2Modern FishesThe term fish refers to any member of one of three general categories of vertebrates: Agnatha (jawless fishes), Chondrichthyes (cartilaginous fishes), and Osteichthyes (bony fishes).The great diversity of fishes found today reflect various adaptations that enable fishes to live in the oceans and fresh waters around the world.Fishes vary in size from whale sharks longer than a moving van to gobies no larger than your fingernail.
3Characteristics of Modern Fishes Despite the variation seen among fishes, all share certain key characteristics.Gills. Fishes normally obtain oxygen from the oxygen gas dissolved in the water around them. They do this by pumping a great deal of water through their mouths and over their gills.Single-loop Blood Circulation. Blood is pumped from the heart to the capillaries in the gills. From the gills, blood passes to the rest of the body and then returns to the heart. (Lungfishes, which have a double-loop circulation, are an exception.)
4Characteristics of Modern Fishes Vertebral Column (backbone). All fishes have an internal skeleton made of either cartilage or bone, with a vertebral column surrounding the spinal cord. The brain is fully encased within a protective covering called the skull or cranium.
5GillsIf you look closely at the face of a swimming fish, you will notice that as it swims, the fish continuously opens and closes its mouth, as if it were trying to eat the water.What looks like eating is actually breathing.The major respiratory organ of a fish is the gill.
6GillsGills are made up of rows of gill filaments – fingerlike projections through which gases enter and leave the blood.The gill filaments hang like curtains between a fish’s mouth and cheeks.At the rear of the cheek cavity is an opening called a gill slit.When a fish ‘swallows,’ water is forced over the gills and out through the gill slits.
7GillsIn countercurrent flow, water passes over the gills in one direction as blood flows in the opposite direction through capillaries in the gills.Countercurrent flow ensures that oxygen diffuses into the blood over the entire length of the capillaries in the gills.Due to this arrangement, the gills of bony fishes are extremely efficient respiratory organs.Fish gills can extract up to 85% of the oxygen in the water passing over them.
9Circulation of BloodChordates that were ancestral to the vertebrates had a simple tubular “heart.”This structure was a little more than a specialized zone of one artery that had more muscle tissue than the other arteries had.When a tubular heart contracts, blood is pushed in both directions and circulation is not very efficient.
10Circulation of BloodFor an organism with gills, such as a fish, a simple tubular heart is not an adequate pump.The tiny capillaries in a fish’s gills create resistance to the flow of blood, so a stronger pump is needed to overcome this resistance.In fishes, the tubular heart of early chordates has been replaced with a simple chamber-pump heart.The chamber-pump heart can be thought of as a tube with four chambers in a row.
12Circulation of BloodSinus Venosus. This collection chamber acts to reduce the resistance of blood flow into the heart.Atrium. Blood from the sinus venosus fills this large chamber, which has thin, muscular walls.Ventricle. The third chamber is a thick-walled pump with enough muscle tissue to contract strongly, forcing blood to flow through the gills and eventually to the rest of the body.Conus Arteriosus. This chamber is a second pump that smoothes the pulsations and adds still more force.
13KidneysVertebrates have a body that is about 2/3 water, and most will die if the amount of water in their body falls much lower than this.Minimizing dehydration (water loss) has been a key evolutionary challenge facing all vertebrates.Even some fishes must cope with the problem of water loss.The ion (salt) concentration of sea water is 3 times that of the tissues of a marine bony fish.As a result, these fishes lose water to the environment through osmosis.To make up for the water they lose, marine bony fishes drink sea water and actively pump excess ions out of their body.
14Kidneys Freshwater fishes have the opposite problem. Because their bodies contain more ions than the surrounding water, they tend to take in water through osmosis.The additional water dilutes their body salts, so freshwater fishes regain salts by actively taking them in from their environment.Although the gills play a major role in maintaining a fish’s salt and water balance, another key element is a pair of kidneys.
15Kidneys Kidneys are organs made up of thousands of nephrons. Nephrons are tubelike units that regulate the body’s salt and water balance and remove metabolic wastes from the blood.Excess water and bodily wastes leave the kidneys in the form of a fluid called urine.Marine fishes excrete small amounts of urine and rid their bodies of ammonia largely through their gills.Freshwater fishes excrete large amounts of dilute urine.
17ReproductionThe sexes are separate in most fishes, and generally fertilization takes place externally.In a process called spawning, male and female gametes are released near one another in the water.A yolk sac within each egg contains nutrients the developing embryo will need for growth.
18ReproductionThe yolk sac remains attached to the hatchling fish but is quickly used up.Then, the growing fish must seek its own food.More likely than not, it will become food for some larger animal.Many species of fish release large numbers of eggs that are fertilized in a single spawning season.This practice helps ensure that some individuals will survive to maturity.
19ReproductionThe eggs of sharks, skates, and rays are fertilized inside the female’s body.During mating, the male uses two organs called claspers to insert sperm into the female.In most species, the eggs develop inside the female and the young are born alive.A few species of sharks lay eggs.
20Jawless FishesPerhaps the most unusual fishes found today are the surviving jawless fishes, the lampreys and hagfishes.These primitive creatures have changed little over the past 330 million years.Little is known about hagfishes because they live on the ocean floor at depths as great as 1,700 m (about 1 mi).Lampreys are better understood and are found in both salt and fresh water.All species of lampreys must return to fresh water to reproduce, suggesting that their ancestors lived in fresh water.
21Jawless FishesLampreys and hagfishes have scaleless, eel-like bodies with multiple gill slits and unpaired fins.Their skeletons are made of cartilage, a strong fibrous connective tissue, and both kinds of fishes retain their notochord into adulthood.Hagfishes are scavengers of dead and dying animals on the ocean bottom.Because of this behavior, they are sometimes called the “vultures of the sea.”When threatened, a hagfish can produce huge quantities of slime from its roughly 200 slime glands.
22Jawless Fishes Most lampreys are parasitic on other living fishes. A lamprey has a suction-cup-like structure around its mouth that it uses to attach itself to its host.After attachment, the lamprey gouges out a wound with its rough tongue, feeding on blood and bits of flesh from the wound.
23Cartilaginous FishesSharks, skates, and rays are cartilaginous fishes.Their skeletons are made of cartilage strengthened by the mineral calcium carbonate.Calcium carbonate is deposited in the outer layers of cartilage and forms a thin layer that reinforces the cartilage.The result is a very light but strong skeleton.
24Cartilaginous FishesThe shark’s light, streamlined body allows it to move quickly through the water in search of prey.The shark’s skin contains cone-shaped placoid scales, which give the skin a rough texture.The shark’s scales and teeth are quite similar in structure.This is because the teeth are actually modified scales.
25Teeth of SharkThe shark’s teeth are arranged in 6 to 10 rows along the shark’s jaw.The teeth in front are pointed and sharp and are used for biting and cutting.Behind the front teeth, rows of immature teeth are growing.When a front tooth breaks or is worn down, a replacement tooth moves forward.One shark may use more than 20,000 teeth during its lifetime.This system of tooth replacement guarantees that the teeth being used are always new and sharp.
26Cartilaginous FishesTwo smaller groups of cartilaginous fishes, the skates and rays have flattened bodies that are well adapted to life on the sea floor.Rays are usually less than 1 m (3.3 ft) long, while skates are typically smaller.The giant manta ray may be up to 7 m (23 ft) wide.Most species of skates and rays have flattened teeth that are used to crush their prey, mainly small fishes and invertebrates.
27Bony FishesJawless and cartilaginous fishes are not as diverse as bony fishes, which are the most numerous of all the fishes.Bony fishes have a strong, internal skeleton made completely of bone, bony fishes and have a series of unique structural adaptations that contributed to their success.
28Lateral Line SystemBony fishes have a fully developed lateral line system.The lateral line is a sensory system that extends along each side of a bony fish’s body.As moving water presses against the fish’s sides, nerve impulses from ciliated sensory cells within the lateral line permit the fish to perceive its position and rate of movement.
29Lateral Line SystemThe lateral line system also enables a fish to detect a motionless object by the movement of water deflected by that object.The way that a fish detects an object with its lateral line and the way that you hear music with your inner ear are quite similar.Both processes share the same basic mechanism, sensory cells with cilia detect vibrations and send this information to the brain.
30Gill CoverMost bony fishes have a hard plate, an operculum that covers the gills on each side of the head.Movements of certain muscles and of the opercula permit a bony fish to draw water over the gills, which enables the fish to take in oxygen.
31Gill CoverThrough opening and closing the operculum, most bony fishes can move water over their gills while remaining stationary in the water.A bony fish doesn’t have to swim forward with its mouth open to move water over its gills.This ability to respire without swimming enables a bony fish to conserve energy that can be spent chasing after prey and escaping from predators.
32Swim BladderThe density of the fish body is slightly greater than that of sea water.Bony fishes contain a special gas sac called a swim bladder.By adjusting the gas content of the swim bladder, bony fishes can regulate their buoyancy.As the swim bladder fills, the fish rises, and as it empties, the fish sinks.
33Swim BladderThe swim bladder of early bony fishes was connected to their throat, and they gulped air to fill it.The swim bladder of modern bony fishes does not have a direct passage to the mouth.Instead, gas is exchanged between the bloodstream and the swim bladder.This permits the fish to maintain or change its depth in the water.
34Ray-Finned Bony Fishes Ray-finned bony fishes comprise the vast majority of living fishes.Their fins are supported by bony structures called rays.Teleosts, such as the yellow perch are the most advanced of the ray-finned bony fishes.Teleosts have highly mobile fins, very thin scales, and completely symmetrical tails.About 95% of all living fish species are teleosts.
37Lobe-Finned Bony fishes Only several species of lobe-finned fishes survive today.One species is the coelacanth and the other six species are all lungfishes.Coelacanth can reach up to nearly 3 m (9.8 ft in length).The lobe-finned fishes have paired fins that are structurally very different from the fins of ray-finned fishes.In many lobe-finned fishes, each fin consists of a long, fleshy, muscular lobe that is supported by a central core of bones.These bones are connected by joints, like the joints between the bones in your hand.Bony rays are found only at the tips of each lobed fin.Muscles within each lobe can move the bony rays independently of each other.
38Lobe-Finned Bony fishes Scientists have debated whether the direct ancestor of amphibians was a coelacanth or a lungfish.Recent evidence has led biologists to believe that it was neither.The ancestor of the amphibians most likely was a third type of lobe-finned fish that is now extinct.
39AmphibiansClass amphibia contains three orders of living amphibians: order Anura (frogs and toads), order Urodela (salamanders and newts), and order Apods (caecilians).Most amphibians share five key characteristics.Legs. The evolution of legs was an important adaptation for living on land. Frogs, toads, salamanders, and newts have four legs. Caecilians lost their legs during the evolutionary course of adapting to a burrowing existence.
40AmphibiansLungs. Although larval amphibians have gills, most adult amphibians breathe with a pair of lungs. Lungless salamanders are an exception.Double-loop circulation. Two large veins called pulmonary veins return oxygen-rich blood from the lungs to the heart. The blood is then pumped to the tissues at a much higher pressure than in the fish heart.
41AmphibiansPartially divided heart. The atrium of the amphibian heart is divided into left and right sides, but the ventricle is not. A mixture of oxygen-rich and oxygen-poor blood is delivered to the amphibian’s body tissues.Cutaneous respiration. Most amphibians supplement their oxygen intake by respiring directly through their moist skin. Cutaneous respiration (‘skin breathing’) limits the maximum body size of amphibians because it is efficient only when there is a high ratio of skin surface area to body volume.
42Amphibian LungsAlthough air contains about 20 times as much oxygen as sea water, gills cannot function as respiratory organs when out of water.One of the major challenges that faced the first land vertebrates was that of obtaining oxygen from air.The evolutionary solution to this challenge was the lung.
43Amphibian LungsA lung is an internal, baglike respiratory organ that allows oxygen and carbon dioxide to be exchanged between the air and the bloodstream.The amount of oxygen a lung can absorb depends on its internal surface area.The greater the surface area, the greater the amount of oxygen that can be absorbed.In amphibians, the lungs are sacs with folds on their inner membrane that increase their surface area.
44Amphibian LungsWhich each breath, fresh oxygen-rich air is drawn into the lungs.There it mixes with a small volume of air that has already given up most of its oxygen.Because of this mixing, the respiratory efficiency of lungs is much less than that of gills.Because there is much more oxygen in air than there is in the water, lungs do not have to be as efficient as gills.Many amphibians also obtain oxygen through their thin, moist skin.
45Double-Loop Circulation As amphibians evolved and became active on land, their circulatory system changed, resulting in a second circulatory loop.This change allowed more oxygen to be delivered to their muscles.
46Double-Loop Circulation Amphibians have a pair of blood vessels not found in fishes, the pulmonary veins.The pulmonary veins carry oxygen-rich blood from the amphibian’s lungs to its heart.The heart pumps the oxygen-rich blood to the rest of the body.The advantage of this arrangement is that oxygen-rich blood can be pumped to the amphibian’s tissues as a much higher pressure than it can in fishes.
47Circulation of BloodNot only did the path of circulation in amphibians change, but several important changes occurred in the heart.The sinus venosus continues to deliver oxygen-poor blood from the body to the right side of the heart.Oxygen-rich blood from the lungs enters the left side of the heart directly.
48Circulation of BloodA dividing wall known as the septum separates the amphibian atrium into right and left halves.The septum prevents the complete mixing of oxygen-rich and oxygen-poor blood as each enters the heart.Both types of blood empty into a single ventricle, where some mixing of blood occurs.Due to the anatomy of the ventricle, the two streams of blood remain somewhat separate.Oxygen-rich blood tends to stay on the side that directs blood toward the body.Oxygen-poor blood tends to stay on the side that directs blood toward the lungs.
49Circulation of BloodA number of amphibians have a spiral valve that divides the conus arteriosus.The spiral valve also helps to keep the two streams of blood separate as they leave the heart.
50Frogs and ToadsThe order Anura is made up of frogs and toads that live in environments ranging from deserts to rain forests, valleys to mountains, and ponds to puddles.Adult anurans are carnivorous, eating a wide variety of small prey.Some species have a sticky tongue that they extend rapidly to catch their prey.
51Frogs and ToadsThe frog body is adapted for jumping, with its long muscular legs providing the power.Toads are very similar to frogs but have squat bodies and shorter legs.The toad’s skin is not smooth like that of a frog but is covered with bumps.
52Reproduction in FrogsLike most living amphibians, frogs depend on the presence of water to complete their life cycle.The female releases her eggs into the water and a male’s sperm fertilizes them externally.After a few days, the fertilized eggs hatch into a swimming, fishlike larval form called tadpoles.Tadpoles breathe with gills and feed mostly on algae.After a period of growth, the body of the tadpole changes into that of an adult frog.The rate at which tadpoles develop depends on the species and the availability of food.This process of physical change is called metamorphosis.
54SalamandersSalamanders have elongated bodies, long tails, and smooth, moist skin.There are about 369 species of salamanders all belonging to the order Urodela.They typically range from 10 cm to .3 m (4 in to 1ft) in length.However, giant Asiatic salamanders of the genus Andrias grow as long as 1.5 m (5 ft) and weight up to 41 kg (90 lbs).Because salamanders need to keep their skin moist, most are unable to remain away from water for long periods, although some salamander species manage to live in dry areas by remaining inactive during the day.
55Salamander Reproduction Salamanders lay their eggs in water or in moist places.Fertilization is usually external.A few species of salamanders practice a type of internal fertilization in which the female picks up a sperm packet that has been deposited by the male and places it in her cloaca.Unlike frog and toad larvae, salamander larvae do not undergo a dramatic metamorphosis.
56Salamander Reproduction The young that hatch from salamander eggs are carnivorous and resemble small versions of the adults, except that the young usually have gills.A few species of salamanders, such as the North American mudpuppy and the Texas spring salamander, retain their external gills as adults.
57CaeciliansCaecilians (order Apoda) are a highly specialized group of tropical, burrowing amphibians with small, bony scales embedded in their skin.They feed on small invertebrates found in the soil.These legless, wormlike animals grow to about .3 m (1 ft) long, although some species can be up to 1.2 m (4 ft) long.During breeding, the male deposits sperm directly into the female.Depending on the species, the female may bear live young or lay eggs that develop externally.Caecilians are rarely seen, and scientists do not know much about their behavior.