Sharks

by Andrea Gibson, Robin Carter (Illustrator)

Overview

This eBook is best viewed on a color device.

At last, a fantastically illustrated new addition to the classic nature guide series.

A Golden Guide to Sharks, like all the books in the series, presents a general overview of its subject in accessible prose, illuminated by detailed illustrations of the species discussed. Its directness enables it to be appreciated by young and mature audiences alike.

Product Details

ISBN-13:	9781466864863
Publisher:	St. Martin's Publishing Group
Publication date:	02/25/2014
Series:	A Golden Guide from St. Martin's Press
Sold by:	Macmillan
Format:	eBook
Pages:	160
File size:	17 MB
Note:	This product may take a few minutes to download.

About the Author

Golden Guides first appeared in 1949 and quickly established themselves as authorities on subjects from Natural History to Science. Relaunched in 2000, Golden Guides from St. Martin's Press feature modern, new covers as part of a multi-year, million-dollar program to revise, update, and expand the complete line of guides for a new generation of students.

Robin Carter is a versatile freelance illustrator from the United Kingdom whose work includes nature and wildlife, children's books and cartoons, technical and scientific illustration, and fine art paintings.

Read an Excerpt

Sharks

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By Andrea Gibson, Robin Carter

St. Martin's Press

Copyright © 2002 St. Martin's Press
All rights reserved.
ISBN: 978-1-4668-6486-3

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CHAPTER 1

WHAT IS A SHARK?

Unlike most vertebrates, whose bodies are supported by a skeleton of hard bone, sharks have a softer, more flexible skeleton of cartilage, or gristle. In zoological terms they are fish of the class Chondrichthyes. Together with their close relatives the rays, sharks constitute the subclass Elasmobranchii. Their more distant relatives the chimeras form the subclass Holocephalii.

There are more than 420 known species of shark and more than 550 species of ray. Since their appearance in the fossil record more than 350 million years ago, sharks appear to have changed little, however, they have undergone several dramatic changes. These have enabled the sharks to inhabit niches in every ocean and sea, from the icy polar waters to the tropical seas and even freshwater bodies.

Equipped with superb senses, capable of extremely rapid movement, and armed with rows of lethally sharp teeth, the typical shark is the top predator in its food chain.

But these very qualities that have shaped the shark for survival have also led to a downturn in its fortunes.

Most people see the shark as a ruthless killing machine; a fearsome predator prowling the seas, ready to attack and eat anything that it encounters – including people. This exaggerated image, promoted and glorified by a sensationalist media, has earned the shark a truely undeserved reputation as one of nature's true villains.

Today, people are clearing the seas of sharks, leaving many species struggling for survival in an environment they had mastered long before humans walked the earth.

In truth, the majority of shark species are harmless, unless provoked. Most are small, placid creatures, and as the phrase goes, "you are more likely to be bitten by a New Yorker than a shark."

CHAPTER 2

ADAPTATIONS

Though there are exceptions to the rule, most sharks follow a common body plan. The typical shark has a streamlined, spindle-shaped body with a muscular tail and fins. These propel it efficiently and rapidly through water, suiting its lifestyle as an active predator at the top of the oceanic food chain.

The streamlined body plan is typical of pelagic (open-water) fish species. The Mackerel Sharks, family Lamnidae, are typically stouter than other species, their natural design exhibiting almost perfect hydrodynamics. The snout is pointed and laterally flattened, and the tail is designed to give maximum thrust and minimum drag. Mackerel Sharks are the pinnacle of shark design, and the Shortfin Mako (Isurus oxyrinchus) is probably the fastest-swimming of any shark, reaching speeds of well over 50 mph (80 km/h).

TAIL-FIN SHAPE

The tail fin, otherwise known as the caudal fin, in addition to being a key identifying feature, can also reveal a lot about the shark's lifestyle and swimming habits.

TIGER SHARK

A strong upper lobe delivers power for cruising or for sudden, short sprints. This shark relies on this high maneuverability while hunting for its varied diet of turtles, fish, stingrays, and other shark species.

NURSE SHARK

A species of the shallow waters, spending most of the daylight hours resting on the seabed. It swims slowly, with a sinuous, eel-like motion and broad sweeps of its long tail, as it searches for crustaceans and small fish.

THRESHER SHARK

An active and powerful hunter of squid and fish, which it herds and stuns using its powerful and elongate upper tail lobe.

GREAT WHITE SHARK

Chiefly a coastal species that relies on its equal-sized tail lobes for slow cruising and high-speed sprints after its often fast-moving and agile prey.

COOKIECUTTER SHARK

A tiny but savage species, fastening its needlelike teeth on a range of prey items, from crustaceans to marlins, dolphins and larger sharks. The tail, which has broad upper and lower lobes of similar size, is ideally suited to its deep-water, active lifestyle.

CHAPTER 3

PREHISTORIC SHARKS

IN THE BEGINNING

Long before dinosaurs roamed the Earth, sharks had already undergone a transformation from small, sucker-mouthed scavengers into top predators.

The first cartilaginous fish appeared more than 400 million years ago in the Silurian period of the Paleozoic era. By the Devonian period, 360 million years ago, bony and cartilaginous fish had diversified into all parts of the world. This period has often been termed the Age of Fishes.

Shark evolution is still the subject of much debate. They are poorly represented in the fossil record because their cartilaginous skeletons do not fossilize well. The knowledge we have gained comes mainly from studying fossilized teeth, which, with their covering of hardened enamel, are the main calcifying materials in a shark's body. Occasionally, body parts such as tiny fin scales are found, although these are so small that they require microscopic examination. There are a few examples of whole carcasses being preserved, but they are also rare due to the fortuitous conditions required to preserve their soft tissues.

Sharks may be related to an extinct Paleozoic group of jawless fishes called thelodonts, which had scales similar to today's species, or they may have evolved alongside placoderms: giant jawed fishes with protective bony armor. Alternatively, they may have an entirely different ancestor.

The oldest-recognized sharklike fish is of the extinct mid-Devonian genus Cladoselache. Cladoselache was probably only 5–6 feet (1.5–1.8 m) long. It had a streamlined body with five pairs of gill slits; stiffened, paired, pectoral and pelvic fins; and long, slender jaws bearing distinctive teeth. It was probably a swift predator, actively hunting down prey items, swallowing them tail first. Although strikingly similar to today's sharks, Cladoselache is not thought to have given direct rise to modern sharks, mainly due to the absence of claspers (reproductive organs developed in all modern male sharks) and the fact that it did not replace its teeth as frequently as modern sharks do.

ODDBALLS

During the Carboniferous and Permian periods (370–220 mya), sharks underwent a second phase of dramatic changes, some of which were quite bizarre.

Iniopterygians, with their long, winglike pectoral fins, have been called the "missing link" between sharks and bony fish. Stethacanthids included species with elaborately modified dorsal fins that projected over their head. The teeth of Helicoprion, an edestid, were arranged in an outward spiral whorl, with the newest and largest teeth inside the mouth. Some of these species may have grown very large. Among the oddest were the petalodontids, which had deep, bulbous bodies, small heads, and flat teeth for crushing.

Most of these evolutionary oddballs had vanished by the Mesozoic era (220 mya). By the middle Mesozoic, the first rays had appeared.

Most of the sharks of 220–150 mya fall into two groups. The xenacanths lived mainly in fresh water. They were long and large, with dorsal spines and tapering tails. They had a worldwide distribution until their demise 200 mya.

The second group, the hybodonts, were mostly oceanic; they dominated from 220 mya to 65 mya, when almost all other shark groups also died out.

Resembling Cladoselache, hybodonts had a number of advanced features linking them more closely to modern sharks. Hybodus had a blunt snout and an elongate body. Blunt teeth at the rear of the mouth and sharp, pointed teeth at the front suggest that Hybodus fed on the move.

These prehistoric predators ruled the oceans until the end of the Cretaceous period (65 mya). Hybodus couldn't compete with "modern sharks" and gradually disappeared at about the same time as the last dinosaurs.

CHAPTER 4

THE RISE OF MODERN SHARKS

With the extinction of dinosaurs and the rise of mammals, sharks underwent their third period of dramatic change. This led to the diversity of sharks we recognize today.

Since then, the diversity of the large, modern, pelagic sharks has increased with no major declines or extinctions. The oldest white shark teeth stem from this period, when two distinct lines of shark form appeared. One line had rough, serrated teeth, thought to have given rise to modern Great White Sharks; the other seems to have had a tendency to grow to enormous sizes. This group of giant sharks appears to have reached their pinnacle during the Miocene epoch (late Tertiary period), 25–10 mya. They included probably the most spectacular marine predator ever to have cruised the oceans – the giant Carcharocles megalodon, otherwise known as "Megatooth". Their fossils are found in worldwide deposits up until 3 mya. These terrifying super-predators had huge, serrated, triangular teeth around 7 inches (17.5 cm) tall and bodies 40–50 feet (12–15 m) in length.

Recent opinions are that sharks are not primitive animals as popularly thought. They actually display an advanced design with such evolutionary advantages as larger brains, specialized teeth, and extremely well-developed sensory systems, placing them with birds and mammals in terms of evolutionary complexity.

Modern studies on shark evolution make regular advances. As technologies improve, scientists are able to study minute details in fossilized shark teeth. They are now focusing on the layers laid down in tooth manufacture, and it is hoped that this will improve our understanding of sharks' evolutionary relationships.

CHAPTER 5

ANATOMY

Some basic anatomical features are common to all sharks. The mouth opens at the tip of the head or is underslung, and the pectoral and pelvic fins are paired. Some species have an anal fin, and on the back there may be one or two dorsal fins. The backbone extends into the upper tip of the tail, so the upper tail lobe is often longer than the lower. Male sharks also have claspers. These are adaptations of the pelvic fins, used in transferring sperm during copulation.

Aside from their cartilage skeleton, other features separate sharks from bony fish. They lack an opercular flap, the bony covering of the gill slits which keeps the gills hidden in most fish. In sharks, the gill slits – numbering five, six, or seven per side – are exposed.

Sharks lack a swim bladder, the mechanism in bony fish that keeps them afloat. Instead sharks have a large, oil-rich liver. In some species, such as the Blue Shark, the liver makes up to 20% of their body weight. Deep-water sharks need larger livers in order to control their bouyancy.

It is a myth that all sharks must keep swimming in order to avoid drowning. Many bottom-dwelling species breathe with a pumplike mechanism which allows them to force water through their gills by opening and closing their mouth. This allows oxygen uptake even when stationary.

In most sharks, tiny scales are embedded in the skin, giving it a prickly texture. These placoid scales are thought to have evolved from teeth and are even made of similar materials – a pulpy cavity surrounded by dentine and capped with enamel.

SENSES

Far from being the mindless killers of popular legend, sharks are advanced animals with highly developed, acute senses. These are essential navigational, communication and hunting aids in the reduced light below the surface.

Vision

Sharks' eyes closely resemble those of other vertebrates, even to the extent of seeing in color. Mounted on the sides of the head, they provide a wide field of vision. In some species an eyelid, known as the nictitating membrane, can be flicked up to protect the eye when required. Blue Sharks, for example, are often witnessed covering their eyes with a white screen in the final moments before seizing prey in their jaws. White sharks do not have this membrane; they roll their eyes back for protection.

SMELL

Inside the nostrils are two olfactory sacs. Covering each sac is a flap of skin that channels water, and any particles suspended in it, into the nasal chamber. Here, the water flows between lamellae (small plates) lined with olfactory receptor cells. Nerve fibers lead from the cells to the brain, passing on highly detailed information about the surrounding water.

TASTE

Like the smell receptors, a shark's taste receptors can detect minute biological particles. But since the taste receptors are located inside the mouth, they come into play only when potential prey is already inside the mouth. Only then does the shark decide whether the food is palatable.

OTHER SENSES

Further senses give the shark an exceptional awareness of its surroundings. The vestibular system aids balance; the auditory system provides hearing; and electrosensory systems helps incredibly accurate location of prey.

The vestibular system is a series of fluid-filled chambers within the inner ear. This works rather like a spirit level, in that the fluid presses at various pressure points on vertical and horizontal nerve cells as the shark rolls and pitches. Using signals from the nerve cells, the shark is always aware of its body orientation relative to gravity.

Since sharks do not have external ears, they detect disturbances in the water through organs on the top of their head that lead to an inner ear.

Like all other fish (except hagfish), sharks have a lateral line system. This is a line along each flank of open pores connected to a water-filled canal system beneath the skin. Arranged along this canal are several pressure-sensitive cells known as neuromasts. The linear arrangement enables the shark to detect pressure differentials between one point and another. It can sense water movement and acceleration, and use the information to locate nearby objects, such as other, larger sharks or prey.

Sharks have an additional sense, unique to them, which allows them to detect weak electrical voltages in the water immediately around them.

Clusters of sacs, known as the ampullae of Lorenzini, lie beneath the skin of the snout and lower jaw. From each ampulla, a canal filled with conductive jelly leads to a pore on the skin. Using this system, a shark can pick up the tiny electrical signals that are involuntarily discharged by moving prey. Signals pass from each pore to the ampullae, and onward to the central nervous system, where they form an electrical "map" of the surroundings.

This electrosensory system is ineffective at long range, but works superbly within a range of 1 foot (30 cm) or so. It enables bottom-feeding species, for example, to strike accurately at prey concealed in shallow sand.

CHAPTER 6

REPRODUCTION

Sharks do not live in pairs, and mating is sometimes the only time when members of the same species come together. With their naturally aggressive tendencies, sharks show no recognizable courtship displays. Although males do tend to follow pheromone-producing females, looking for some kind of recognition prior to mating, this possibly serves only to reduce aggression once mating begins. Even so, both parties may suffer serious bites and other injuries during the act itself.

Relative to bony fish, which practice external fertilization, sharks produce few offspring. It is not unusual for a female bony fish to release millions of eggs into the water, where they are fertilized by the male's free-floating sperm. In sharks, fertilization occurs internally. The male uses his claspers to transfer sperm into the female's cloaca. Anywhere from one to a few dozen offspring are produced. The gestation period of young sharks varies greatly, from a few months to nearly two years in the Spiny Dogfish – the longest known gestation period of any animal.

Sharks also produce their offspring at different stages in their development. There are three methods: egg-laying (oviparity), live young developed internally without a placenta (ovoviviparity) and live young developed internally with a placenta (viviparity).

OVIPARITY

Sharks that lay eggs are termed oviparous. They include the Hornsharks, Catsharks, Zebra Sharks, and Carpet Sharks. Once fertilized, the eggs are enclosed in a strong, protective casing and usually laid in shallow, sheltered areas on the seabed.

(Continues...)

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Excerpted from Sharks by Andrea Gibson, Robin Carter. Copyright © 2002 St. Martin's Press. Excerpted by permission of St. Martin's Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

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Table of Contents

Contents

Introduction,
What Is a Shark?,
Adaptations,
Prehistoric Sharks,
The Rise of Modern Sharks,
Anatomy,
Reproduction,
Where Sharks Live,
Feeding,
Defensive Behavior,
When Sharks Attack,
Conservation,
Classifying Sharks,
The Shark Species,
Shark Cousins,
Index,