Carolyn C. Smith
Stop! Look! Listen! These three basic commands can be associated with any adventure that a person wishes to undertake. As this unit is being presented those three little words will play a significant role.
The changes which occur on this earth are so gradual that we don’t notice them immediately. However, if we were to ignore our surroundings for a few days or weeks, we would be able to recognize a difference of some kind almost instantly. When we finally notice the changes we are quick to say, “It happened overnight!”
Scientists are constantly recording the changes which are occurring on this earth. For centuries man has theorized why, when, where, who, what, and how something is done. Ever so often new evidence is presented and he is forced to change his explanations when results contradict his previous understanding.
This unit deals with the results of a chain of events which is believed to have started 4-1/2 billion years ago. We are going to take a look at a species of animals which is found in the Arthropoda Kingdom—“The Crab”. You will find that these little creatures are quite fascinating in their developmental and behavior patterns.
After studying this unit the students will be able to:
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1. identify the Phylum Arthropoda.
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2. discuss the Class Crustacea.
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3. discuss the evolution of the crabs.
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4. discuss the stages of development of crabs.
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5. Identify some of the different species of crabs and tell how they adapt themselves to their environment.
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6. distinguish the male and female of some species of crabs.
This unit is designed to be used in a science class of grades 5-8. The activities provided should enable you to correlate the information with other subject areas, especially mathematics and reading. This unit can be completed in 2-3 weeks.
Origin of Arthropods
All animals that inhabit the earth are classified as vertebrates or invertebrates. Vertebrates are animals with a row of bones along the middle of their back as well as some appendicular bones. Invertebrates are animals without a backbone.
The arthropoda are considered the most advanced of invertebrates. There is evidence that the arthropods began their development during the pre-Cambrian period which is the first era defined in the geological time. By the Cambrian period, which began about 600 million years ago, arthropods were well established and had found their place on earth.
The most abundant fossils of the Cambrian period are those of the trilobites. These animals are the ancestors of modern day crabs. It is believed that one of the reasons trilobites are so plentiful is that they frequently molted and discarded exoskeletons in large numbers. These exoskeletons are mineralized tissues which contain large amounts of calcium carbonate (CaCO3) which enhanced their capacity to remain unaffected over this vast time interval. The name trilobite means “three-lobed” and makes reference to the fact that the dorsal surface of the body is divided into three lobes. This dorsal surface is the part of the body we normally see on today’s crabs. The three body segments can be identified as: 1) a head; 2) the thorax, which is the middle flexible portion; and 3) the abdomen, the posterior region which consists of a number of fused segments. In some trilobites, this fused segment extends into a spine. On the head there is a pair of similar jointed, two-branched appendages. These projections were modified and may have enabled the arthropods to hold their food. Similarities can be seen on the horseshoe crabs of today. The function of these appendages vary according to the habitat of each species. The protruding legs of some trilobites show segments as well. The outer branches of legs are flat with a row of bristles along the back edge. These flat branches are believed to have been used for respiration and swimming. It is also believed that the inner branches were probably used for walking. At the base of each appendage there are inward projections which probably also aided the arthropod’s movement.
The habits of trilobites can only be inferred. However, it is believed that they lived in the sea because their remains are always found with corals, crinoids, brachiopods, and other exclusively marine animals.
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Most trilobites probably frequented the shallow waters and crawled on the bottom of the floor of the sea. It is believed that they fed on various seaweeds, sponges, brachiopods, and mollusks which are known to inhabit the same places. They probably scavenged organic matter by plowing through the mud. The analogy of the modern crabs makes these projections seem plausible.
The trilobites were the dominant invertebrates during the Cambrian period. They continued to flourish during the Ordovician period but then began to decline. The last of the trilobites died out in the Permian period which was about 275 million years ago. The trilobites did not directly give rise to any other group of arthropods, however, their morphology and habitat seem to be closely related to the animals which are found in the crustacean class of arthropods of today. (See Diagram Below)
Arthropods—Past and Present
(figure available in print form)
Today’s Arthropoda Kingdom
One of the very large phylum in the invertebrate animal kingdom is called the arthropoda. The animals found in this phylum are called arthropods. Arthropods of today have very close evolutionary links to the annelids or worms which is a group of segmented animals. If we take a close look at the diagram of the first arthropods, we can definitely see this relationship. We could turn an annelid into an arthropod with only a few basic structural changes. Arthropods are animals with six or more jointed legs. There is no other group of animals that includes so many different kinds of animals as the arthropods. There are over 800,000 species in the phylum arthropoda. Insects crustaceans, and spiders are all members of the arthropoda kingdom. All have a jointed, chitinous procuticle, serving both as a protective armor and an exoskeleton.
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The animals in this group range in size from very tiny mites to the giant crabs, which may be about eleven and a half feet from leg tip to leg tip. Arthropods can be found almost everywhere—in the sea, in fresh water, in the ocean and on land. Many arthropods can fly such as the dragon fly and the grasshopper. The arthropods are referred to as armored animals. This means that they have an external covering which provides both protection and support. This protective covering, called the exoskeleton, is discussed later on in this unit.
General Characteristics
Arthropods with a rigid exoskeleton and jointed limbs have internal muscles arranged as flexors and extensors of each joint. The arthropods can propel themselves through the environment, whatever the medium, by interaction of a systems of levers which are its jointed appendages.
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On the basis of the mechanical functions and of inferred phylogeny from the characteristic features, arthropods can be divided into two distinct lines. The first group are the animals with characteristics closely similar to those of the phylum annelid showing parallel aspects of development and metamerism. The second group are those that possess an exoskeleton composed of chitin and an epicuticle. By examining these invertebrates more closely you will find that the cuticle forms a series of heavy, skeletal plates or rings (exoskeleton), connected by a thin flexible membrane (cuticle) which permits the animal to move freely.
Growth of the Arthropods: Molting
The growth of animals usually means a gradual increase in size, accompanied by a gradual change in shape, if necessary, until the form of the fully grown adult is reached. This process is difficult in the arthropods because of their exoskeleton. Once an arthropod’s exoskeleton has hardened which is caused by sclerotization and calcification, growth in arthropods must proceed through a series of molts. The arthropods have a natural source of calcium (Ca) which is quite useful during this phase of development. The molting process involves the secretion of a new cuticle and shedding of the old exoskeleton. Actually the new carapace or shell is made inside and before the old one is discarded. During this short period, the arthropod shows a rapid increase in bulk, which usually involves the intake of water or air into internal spaces. The arthropod must now grow new tissues to fill the new armor.
During the molting cycle, the behavior of all arthropods change. Once the old shell has been shed the arthropod is vulnerable to attack by predators. Most of them go into hiding when they are unprotected by their usual exoskeleton. Due to the fact that the reproduction has a cycle similar to the molting cycle, the population growth of these animals is affected during the molting process.
The Molting Cycle of Crabs
(figure available in print form)
Physiology of Arthropods
The circulatory system of arthropods is very simple. Although the animal has a heart it functions somewhat differently from those found in humans. The heart is a muscle but in the form of a pulsating tube which lies in a dorsal pericardial sinus. The blood circulates and fills cavities which are found throughout the body. Within these cavities, the blood bathes the various organs. The blood is not necessarily confined within blood vessels at all times creating an open circulatory system.
Most terrestrial arthropods have a system of branching air tubes which make up the respiratory system. These air tubes are formed through in-growths of the surface ectoderm. The ectoderm which contain specialized cells secretes an inner lining of cuticle which strengthens the walls of the delicate tubes and prevents them from collapsing. Openings on the sides of the body permits air to enter and leave the tubes. The air is piped directly from tubes to the tissues almost completely replacing the respiratory function of the circulatory system as we know it from higher animals.
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Most aquatic arthropods breathe by means of gills which are thin walled extensions of the body wall. These gills allow the gases, carbon dioxide and oxygen, to pass readily.
The nervous system of the arthropods consists of a brain which is connected to the nerve cord by a ring of nervous tissue which circles the digestive tract.
The cuticle of the arthropods also forms an important part of the digestive system. During growth and development, the ectoderm turns in at the mouth and anus and lines the anterior and posterior areas of the digestive tube with the cuticle it secretes. In the anterior region the cuticle may produce hard teeth for grinding up the food. In many of the arthropods the anus serves as a means of expelling nitrogenous wastes since the excretory organs are directly connected to the digestive tube.
Like most invertebrates, arthropods lay large numbers of small size eggs. The eggs are richly supplied with yolk, and few show traces of spiral cleavage or mesoderm stem cell. The body typically forms from a primitive streak that serves as the growth center of the embryo.
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The young hatch from the egg in an immature state and must be able to move to obtain the necessary materials for further growth.
Exoskeleton
The typical arthropod’s body is a series of segments that have movement through the action of muscles. Each segment is provided with a pair of jointed legs. This body structure allows the arthropods segments and jointed legs to perform many varied functions and no doubt this characteristic has contributed to their evolutionary divergence into various environments.
One of the most unique features of arthropods is their exoskeleton. This rigid structure has been the concern of scientists for extensive investigation. Due to the fact that the skeleton occupies the outside of the body and is heavy and bulky, this has limited the size of arthropods.
Arthropods often display a range of color patterns typifying the different groups. This is especially true of insects and crustaceans.
The true pigments and structural colors are represented: The brilliant yellow, orange, and red colors found among these animals are due to a variety of nitrogen-free, lipochrome pigments found in the tissues, while shades of blue, green, and some other colors are the results of the effects of light interference on the original chromatic materials.
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Combinations of two kinds of color might produce many shades resulting from the interaction of both coloring agencies. This color interaction, a characteristics of arthropods, gives them an opportunity for camouflage thus creating a protective environment.
Crustacea
Crustaceans include the lobsters, crabs, shrimps, and sea dwellers which are well known as food for humans. Although they are quite familiar to us, there are other varieties that are unknown to most persons. Many crustaceans are land animals such as the millipedes, centipedes, spiders, and most adult insects. However, most crustaceans have remained marine animals. Their habits and shapes make it easy for them to adapt to the waters.
The major characteristics of the crustacea are:
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1. They live predominantly in water and use gills for respiration.
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2. They have five pairs of appendages.
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3. The body is divided into segments but usually has a recognizable head, thorax, and abdomen region.
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4. The posterior region contains an anus but has no appendages.
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5. The circulatory system is that typical of the arthropod, an open circulatory system.
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6. The excretory system contains either antennal or maxillary glands or both.
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7. There is a median eye and usually a pair of lateral eyes.
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8. The sexes are usually separable, with indirect development and larva stages.
Crustacea does not contain many species. About 26,000 are known, however, those that exist are abundant and many of them are relatively large. These animals are just as prevalent in the water as they are on land. Let us take a look at one of the crustaceans which we can readily find along the Atlantic coast and, even closer to home, along the coast of Connecticut and other New England states.
The Crab
The crab is an animal which belongs to the phylum arthropoda because it has an exoskeleton and jointed legs. It belongs to the class crustacea because its body is segmented. Due to the fact that it has ten legs it belongs to the group called the decapods.
The crab is a curious creature. Man has become interested in them because of the large variety and also because they are edible. Anyone who has visited the seashore has seen crabs. They live all over the world from the tropics to the arctic. They make their homes on land, in trees, in shallow water and the deep waters of the ocean. Some crabs live in burrows and even inside other sea animals. These strange creatures have different manners in which they move around from place to place. These maneuvers are characteristic of their environment, shape, and appendages. They can move fast or slow, walk, run, and swim depending on their traits, physiology, environment, and habits.
The largest of this class of animals is the imaginary crab which is found in the sky. This crab is the constellation and the astrological sign, Cancer, which is made up of eight stars. It is so named, the crab, because it appears to move backwards and forward equally as well. This imaginary crab can be seen by studying the stars toward the south on spring nights.
Crabs use many methods of protecting themselves from their enemies. One of the best ways is through camouflage. There is a wide variety of colors and textures of the crab and this enable them to naturally blend in well with the environment in which they live. Some are able to make disguises out of the material around them. If this fascinating creature is unable to hide or disguise himself to avoid a predator, he uses his claws or pinchers as a weapon to ward off his enemy. Some of the crabs that live in the ocean may even use sea anemones for protection. Although sea anemones have a stinging poison in their tentacles, they are not harmful to the crab because of their hard protective shell. However, predators usually keep their distances because of their innate avoidance of this poisonous animal.
Although crabs use all means to protect themselves sometimes they are not fortunate to escape unharmed, especially during the molting cycle. They may lose one of their appendages, however, the crab has the ability to regenerate a lost limb. This biological process is not available to vertebrate orders. If the claw or leg is injured, the crab can make it drop off in a definite place which is called a breaking plane. After molting the crab has a new, usable leg, but much smaller. It takes two or more molting cycles to form a leg or a claw of normal size. Once a crab is old, it can no longer molt and therefore does not regenerate a lost limb. (See Diagram Below)*
(figure available in print form)
All animals must reproduce in order not to become extinct. The sex cells of male and female must unite in order for new life to begin. Believe it or not many shore crabs go through special courtship ceremonies to start the process. The males will begin by waving their claws to attract females of their species. Some males will use their claws to tap on rocks which produce a drumming sound to attract the females. Once a male finds a willing female, the two will mate. Shortly after mating, most female shore crabs are ready to lay their eggs. Depending on the species, a female crab may carry her eggs around in the abdomen for as short as two weeks to as long as several months. The individual eggs are smaller than the period at the end of this sentence. While the eggs are being carried by the female, she stays near the water because the eggs must be kept moist during their development. The female ghost crab has a problem during this period because she lives on the dry beaches. After she has deposited her eggs in her abdomen, she constantly runs into the water to wet them. When the time comes for the thousands of eggs to hatch, the female enters the shallow water near the shore. The eggs leave the female’s body through an opening which is found near the end of the thorax and are covered with a transparent jelly-like substance. After the female has entered the water she moves her abdomen rapidly from side to side. This movement causes the eggs to break open and the little creatures or larvae lmerge. This is the first stage of development of a crab and they do not resemble adult crabs at all. The crab larva must go through a series of changes before it becomes an adult. (See Diagram on the following page)*
Stages of Development
(figure available in print form)
The crab larva has two main stages of development, the zoea and the megalops. In the first part of the stage the crab looks like a shrimp and is called a zoea.
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This small animal feeds mainly on the larvae of other sea animals such as oysters and starfish and will molt about four to fives times. Each time it molts, more appendages are added to its body. The second stage of the larva crab is known as the megalops which means “large eyes”.
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It is during this stage that the animal acquires the claws and other appendages of an adult crab. After the larvae have reached the megalops stage they molt one more time. All ocean dwelling crabs complete this stage on the ocean floor. The land and shore crabs usually come out of the water at the beginning or at the end of this stage. This is very important to them because it is at this time that they adapt themselves to their environment.
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When the megalops molt all the basic parts of an adult crab are obvious. Although the larvae development is complete, the new little crab will continue to molt through its life.
Some Shapes of Crab*
(figure available in print form)
The respiratory system of the crab is similar to that of the fish. Crabs breathe by means of gills. The number of gills found on crabs vary with the species. For instance, the blue crabs have eight gills on each side of their bodies while the small pea crabs have only three. Oxygen which is essential to the living crab is extracted from the water as it passes over the myriads of blood vessels in the crab’s gills. A current of water is produced by small paddles called gill bailers. Within the gills there are brushes or hair-like appendages, which are used to remove dirt from the gills. (See Diagram)*
Respiratory System
(figure available in print form)
The gills of the terrestrial crabs are smaller than those of the marine crabs, however, the blood vessels are larger to maximize the absorption of oxygen. The crabs that move back and forth from water to land fill their gill chambers with water. While they are on land, a current of air set up by the gill bailers adds oxygen to the water that is being carried in their gills.
The circulatory system of the crab is relatively simple. As the blood flows through the gills, oxygen is taken out and carbon dioxide is released. Crabs have blue blood which is caused by a copper chemical that carries the oxygen. As the blood moves from the gills it enters the heart, goes around the body and back to the gills. This process takes about 40 to 60 seconds in a large crab.
Circulatory system*
(figure available in print form)
The digestive system of the crab appears complex, but it is very simple. They use their pinchers or claws to catch their food. The pinchers pass the food to the mouth which has three pairs of hard jaws. The first pair of jaws helps to hold the food while the other pairs cut and tear it into smaller pieces. The food then passes into a double stomach where 1) a grinding action takes place and 2) digestive juices aid in breaking down food so that it can be digested. All food that is not digested passes through the intestine and out through the anus.
*Digestive System
(figure available in print form)
Crabs have a small, limited brain. It is connected to the nerves that go to sense organs and to all parts of the body of the crab. The eyes are important to the crab. They are usually found on stalks and can make a complete revolution of 360 degrees. They can also be pulled down into eye sockets. These eyestalks also make messenger chemicals which, when combined with the nerve cells, signal changes in the color of the crab’s shell. This color change helps the crab to blend better into his environment. The red and yellow colors seem to be more controlled by the messenger chemical while the black and white colors are under the control of the nerves. (See Diagram Below) Scientists are mystified by these color adaptive mechanism as well as the daily activities of the crab. They suspect that these changes are due to a biological change but it is still being studied.
*Nervous System
(figure available in print form)
Hormones produced by the crab are responsible for its size, growth, as well as color change. These hormones also control the amount of salt and water that is found within the body of crabs. If a crab changes from salt water to fresh water too rapidly, it will die. The crabs that move back and forth from salt water to fresh water and vice versa depend on their gills to make the adjustment. The gills respond to hormone signal to let in more or less salt water. An adjustment period of several hours to several days is needed for crabs that move to and from the salt water. (See Diagram Below)*
Color Influenced by Hormones from Eye Stalks
(figure available in print form)
Species of Crabs
Plants and animals are divided into groups within each phylum. The species name follows immediately after the name of the genus or category of that plant or animal. There are about 4,500 different kinds of crabs found all over the world. Many of these species are valued to man because they are used for food. Below you will find the names and habits of some common crabs.
