Characteristics they do have include:
A notochord, both in larvae and adults.
Seven or more paired gill pouches.
The  branchial arches, a series of arches that support the gills of aquatic  amphibians and fishes, lie close to the body surface.
A light sensitive pineal eye, an eye-like structure that develops in some cold-blooded vertebrates.
A cartilaginous skeleton, or a skeleton made of bone-like material called cartilage.
A heart with two chambers.
Reproduction using external fertilization.
They are ectothermic.
Many  jawless fish in the fossil record were armored with heavy bony-spiky  plates. The first armored fish in this class evolved before bony fish  and tetrapods, including humans.
Cartilaginous Fishes
So why did fish eventually evolve to have jaws? Such an adaptation would  allow fish to eat a much wider variety of food, including plants and  other organisms. The cartilaginous fishes are jawed fish with paired  fins, paired nostrils, scales, two-chambered hearts, and skeletons made  of cartilage rather than bone. Cartilage does not have as much calcium  as bones, which makes bones rigid. Cartilage is softer and more flexible  than bone.
The 1,000 or so species of cartilaginous fish are  subdivided into two subclasses: the first includes sharks, rays, and  skates; the second includes chimaera, sometimes called ghost sharks.  Fish from this group range in size from the dwarf lanternshark, at 6.3  inches, to the 50-foot whale shark shown in Figure above .
Blood, Skin, and Teeth
Since they do not have bone marrow (as they have no bones), red blood  cells are produced in the spleen, in special tissue around the gonads,  and in an organ called Leydig’s Organ, only found in cartilaginous  fishes. The tough skin of this group of fish is covered with dermal  teeth, or placoid scales. In adult chimaera, the  placoids are very small, making it feel like sandpaper. The teeth found  in the mouth of cartilaginous fish evolved from teeth found on the skin.
Superorders
The sharks, rays and skates are further broken into two superorders:
Rays and skates.
Sharks (Figure below ).
Sharks are some of the most frequently studied cartilaginous fish. Sharks are distinguished by such features as:
The number of gill slits.
The numbers and types of fins.
The type of teeth.
An elongated, toothed snout used for slashing the fish that they eat, as seen in sawsharks.
Teeth used for grasping and crushing shellfish, a characteristic of bullhead sharks.
A whisker-like organ named barbels, a characteristic of carpet sharks.
A long snout (or nose-like area), characteristic of groundsharks.
Large  jaws and ovoviviparous reproduction, where the eggs develop inside the  mother’s body after internal fertilization, and the young are born  alive. This trait is characteristic of mackerel sharks.
A spotted Wobbegong shark, at Shelly Beach, Sydney, Australia, showing skin flaps around the mouth and camouflage coloration.
Bony Fishes
There are almost 27,000 species of bony fish, which are divided  into  two classes: ray-finned fish and lobe-finned fish. Most bony fish are  ray-finned. There are only eight living species of lobe-finned fish,  including lungfish (Figure below ) and coelacanths (Figure below ).
One of the only eight living species of lobe-finned fish, the lungfish.
One of the eight living species of lobe finned fish, the coelacanth.
Most fish are bony fish, making them the largest group of vertebrates in existence today. They are characterized by:
A head and pectoral girdles (arches supporting the forelimbs) that are covered with bones derived from the skin.
A  lung or swim bladder, which helps the body create a balance between  sinking and floating by either filling up with or emitting gases such as  oxygen
Jointed, segmented rods supporting the fins.
A cover over the gill, which helps them breathe without having to swim.
The ability to see in color, unlike most other fish.
One  of the most interesting adaptations of these fish is their ability to  produce replacement, bone, by replacing cartilage from within, with  bone.  They also produce “spongy," bone.  This means that the fish have a  lightweight, flexible bone on the inside, surrounded by stronger and  more rigid bone on the outside.  This type of bone allows the fish to  move in different ways compared to the cartilaginous fish.
How Big Are Bony Fish?
The ocean sunfish is the most massive bony fish in the world, up to 11 feet long, and weighing up to 5,070 pounds (Figure below ).   Other very large bony fish include the Atlantic blue marlin, the black  marlin, some sturgeon species, the giant grouper and the goliath  grouper. In contrast, the dwarf pygmy goby measures only 0.6 inches.
An ocean sunfish, the most massive bony fish in the world, up to 11 feet long and weighing 5,070 pounds!
Why Fish are Important
How are fish important? Of course, they are used 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, a type of farming  known as aquaculture.  Fish are also caught for recreation to display in the home and or in public aquaria.
Workers harvest catfish from the Delta Pride Catfish farms in Mississippi.
Lesson Summary
The general traits of fish help adapt them for living in an aquatic  environment, mostly for swimming, and also for absorbing oxygen.
Fish are ectothermic (cold-blooded), although some show warm-blooded adaptations.
Jawless fish do not have bone, but they do have cartilage.
Fish with jaws consist of both the cartilaginous fish and the bony fish.
Cartilaginous fishes include sharks, rays, skates and chimaera.
Bony fish form the largest group of vertebrates in existence today, and have true bone that can regenerate.
Fish are an important food source for humans.
Review Questions
1. What are three traits that all fish have in common?
2. What is an example of one exception to the general traits of fish?
3. What are three characteristics of jawless fish?
4. List three ways that fish are important.
Apply Concepts
5. Between the hagfish and a cartiliaginous fish, the structure of the  jaw evolved? Why do you think this major structure evolved in fish? How  does it benefit them?
6. What is the major difference between  cartilaginous fish and bony fish? Why do you think the bony fish evolved  to be different?
Critical Thinking
7. Mudskippers are  an example of a fish species that must absorb oxygen across the  skin,  instead of through gills, since they spend most of their time out of  water.  What kind of environment would pressure fish to evolve to  breathe in air instead of breathing in the water?
Further Reading / Supplemental Links
valarian respiration
Characteristics of Amphibians
What group of animals begins its life in the water, but then spends  most of its life on land?  Amphibians! Amphibians are a group of  vertebrates that has adapted to live in both water and on land.  Their  ancestors evolved from living in the sea to living on land. There are approximately 6,000 species of amphibians, of many different body types,  physiologies, and habitats, ranging from tropical to subarctic regions.  Frogs, toads, salamanders, newts, and caecilians are all types of  amphibians (Figure below ).
One of the many species of amphibian is this dusky salamander.
Like fish, amphibians are ectothermic vertebrates. They belong to the class Amphibia. There are three orders:
Urodela, containing salamanders and newts.
Anura, containing frogs and toads.
Apoda, containing caecilians.
Amphibian  larvae are born and live in water, and they breathe using gills. The  adults live on land for part of the time, and breathe both through their  skin and using lungs.
Where do Amphibians Live?
Most  amphibians live in fresh water, not salt water. Although there are no  true saltwater amphibians, a few can live in brackish (slightly salty)  water. Some species do not need any water at all, and several species  have also adapted to live in drier environments. Most amphibians still  need water to lay their eggs.
How do Amphibians Reproduce?
Amphibians reproduce sexually. The life cycle of amphibians happens in the following stages:
Egg  Stage: Amphibian eggs are fertilized in a number of ways. External  fertilization, employed by most frogs and toads, involves a male  gripping a female across her back, almost as if he is squeezing the eggs  out of her. The male releases sperm over the female's eggs as they are  laid. Another method is used by salamanders, whereby the male deposits a  packet of sperm onto the ground. The female then pulls it into her  cloaca, where fertilization occurs internally. By contrast, caecilians  and tailed frogs use internal fertilization, just like reptiles, birds  and mammals. The male deposits sperm directly into the female's cloaca.
Larval stage: When the egg hatches, the organism is legless, lives in water and breathes with gills.
During  the larval stage, the amphibian slowly transforms into an adult by  losing its gills and growing four legs. Once development is complete, it  can live on land.
How did Amphibians Adapt to Living on Land?
In order to live on land, amphibians replaced gills with another respiratory organ, the lungs.
Other adaptations include:
A glandular skin that prevents loss of water.
Eyelids that allow them to adapt to vision outside of the water.
An eardrum developed to separate the external ear from the middle ear.
In frogs and toads, the tail disappears in adulthood.
Salamanders
Salamanders belong to a group of approximately 500 species of  amphibians. The order Urodela, containing salamanders and newts, is  divided into three suborders:
Giant salamanders (including the hellbender and Asiatic salamanders).
Advanced salamanders (including lungless salamanders, mudpuppies, and newts).
Sirens.
Salamanders  are characterized by slender bodies, short legs, and long tails. They  are most closely related to the caecilians, little-known legless  amphibians (Figure below and Figure below ).   Since they have moist skin, salamanders live in or near water or on  moist ground, often in a swamp. Some species live in water most of their  life, some live their entire adult life on land, and some live in both  habitats. Salamanders are carnivorous, eating only other animals, not  plants. They will eat almost any smaller animal. Finally, salamanders  have the ability to grow back lost limbs, as well as other body parts.  This process is known as ecdysis.
The marbled salamander shows the typical salamander body plan: slender body, short legs, long tail and moist skin.
A species of African caecilian,
How Do Salamanders Breathe?
Different salamanders breathe in different ways. In those that have  gills, breathing occurs through the gills as water passes over the gill  slits. Species that live on land have lungs that are used in breathing,  much like breathing in mammals.  Other land-living salamanders do not  have lungs or gills. Instead, they "breathe", or exchange gases, through  their skin. This is known as valarian respiration, and requires blood vessels that exchange gases to be spread throughout the skin.
How Big Are Salamanders?
Salamanders are found in most moist or arid habitats in the northern  hemisphere.  They are generally small, but some can reach a foot or  more, as in the mudpuppy of North America.  In Japan and China, the  giant salamander reaches 6 feet and weighs up to 66 pounds (Figure below ).
The Pacific giant salamander can reach up to 6 feet in length and weigh up to 66 pounds.
Frogs and Toads
Frogs and toads (Figure below )  are amphibians in the order Anura. In terms of classification, there is  actually not a big difference between frogs and toads. Some amphibians  that are called "toads" have leathery, brown colored, wart covered skin,  but they are still in the same order as frogs.
A toad, showing typical characteristics of leathery and warty skin, and brown coloration.
Frogs  are found in many areas of the world, 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 less than 0.5 inches in species in Brazil and Cuba to 1-foot  goliath frog of Cameroon.
Characteristics of Frogs
Adult frogs are characterized by long hind legs, a short body, webbed finger-like parts, and the lack of a tail (Figure below ).  They also have a three-chambered heart, as do all tetrapods except  birds and mammals. Most frogs live part of the time in water and part of  the time on land, and move easily on land by jumping or climbing. To  become great jumpers, frogs evolved long hind legs and long ankle bones.  They also have a short backbone with only 10 vertebrae. Frog and toad  skin hangs loosely on the body, and skin texture can be smooth, warty,  or folded.
In order to live on land and in water, frogs have  three eyelid membranes: one is see-through to protect the eyes  underwater, and two other ones let them see on land.  Frogs also have a tympanum, which acts like a simple ear. They are found on each side of the head. In some species, the tympanum is covered by skin.
A tree frog. Notice the powerful muscles in the limbs, and the coverings around the eyes.
"Ribbiting"
Frogs typically lay their eggs in puddles, ponds or lakes. Their  larvae, or tadpoles, have gills, and the frogs develop in water. You may  hear males "ribbiting," producing a mating call used to attract females  to the bodies of water best for mating and breeding.  Frogs calls can  occur during the day or night.
Listen here http://www.youtube.com/watch?v=mISMwN-0ggE&#38;feature=fvw for these distinctive sounds (3:30).
Eating
Adult frogs are meat-eaters and eat mostly arthropods, annelids, and  gastropods.  Frogs do not have teeth on their lower jaw, so they usually  swallow their food whole, using the teeth they do have to hold the prey  in place. Other frogs do not have any teeth, so they must swallow their  prey whole.
Roles of Amphibians
Amphibians as Foods
Frogs are raised as a food source. Frog legs are a delicacy in China,  France, the Philippines, northern Greece and the American south,  especially Louisiana.
Amphibians in Research
Amphibians  are used in cloning research and other branches of embryology, because  their eggs lack shells, so it is easy to watch their development.
The African clawed frog, Xenopus laevis, is a species that is studied to understand certain biological phenomena  in developmental biology, because it is easy to raise in a lab and has a  large and easy to study embryo.  Many Xenopus genes have been identified and cloned, especially those involved in development.
Many  environmental scientists believe that amphibians, including frogs,  indicate when an environment is damaged. Since they live on land and  water, when species of frogs begin to decline, it often indicates that  there is a bigger problem with the ecosystem.
Amphibians in Popular Culture
Amphibians can be found 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 live 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.
Lesson Summary
Amphibians have adaptations for both aquatic (gills), and terrestrial (lungs and moist skin) lifestyles.
Most amphibians must reproduce in water.
Development includes a shell-less egg, larval stage, and adult stage.
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 finger-like parts) 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 as a food source, are used in various types of  biological research, can serve as indicators of ecosystem health, and  are found in folklore and popular culture.
Review Questions
1. Describe three general traits of amphibians.
2. Describe the life cycle of amphibians from egg stage to adult stage.
Apply Concepts
3. What are two ways that amphibians have adapted to living on land?
4. Why would a scientist want to use a frog for research?
5. Name one way amphibians have evolved to avoid predation.
Critical Thinking
6. 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?
Further Reading / Supplemental Links
http://www.epa.gov/gmpo/education/photo/amphibians.html
Points to Consider
Future studies of molecular genetics should soon provide further  insights to the evolutionary relationships among frog families.  These  studies will also clarify relationships among families belonging to the  rest of vertebrates. We discuss reptiles next.
Although care  of offspring is poorly understood in frogs, it is estimated that up to  20% of amphibian species care for their young, and that there is a great  diversity of parental behaviors.  As you begin to examine the  reproductive system of reptiles in the next lesson, think about what  kinds of parental behaviors reptiles might have and how they compare to  that of amphibians.
Reptiles
poikilothermic
Traits of Reptiles
What reptiles do you know? Snakes, alligators, and crocodiles are all reptiles. Reptiles are tetrapods and amniotes,  which means their embryos are surrounded by a thin membrane. Modern  reptiles live on every continent except Antarctica. They range in size  from the newly-discovered Jaragua Sphaero, at 0.6 inches, to the  saltwater crocodile, at up to 23 feet.
There are four living orders of reptiles:
Squamata, which includes lizards, snakes, and amphisbaenids (or “worm-lizards”).
Crocodilia, which include crocodiles, gharials (Figure below ), caimans, and alligators.
Testudines, which includes turtles and tortoises.
Sphenodontia, which includes tuatara (Figure below ).
An Indian gharial crocodile.
A tuatara.
Reptiles  are air-breathing, ectothermic vertebrates that have skin covered in  scales. Most reptiles have a closed circulatory system with a  three-chambered heart. All reptiles breathe using lungs. They also have  two small kidneys. Usually their sense organs, like ears, are well  developed, though snakes do not have external ears (middle and inner  ears are present). All reptiles have advanced eyesight.
How do Reptiles Reproduce?
The majority of species are egg-laying, although certain species of  squamates can give birth to live young.  This is achieved either by  oviparity (the egg stays in the female until birth), or viviparity  (offspring born without eggs). Many of the viviparous species feed their  fetuses by a placenta, similar to those of mammals. Some reptiles  provide care for their young.
All reptiles have a cloaca, a  single exit and entrance for sperm, eggs, and waste, located at the base  of the tail.  Most reptiles lay amniotic eggs covered with leathery or  calcium-containing shells.  An amnion (the innermost of the embryonic  membranes), chorion (the outermost of the membranes surrounding the  embryo), and allantois (a vascular embryonic membrane) are present  during embryonic life.  There are no larval stages of development.
Most  reptiles reproduce sexually, although six families of lizards and one  snake are capable of asexual reproduction.  In some species of  squamates, a population of females is able to produce a nonsexual  diploid clone of the mother. This asexual reproduction, called  parthenogenesis, also occurs in several species of gecko.
Lizards and Snakes
Lizards and snakes belong to the largest order of reptiles, Squamata.   Lizards are a large group of reptiles, with nearly 5,000 species, living  on every continent except Antarctica.
Characteristics of Squamata
Members of the order are distinguished by horny scales or shields and  movable quadrate bones, which make it possible to open the upper jaw  very wide. Quadrate bones are especially visible in snakes, which are  able to open their mouths very wide to eat large prey (Figure below ).
A corn snake swallowing a mouse.
Characteristics of Lizards
Key features of lizards include:
Four limbs.
A long tail, which they can shed in order to escape from predators.
They eat insects.
Vision,  including color vision, is well-developed in lizards. You may have seen  a lizard camouflaged to blend in with its surroundings. Since they have  great vision, lizards communicate by changing the color of their  bodies. They also communicate by chemical signals called pheromones.
Adult lizards range from 1 inch in length, like some Caribbean geckos, to nearly 10 feet (Figure below ).
A Komodo dragon, the largest of the lizards, attaining a length of 10 feet.
With  40 lizard families, there is an extremely wide range of color,  appearance and size of lizards. Many lizards are capable of regenerating  lost limbs or tails. Almost all lizards are carnivorous, although most  are so small that insects are their primary prey.  A few species are  omnivorous or herbivorous, and others have reached sizes where they can  prey on other vertebrates, such as birds and mammals.
Lizard Behavior
Many lizards are good climbers or fast sprinters.  Some can run on two  feet, such as the collared lizard. Some, like the basilisk, can even run  across the surface of water to escape. Many lizards can change color in  response to their environments or in times of stress (Figure below ). The most familiar example is the chameleon, but more subtle color changes can occur in other lizard species.
A species of lizard, showing general body form and camouflage against background.
Legless Lizards
Some lizard species, including the glass lizard and flap-footed  lizards, have evolved to lose their legs, or their legs are so small  that they no longer work.  Legless lizards almost look like snakes,  though structures leftover from earlier stages of evolution remain.  For  example, flap-footed lizards can be distinguished from snakes by their  external ears.
Characteristics of Snakes
All snakes are  meat-eaters, and are different from legless lizards because they do NOT  have eyelids, limbs, external ears, or forelimbs.  The more than 2,700  species of snake can be found on every continent except Antarctica and  range in size from the tiny, 4-inch-long thread snake to pythons and  anacondas that are over 17 feet long (Figure below ).
In  order to fit inside of snakes’ narrow bodies, paired organs (such as  kidneys) appear one in front of the other instead of side by side.  Snakes’ eyelids are transparent “spectacle” scales which remain  permanently closed. Most snakes are not venomous, but some have venom  capable of causing painful injury or death to humans.  However, snake  venom is primarily used for killing prey rather than for self-defense.
A species of anaconda, one of the largest snakes, which can be as long as 17 feet.
Most snakes use specialized belly scales, which grip surfaces, to move.  The body scales may be smooth, keeled or granular (Figure below ).  In the shedding of scales, known as molting, the complete outer layer of skin is shed in one layer (Figure below ).  Molting replaces old and worn skin, allows the snake to grow, and helps it get rid of parasites such as mites and ticks.
A close up of snake scales of a banded krait, showing black and yellow alternating bands and spaces between scales.
A northern water snake shedding its skin.
Although  different snake species reproduce in different ways, all snakes use  internal fertilization. The male uses sex organs stored in its tail to  transfer sperm to the female. Most species of snakes lay eggs, and most  species abandon these eggs shortly after laying.
How do Snakes Eat?
All snakes are strictly carnivorous, eating small animals including  lizards, other snakes, small mammals, birds, eggs, fish, snails or  insects. Because snakes cannot bite or tear their food to pieces, prey  must be swallowed whole.  The body size of a snake has a major influence  on its eating habits.  The snake’s jaw is unique in the animal kingdom.   Snakes have a very flexible lower jaw, the two halves of which are not  rigidly attached. They also have many other joints in their skull,  allowing them to open their mouths wide enough to swallow their prey  whole.
Some snakes have a venomous bite, which they use to kill  their prey before eating it. Other snakes kill their prey by strangling  them, and still others swallow their prey whole and alive.  After  eating, snakes enter a resting stage, while the process of digestion  takes place.  The process is highly efficient, with the snake’s  digestive enzymes dissolving and absorbing everything but the prey’s  hair and claws, even when the prey is swallowed whole!
Alligators and Crocodiles
Crocodilia, containing both alligators and crocodiles, is an order of  large reptiles.  Reptiles belonging to Crocodilia are the closest living  relatives of birds. Reptiles and birds are the only known living  descendants of the dinosaurs. Think about how organisms with the same  ancestors can evolve to be so different.
The basic crocodilian body plan (Figure below )  is a very successful one, and has changed little over time. Modern  species actually look very similar to their Cretaceous ancestors of 84  million years ago.
Nile crocodiles display the basic crocodilian body plan.
Characteristics of Crocodiles
Crocodilians have a flexible, semi-erect posture.  They can walk in  low, sprawled “belly walk,” or hold their legs more directly underneath  them to perform the “high walk.”  Most other reptiles can only walk in a  sprawled position.
All crocodilians have, like humans, teeth set  in bony sockets, but unlike mammals, they replace their teeth  throughout life.  Crocodilians also have a secondary bony palate that  enables them to breathe when under water, even if the mouth is full of  water.  Their internal nostrils open in the back of their throat, where a  special part of the tongue called the palatal valve closes off their  respiratory system when they are underwater, allowing them to breathe.
Crocodiles  and gharials (large crocodilians with longer jaws) have salivary glands  on their tongue, which are used to remove salt from their bodies.   Crocodilians are often seen lying with their mouths open, a behavior  called gaping. One of its functions is probably to cool them down, but  it may also have a social function.
Crocodilians are known to  swallow stones, known as gastroliths, which help digest their prey.  The  crocodilian stomach is divided into two chambers. The first is powerful  and muscular.  The other stomach is the most acidic digestive system of  any animal. It can digest mostly everything from their prey, including  bones, feathers, and horns!
The sex of developing crocodilians is  determined by the temperature of the eggs during incubation (eggs are  kept warm before they hatch).  This means that the sex of crocodilians  is not determined genetically. If the eggs are kept at a cold or a hot  temperature, then their offspring may be all male or all female. To get  both male and female offspring, the temperature must be kept within a  narrow range.
Evolving More Complex Structures
Like all  reptiles, crocodilians have a relatively small brain, but the  crocodilian brain is more advanced than those of other reptiles.  As in many other aquatic or amphibian tetrapods, the eyes, ears, and nostrils  are all located on the same "face" in a line one after the other.  They  see well during the day and may even have color vision, but they have  excellent night vision. A third transparent eyelid, the nictitating membrane, protects their eyes underwater.
While  birds and most reptiles have a ring of bones around each eye which  supports the eyeball, crocodiles lack these bones, just like mammals and  snakes.  The eardrums are located behind the eyes and are covered by a  movable flap of skin.  This flap closes, along with the nostrils and  eyes, when they dive, preventing water from entering their external head  openings.  The middle ear cavity has a complex of bony air-filled  passages and a branching tube.
The upper and lower jaws are  covered with "sensory pits," which hold bundles of nerve fibers that  respond to the slightest disturbance in surface water.  Crocodiles can  detect vibrations and small pressure changes in water, making it  possible for them to sense prey and danger even in total darkness.
Like  mammals and birds, and unlike other reptiles, crocodiles have a  four-chambered heart. But, unlike mammals, blood with and without oxygen  can be mixed.
Turtles
Turtles are reptiles in the order  Testudines. If you have seen turtles before, what is the most  noticeable thing about them? Their shells. Most turtle bodies are  covered by a special bony or cartilaginous shell developed from their  ribs. About 300 species are alive today, and some are highly endangered.  Like other reptiles, turtles are poikilothermic, meaning their temperature changes in response to their environment.
Turtles are broken down into two groups, based on how they bring their neck back into their shell:
Cryptodira, which can draw their neck inside and under their spine.
Pleurodira, which fold their necks to one side.
Characteristics of Turtles
Although many turtles spend large amounts of their lives underwater,  they can also spend much of their lives on dry land and breathe air.  Turtles cannot breathe in water, but can hold their breath for long  periods of time. Turtles must surface at regular intervals to refill  their lungs.
Turtles don’t lay eggs underwater. Turtles lay  slightly soft and leathery eggs, like other reptiles. The eggs of the  largest species are spherical, while the eggs of the rest are longer in  shape (Figure below ).
Turtle eggs.
Most  turtles that spend most of their life on land have their eyes looking  down at objects in front of them.  Some aquatic turtles, such as  snapping turtles and soft-shelled turtles, have eyes closer to the top  of the head. These species of turtles can hide from predators in shallow  water, where they lie entirely submerged in water except for their eyes  and nostrils.
Sea turtles (Figure below ) have glands near their eyes that produce salty tears, which remove excess salt taken in from the water they drink.
A species of sea turtle, showing placement of eyes, shell shape, and flippers.
Turtles  have exceptional night vision due to the unusually large number of  cells that sense light in their eyes. Turtles have color vision.
In  some species, temperature determines whether an egg develops into a  male or female. Large numbers of eggs are placed by the female in holes  dug into mud or sand. They are then covered and left to grow and develop  by themselves. When the turtles hatch, they squirm their way to the  surface and head toward the water.
How do Turtles Eat?
Turtles have a rigid beak and use their jaws to cut and chew food.  Instead of teeth, the upper and lower jaws of the turtle are covered by  horny ridges. Carnivorous turtles usually have knife-sharp ridges for  slicing through their prey.  Herbivorous turtles have serrated ridges  that help them cut through tough plants.
How Big Are Turtles?
The largest turtle is the great leatherback sea turtle (Figure below ),  which can have a shell length of 7 feet and can weigh more than 2,000  pounds.  The only surviving giant tortoises are on the Seychelles and  Galapagos Islands and can grow to over 4 feet in length and weigh about  670 pounds (Figure below ).  The smallest turtle is the speckled padloper tortoise of South Africa,  measuring no more than 3 inches in length, and weighing about 5 ounces.
The leatherback turtle can reach up to 7 feet in length and weigh over 2,000 pounds.
A Galapagos giant tortoise can grow to over 4 ft in length and weigh about 670 lb.
Importance of Reptiles
The chief impact of reptiles on humans is their role as predators of  pest species. For example, in many different countries, like India,  snakes kill rats that can be pests and carriers of disease. Also, since  turtles live for hundreds of years, genetic researchers are examining  the turtle's DNA for possible genes involved in a long life.
Reptiles as Food
Reptiles are also important as food sources:
Green iguanas are eaten in Central America.
The tribals of Irulas from Andhra Pradesh and Tamil Nadu in India are known to eat some of the snakes they catch.
Cantonese snake soup is consumed by local people in the fall to prevent colds.
Cooked rattlesnake meat is commonly consumed in parts of the Midwestern United States.
Turtle soup is consumed throughout the world.
Reptiles as Pets
Reptiles also make good pets. In the Western world, some snakes,  especially less aggressive species, like the ball python or corn snake,  are kept as pets. Turtles, particularly small land-dwelling and  freshwater turtles, are also common pets.  Among the most popular are  Russian tortoises, Greek spur-thighed tortoises and terrapins.
Reptiles in Art and Culture
Finally, reptiles play a significant role in folklore, religion and  popular culture. The Moche people of ancient Peru worshipped reptiles  and often put lizards in their art. Snakes or serpents are connected to  healing and to the Devil. Since snake's shed and then heal again, they  are a symbol of healing and medicine, as shown in the Rod of Asclepius (Figure below ).
In Egyptian history, the Nile cobra is found on the crown of the pharaoh.  This snake was worshipped as one of the gods.
The Rod of Asclepius, where the snake is a symbol of healing and medicine.
Lesson Summary
Reptiles are air-breathing, cold-blooded vertebrates characterized by a scaly skin.
Reptiles have a variety of reproductive systems, with different strategies for providing nutrition to developing young.
Lizards and snakes are distinguished by a unique type of scaly skin and movable quadrate bones.
Lizards have some unique adaptations, including regeneration of lost limbs or tails and changing color.
Snakes are distinguished by lack of eyelids, limbs, external ears and vestiges of forelimbs.
Snakes have various adaptations for killing and eating their prey.
Crocodilia have a flexible semi-erect posture, lifelong replacement of teeth, and a secondary bony palate.
The sex of developing crocodilians is determined by the incubation temperature of the eggs.
Other crocodilian traits, such as salt glands, nictitating membranes,  ear flaps, and sensory pits, are adaptations for aquatic living.
Turtles are characterized by a special bony or cartilaginous shell.  They have specialized adaptations for aquatic living, such as eye  placement and salt glands, and adaptations for terrestrial living as  well, like placement of eyes and protection of eggs.
Reptiles  play important roles as predators of pest species, food sources, pets,  in medical and scientific research, and in folklore, religion and  popular culture.
Review Questions
1. Describe three general traits of reptiles.
2. Describe two different types of reproduction in reptiles.
3. How are snakes different from legless lizards?
Apply Concepts
4. Name two adaptations of a crocodilian stomach which help it in digestion.
5. Name two organs that a turtle or a snake have that closely resemble organs in humans. Why are they similar?
Critical Thinking
6. The shape and structure of a turtle’s shell can give its inhabitant  advantages for avoiding predators, aid in swimming and diving, and for  walking on land.  Given what you know about a turtle’s shell, explain  how the structure and shape could help the turtle in the above  situations.
Further Reading / Supplemental Links