Chapter 31: Fishes and Amphibians

Section 1: Fishes

What is a Fish?

• Fishes and other vertebrates have at some time during their development a notochord, a hollow dorsal nerve cord, and pharyngeal slits

• In most vertebrates, the notochord is replaced during development by a backbone, or vertebral column, which encloses and protects much of the nerve cord

• In addition, most vertebrates have two sets of paired appendages, a closed circulatory system with a ventral heart, and either gills or lungs for breathing

What is a Fish?

• Fishes can be defined as aquatic vertebrates that are characterized by scales, fins, and pharyngeal gills

• There are so many fishes, living and extinct, that their correct scientific classification is complicated

• For our purposes, we can say that the living fishes fall into three main groups: jawless fishes, sharks and their relatives, and bony fishes– Sharks and their relatives are also known as

cartilaginous fishes because their skeletons are made up of soft, flexible cartilage rather than bone

In ocean water deep below the reach of sunlight live fishes that sparkle with light produced by their own body. The lights on the viperfish may serve to attract prey or distract predators.

Representatives of the three main groups of living fishes are shown here. The lamprey is a parasitic jawless fish. The blue-spotted stingray is a venomous bottom-dwelling cartilaginous fish. The Potter’s angelfish, which is found only in Hawaiian coral reefs, is a bony fish.

Evolution of Fishes• Fishes are the most primitive living vertebrates

• The first fishes were odd-looking jawless creatures whose bodies were covered with bony plates

– Lived in the oceans of the late Cambrian period (540 MYA)

– For over 100 MY, fishes retained the basic armored jawless body plan

Evolution of Fishes

– Then, during the Ordovician and Silurian periods, fishes underwent a major adaptive radiation• Jawless fishes – little armor• Jawless fishes – no armor

–Still other were armored fishes that possessed a feeding adaptation that would revolutionize vertebrate evolution: jaws

Evolution of Fishes

• Jawless fishes are limited to eating small particles of food

• Jaws made it possible for vertebrates to nibble on plants, munch on other animals, and defend themselves by biting

• Also evolved pectoral and pelvic fins

– More control over their movement

Form and Function in Fishes

• Fishes have entered many environments and evolved adaptations that enable them to survive a tremendous variety of conditions

Feeding

• Every mode of feeding is seen in fishes– Herbivores, Carnivores, Parasites, Filter feeders,

Detritus feeders• A single fish may exhibit several modes of feeding,

depending on what type of food is available• The adaptations for feeding in fishes are often

remarkable– Sawfish– Parrotfish

Feeding

• Most fish do not chew their food

• Instead, they tear their food into chunks or swallow their prey whole

Feeding

• From the mouth food passes through the esophagus and into the stomach, where it is partially broken down

• In many fishes, the food is further processed in the pyloric ceca, which are located at the point where the stomach and the intestine meet

– Secrete digestive enzymes and absorb nutrients from the digested food

Feeding

• The intestine receives partially digested food from the stomach and pyloric ceca and completes the process of digestion and nutrient absorption

• Any materials that remain undigested after passing through the intestine are eliminated through the anus

Fishes are adapted to many modes of feeding. Some male anglerfishes are parasites that are nourished by the blood of their much larger mates. Note the male attached to the head of this female. The parrotfish uses its “beak” to bite off chunks of algae covered coral. It digests the plant material and passes the crushed coral dust through its gut. The sawfish slashes its way through schools of fishes, then doubles back to devour the dead or wounded prey.

The internal organs of a typical bony fish are seen here.

Respiration

• Most fishes breathe with gills that are located on either side of the pharynx

• Most fishes breathe by pumping water through the mouth, over the gill filaments, and out through slits in the sides of the pharynx

• A number of fishes have an adaptation that allows them to survive in oxygen-poor water or in areas where bodies of water often dry up

Respiration

• They have specialized organs that serve as lungs by obtaining oxygen from the air

– Modified swim bladder

• Swim bladder is a gas filled sac that controls the fishes buoyancy

In some fishes, such as sharks, the gill chambers open to the outside through a number of slits. In other fishes, the gill chambers empty through a single opening that is covered by a protective flap.

Internal Transport

• Typically have closed circulatory systems with a heart that pumps blood around the body

• The heart consists of two muscular pumping chambers– Atrium– Ventricle

• Pumps blood out of the heart into a muscular vessel called the aorta

Almost all fish have a closed circulatory system in which a two-chambered heart pumps oxygen-poor blood from the body to the gills. Oxygen-rich blood then travels from the gills to all parts of the body.

Excretion

• Most fishes get rid of nitrogenous wastes in the form of ammonia

• Some wastes diffuses through the gills into the surrounding water

• Others are removed by the kidneys, which are excretory organs composed of many tubules that filter nitrogenous wastes from the blood and concentrate them

• Kidneys help fishes control the amount of water in their body

Response• Fishes have a fairly well-developed nervous system

organized around a brain• The most anterior parts of a fish’s brain are the

olfactory bulbs, which are connected by stalks to the two lobes of the cerebrum

• Cerebrum – sense of smell, taking care of young, exploring environment

• The optic lobes process information from the eyes• The cerebellum coordinates body movements• The medulla controls many internal organ functions

and maintains balance

Response

• Posterior to the brain is the spinal cord

• In cartilaginous and bony fishes, the spinal cord is enclosed and protected by the vertebral column

• Between each set of vertebrae, a pair of spinal nerves exits the cord and connects with internal organs and muscles

Response

• Most fishes have superbly designed sense organs that collect information about their environment

• Chemoreceptors

• Most fishes have ears inside their head, but they cannot hear sounds well– Detect vibrations

The brain of a typical fish has several clearly visible parts.

The sense organs in fishes are highly developed. A chimaera’s huge silvery eyes enable it to see in the permanent dark of its deep-water home. The lateral line, which appears as a series of tiny dots in the pink stripe of the rainbow trout, detects water movements. Some fishes, such as the elephant fish, are able to detect electricity.

Reproduction

• Most fishes have separate male and female sexes

• A number of fishes are born as males but change to females as they grow older

• Many fishes are oviparous

– Lay eggs

• External fertilization

Reproduction

• Some species of fishes are ovoviviparous

– Young develop inside the mother’s body but are not directly nourished by the mother’s body

• Other species are viviparous

– Truly live-bearing

Some newly hatched fishes, such as salmon, are nourished by a yolk sac on their belly.

Jawless Fishes

• Divided into two classes

– Lampreys and hagfishes

• Only vertebrates that do not have backbones as adults

• Instead, their long, snakelike bodies are supported by a notochord

Modern jawless fishes are divided into two classes: lampreys and hagfishes.

Lampreys• Filter feeders• Adult’s head is completely taken up by a circular

sucking disk with a round jawless mouth in the center• Live by attaching themselves to fishes and scraping

away at the skin with their large teeth and a strong tongue

• Then suck up tissues and body fluids• Rarely kill their host, but do leave it in a weakened

condition with a large open wound that is easily infected

Hagfishes• Most primitive vertebrates alive today

• Pinkish-gray wormlike bodies

• 4 – 6 short tentacles around the mouth

• Lack eyes

• Do have light detecting regions scattered around their body

• Feed on dead and dying fish by using a toothed tongue to scrape a hole into the fish’s side

Hagfishes

• Peculiar traits

– Secrete incredible amounts of slime

– Have six hearts

– Open circulatory system

– Tie themselves into knots

Sharks and Their Relatives

• Class Chondrichthyes• Contains sharks, rays, skates, etc.• All members have an endoskeleton made

entirely of cartilage• 225 living shark species

– Curved tails, torpedo shaped bodies, and rounded snouts

– Enormous number of teeth• 3000 arranged in 6 – 20 rows

Sharks and Their Relatives

• Not all sharks attack people

• Some are filter feeders

• Others have flat teeth adapted for crushing the shells of mollusks and crustaceans

• Each year more people are killed by lightning than by sharks

Sharks and Their Relatives

• Rays and skates are adapted for living on the ocean floor

– Flattened from top to bottom

– Swim by flapping their large wing like pectoral fins

– Most reach a maximum length of about 1 meter

Cartilaginous fishes include sharks and rays. The wobbegong, or carpet shark, is a bottom dweller that feeds primarily on fishes. The leopard shark is one of the most attractive sharks. Its teeth are adapted for crushing the shells of mollusks and crustaceans. The underside of some rays seems to have an almost human face.

Bony Fishes

• Class Osteichthyes

• More species in this class than in any other vertebrate class

• About 40% of all vertebrates are bony fishes

• Somewhere between 15,000 and 40,000 species alive today

Bony Fishes

• Almost all bony fishes belong to the enormous group called the ray-finned fishes

• Includes everything from guppies to groupers, salmon, and eels

• The name refers to the thin bony spines, or rays, that are connected by a thin layer of skin to form the fins

• These fins are adapted to a wide variety of functions

Bony Fishes• Only seven living species of bony fishes are not classified

as ray-finned fishes• These are the lungfishes and the coelacanth• These fishes are of interest because they give us an idea of

what the lungs and limbs may have been like in the ancestors of terrestrial vertebrates

• The six species of lungfishes alive today are found in Australia, Africa, and South America

• When water is available, lungfish use their gills to eliminate carbon dioxide, but they get most of their oxygen by gulping air into a simple sac that functions as a lung

• During the dry season, lungfish burrow in the mud and enter a dormant state

Bony Fishes• The single species of coelacanth alive today is the only

surviving member of the lobe-finned fishes• Unlike ray-finned fishes, which have many bones in the

bases of their fins, coelacanths have few bones in their fin bases

• Attached to those bones are a few large rays that form the fins

• Ancient lobe-finned fishes seemed to have lived in swampy areas where shallow pools alternated with mud flats and sand bars

• The modern coelacanth lives in water about 70 to 400 meters deep in a relatively small area of ocean off the western coast of Africa

Bony Fishes

• Coelacanths were thought to have disappeared with the dinosaurs about 70 million years ago

• In 1938, however, fishermen sailing in the ocean off the coast of Africa caught a coelacanth

• Scientists were enormously excited to find living coelacanths because these animals represent a fascinating piece of evolutionary history– The closest thing we known of to the ancestors

of all land vertebrates

How Fishes Fit into the World• Fishes are vital parts of many biological systems• For many birds and mammals fishes are important

foods• As predators and herbivores, fishes help control

the populations of the organisms they eat• Since prehistoric times, humans have caught

fishes for food and recreation• For at least 4000 years, humans have also raised

fishes in artificial ponds

Bony fishes come in a wide variety of forms and colors. The porcupine fish can inflate itself into a prickly ball when threatened. The moray eel has a narrow snakelike body. The bright colors of angelfish may be a means of communication within its species. The hawk fish's narrow snout enables it to pluck bits of food from crevices.

A few fishes manage quite nicely out of water for brief periods of time. African lungfishes get most of their oxygen from the air, which they gulp into a simple sac that serves as a lung. Mudskippers climb out of the water onto logs and rocks. As you can see, the mudskipper’s bulging eyes are quite mobile, enabling it to appear as if it has eyes on the back of its head.

Chapter 31: Fishes and Amphibians

Section 2: Amphibians

Amphibians• About 4,000 living species• Amphibians are the smallest major group of

vertebrates• Range in size from tiny tropical tree frogs

1cm long to enormous salamanders 170 cm long

• Some have long tails and walk on four legs• Others have no tails and leap from one place

to another with large hind legs

Amphibians

• They are descendents of ancestral organisms that evolved some of the adaptations necessary for life on land

• Nearly all of them are restricted to moist areas, and most of them must return to water to breed

Representatives of the three orders of living amphibians – salamanders, frogs and toads, and legless amphibians – are shown here.

What Is an Amphibian?

• The name amphibians refers to the double life that most amphibians lead– Larvae are fishlike aquatic animals that

breathe through gills– Adults are terrestrial carnivores that

breathe through lungs and skin• Amphibians are strongly tied to the water

because their eggs do not have a shell

What Is an Amphibian?

• Their skin does not have scales or any other protective covering

• The skin of almost all adults is used in respiration and must remain moist

• Amphibians can be defined as vertebrates that are aquatic as larvae and terrestrial as adults, breathe with lungs as adults, have a moist skin that contains many glands, and lack scales and claws

Most amphibians spend the first part of their life in water as gilled larvae. As adults, they usually live on land.

Evolution of Amphibians

• Amphibians first appeared about 360 million years ago

• Amphibians evolved from lobe-finned fishes

• Making the transition from water to land was not easy

• Because natural selection favored individuals that were better able to live on land, early amphibians evolved in ways that went above many problems

– Bones became stronger

– Ribs formed a cage to protect internal organs

– Ears were added to the lateral line system

– Mucous glands, eyelids, and other structures that protect sense organs from drying out developed

Evolution of Amphibians• When amphibians started crawling onto land,

they entered an environment nearly empty of animal life

• Any vertebrates whose legs and lungs allowed them to spend time on land had lots of food and no competitors

• This was short-lived• Climate changes ultimately caused many of

the low, swampy amphibian habitats to disappear

• Most of the amphibian groups became extinct about 245 million years ago

Form and Function in Amphibians

• Living amphibians have evolved many adaptations that help them overcome the problems of living both in water and on land

• As we examine the essential life functions in amphibians, we will focus on frogs

Feeding• Tadpoles are typically filter feeders or herbivores• Tadpoles have to grow quickly

– Those that lag behind may starve or die if their puddle dries out

• Adult amphibians are almost entirely carnivorous• From the mouth, food slides down the esophagus into the

stomach• The stomach connects with the small intestine• The small intestine leads to the large intestine, or colon• At the end of the large intestine is a muscular cavity called

the cloaca, which stores wastes until they are expelled

Respiration• Adults typically breathe using lungs, mouth cavities, and

skin• The lungs are reasonably well developed

– Capillaries• The lining of the mouth cavity and the body skin are thin

and richly supplied with blood vessels• Most carbon dioxide is removed through the skin• Frogs cannot inhale and exhale as we do

– They fill their mouth cavity with air, close their mouth, and force air back through an opening called the glottis into the lungs

• Frogs can also direct some of the air they take in to a pair of vocal sacs

As air moves between the vocal sac and mouth in male frogs it causes the vocal cords to vibrate. Although the resulting sounds may not be music to human ears, a female frog finds them quite attractive.

Internal Transport

• Circulatory system is a double loop– First loop carries oxygen-poor blood from the heart to

the lungs and takes oxygen-rich blood from the lungs back to the heart

– The second loop transports oxygen-rich blood from the heart to the rest of the body and oxygen-poor blood from the body back to the heart

• 3 chambered heart– Left atrium, right atrium, and ventricle

• Tadpoles have two chambered hearts and single-looped circulatory systems

Excretion

• Amphibians use kidneys to eliminate wastes from their bloodstream

• The kidneys are dark colored oval structures that lie against the dorsal part of the body wall

• The excretory product of the kidneys – urine – travels through tubes called ureters into the cloaca

• From there it can be passed directly to the outside or it may be stored in a small urinary bladder

Response• Well developed nervous and sensory systems• Large eyes with a nictitating membrane

– Protects from damage under water– Kept moist on land– Located inside the regular eyelid

• Ears have no external sound collectors– Hearing is vital to their survival and reproduction

• Respond to adverse conditions in many different ways– Being in a sheltered spot, dormant state– Camouflage– Distasteful or toxic chemicals– Warning coloration

The brain of a frog has the same basic parts as that of other vertebrates.

Frogs escape predators in many ways.

Reproduction• When frogs reproduce, the male climbs onto the

female’s back and squeezes• In response to this, the female releases as many as

200 eggs• The embryos are surrounded with a sticky

substance that attaches the egg mass to underwater plants and nourishes the developing embryos

• The eggs typically hatch into tadpoles after 1 – 3 weeks

• Not all amphibians have external fertilization

Reproduction

• Parental care in amphibians varies greatly• Some frogs incubate their young in their

mouth, vocal sac, or stomach• Some have special sacs to store the eggs• In certain tree frogs, tadpoles cling to their

parent’s back with a sucker like mouth and are carried between pools of rainwater that collect among the leaves of certain plants

Salamanders

• These amphibians keep their tails even as adults• Both adults and larvae are carnivores• Most modern salamanders are about 15cm long• Most hatch as fully aquatic larvae with gills• As adults they live in moist woods, where they tunnel

under rocks and rotting logs• Some salamanders never lose their gills and live in

water all their life• Some newts switch back and forth between water and

land

Salamanders are usually terrestrial as adults. However, certain species of salamanders, known as newts, are terrestrial only during an immature phase known as red eft.

Frogs and Toads

• Of the two, frogs are most closely tied to water• Frogs spend much of their time in or near ponds and

streams• Adult toads, on the other hand, often live in moist

woods• These animals burrow deep into moist soil and press

their skin against the walls of their burrows• Many toads and frogs produce potent toxins• One species of poison arrow frog produces a toxin so

powerful that 0.00001 gram can kill an adult human

How Amphibians Fit into the World

• Prey on insects• Frog legs are a delicacy in some cultures• Native hunters tip their arrows with toxins

to kill large animals• Researchers are studying the action of

poison arrow frog toxins for clues to the way the nervous system works

• Studies of regeneration