Section 27–3 27–3 1 FOCUS Objectives f you have ever dug in a garden in the spring, you have probably seen wriggling through the soil. 27.3.1 Describe the defining fea- I Earthworms are annelids, members of the phylum Annelida. tures of annelids. Key Concepts Other annelids include exotic seafloor worms and parasitic, 27.3.2 Identify the characteristics • What are the defining features of annelids? blood-sucking leeches. Because their bodies are long and narrow, of the classes of annelids. • What are the characteristics of some annelids look a bit like flatworms or roundworms. However, 27.3.3 Describe the ecology of the three classes of annelids? the annelids are a distinct group that is probably more closely annelids. Vocabulary related to clams and snails. One piece of evidence for this septum• seta relationship is the fact that annelids,clams, and snails all share crop• gizzard a similar larval stage. closed circulatory system gill• nephridium Vocabulary Preview clitellum What Is an ? Call on volunteers to pronounce each Reading Strategy: The name Annelida (uh-NEL-ih-duh) is derived from the Latin of the Vocabulary words aloud. Using Visuals Before you word annellus, which means “little ring.” The name refers to Correct any mispronunciations, and read, preview Figure 27–16. the ringlike appearance of annelids’ body segments. The body of an How does this animal seem to annelid is divided into segments that are separated by septa note any words that students with differ from the other worms you limited English proficiency have spe- have already studied? Briefly (singular: septum), which are internal walls between each segment. cial trouble pronouncing. summarize any differences you Most segments are similar to one another, although they may be notice. modified to perform special functions. Some body segments may carry one or more pairs of eyes, several pairs of antennae, and Reading Strategy other sense organs. Other segments may be specialized for func- Advise students to compare the tions such as respiration. In many annelids, bristles called setae labeled diagram in Figure 27–16 with (SEE-tee; singular: seta) are attached to each segment. that in Figure 27–3 in Section 27–1. Annelids are worms with segmented bodies. They have a true coelom that is lined with tissue derived from mesoderm. These structures are shown in Figure 27–12. Recall 2 INSTRUCT that flatworms have no coelom, whereas roundworms have a pseudocoelom. Like the roundworms, annelids have a tube- What Is an Annelid? within-a-tube digestive tract that food passes through from the mouth to the anus. Use Visuals Coelom What are some functions performed by specialized Figure 27–12 Divide the class into segments? pairs, and give each pair three Digestive tract different-colored blocks of clay. Then, ask students to make three models, beginning with one that matches the cross section of the annelid shown in this figure. After that is completed, Ectoderm Mesoderm Endoderm students should make similar models Figure 27–12 Annelids are among of the cross sections shown in Figures the simplest animals to have a true 27–1 and 27–7. coelom that is lined with mesoderm. Annelids are also called segmented worms because the body is divided into many Build Science Skills similar segments. The photo shows a Using Models To help students marine annelid. visualize a digestive tract inside a body wall, give each student or group a long sock with the toe cut SECTION RESOURCES off. Tell students to turn the sock halfway inside out to form a tube- within-a-tube. Explain that the inner Print: Technology: layer of the sock represents the diges- • Teaching Resources, Section Review 27–3, • iText, Section 27–3 tive tract, and the outer layer Enrichment • Transparencies Plus, Section 27–3 represents the body wall. Save • Reading and Study Workbooke A, Sectionr 27–3 • Adapted Reading and Studym Workbook B, i Section 27–3 T • Issues and Decision Making, Issues and Decisions 28 • Lesson Plans, Section 27–3

694 Chapter 27 Form and Function in Annelids

How does an Objective Students will be able to pump blood? describe the structure and function 3. Examine the straw using a microscope. Direct light of an earthworm’s circulatory through the straw from below. Look near the front Materials earthworm; dropper pipette; nonchlori- system. of the worm for the large ring blood vessels. Count nated water; large, clear plastic soda straw; dissecting how often these organs beat during a one minute Skill Focus Observing microscope; clock or watch with second hand period. Observe the rest of the circulatory system. Materials earthworm, dropper pipette, nonchlorinated water, large Procedure Analyze and Conclude and clear plastic straw, dissecting 1. Carefully insert an earthworm into a clear plastic 1. Inferring Did you see the worm breathing? straw. Do not force the worm into the straw. Explain your answer. Why must the earthworm’s microscope CAUTION: Handle the earthworm carefully to avoid skin be kept moist? How do your answers relate to Time 15 minutes harming it. Wash your hands after handling the worm. how earthworms live in their environment? Advance Prep Collect earth- 2. Use a dropper pipette to add a drop or two of 2. Observing Is an earthworm’s circulatory system worms outdoors, or buy them from nonchlorinated water into the straw. open or closed? Explain your answer. a bait shop. Pond water, aquarium water, and bottles of spring water are all acceptable forms of nonchlori- Form and Function in Annelids nated water. Tap water can be Annelids have complex organ systems. Many of these systems dechlorinated by boiling and cool- are unique because of the segmented body plan of this group. ing overnight in an open container. Safety Make sure students wash Feeding and Digestion Annelids range from filter feeders their hands with soap and warm to predators. Many annelids get their food using a pharynx. In water after handling the earthworms. carnivorous , such as the Nereis in Figure 27–13, the pharynx usually holds two or more sharp jaws that are used Strategies to attack prey. In annelids that feed on decaying vegetation, • If students have trouble inserting the pharynx is covered with sticky mucus. The worm collects the worm in a straw, try slitting food particles by extending its pharynx and pressing it the straw lengthwise. against the surrounding sediments. Other annelids obtain • Students can also put the earth- nutrients by filter feeding. They fan water through tubelike worm and a few drops of water in burrows and catch food particles in a mucus bag. a petri dish cover, with the bot- In earthworms, the pharynx pumps food and soil into a tube tom of the petri dish inverted on ̆ Figure 27–13 The annelid called the esophagus. The food then moves through the crop, where Nereis uses jaws to capture prey. top to hold the worm in place. Do it can be stored, and through the gizzard, where it is ground When prey approaches, the worm not allow the worms to dry out. into smaller pieces. The food is absorbed farther along in the lunges forward, rapidly extends its Expected Outcomes Students digestive tract, in an organ called the intestine. pharynx, and grabs the prey using its jaws. Inferring How is the should see the earthworm’s dorsal Circulation Annelids typically have a closed circulatory structure of a Nereis’s jaws related to vessel pulsating and that an earth- their function? system, in which blood is contained within a network of blood worm has a closed circulatory system. vessels. An earthworm’s blood circulates through two major Analyze and Conclude blood vessels that run from head to tail. Blood in the dorsal (top) 1. An earthworm has no breathing vessel moves toward the head of the worm. Blood in the ventral movements, because it exchanges (bottom) vessel runs from head to tail. In each body segment,a gases through its skin. The earth- pair of smaller blood vessels connect the dorsal and ventral worm’s skin must be kept moist, blood vessels and supply blood to the internal organs. The dorsal because otherwise it will not be blood vessel functions like a heart because it contracts rhythmi- able to exchange gases. cally and helps pump blood. Earthworms live in moist soil. 2. The earthworm has a closed circu- latory system, which means that SUPPORT FOR ENGLISH LANGUAGE LEARNERS blood never leaves the blood vessels.

Vocabulary: Science Glossary Beginning Write septum, seta, crop, gizzard on students should write sentences using each the board. Underline the syllables as you lead word. the class in pronouncing each word. Define Intermediate Students can expand on the each word, and point to the corresponding science-glossary activity by adding the other Answers to . . . structure in Figure 27–16. Have students then Vocabulary words: closed circulatory system, gill, write the definitions of those terms in their nephridium, clitellum. Have the students form Sample answer: sensing own science glossaries. Students should draw collaborative writing groups and write sen- the environment, respiration and label their own diagrams next to the defi- tences for each word. Call on one student in Figure 27–13 Hooks with sharp nitions. Then, in collaborative writing groups, each group to read the sentences aloud. points are adapted to catch prey.

Worms and Mollusks 695 Respiration Aquatic annelids often breathe through gills.A 27–3 (continued) gill is an organ specialized for the exchange of gases underwater. In feather-duster worms, shown in Figure 27–14, feathery struc- Build Science Skills tures that function as gills protrude from the opening of the Observing Earthworms are among worm’s burrow or tube. Land-dwelling annelids, such as earth- the most familiar of , yet worms, take in oxygen and give off carbon dioxide through their moist skin. These annelids secrete a thin protective coating of probably few students have taken the mucus, which keeps their skins moist. time to observe earthworms closely. To give students this opportunity, Excretion Like other animals, annelids produce two kinds of fill a clear plastic box with about waste. Digestive waste passes out through the anus at the end 2 centimeters of sand. Place about of the digestive tract. Cellular waste containing nitrogen is 7 centimeters of loosely packed top- eliminated by nephridia (nee-FRID-ee-uh; singular: nephrid- soil over the sand. Use pond water to ium), which are excretory organs that filter fluid in the coelom. slightly moisten the soil. (Add more water whenever the soil appears dry.) Response Most annelids have a well-developed nervous Place 6 to 12 earthworms on top of system consisting of a brain and several nerve cords. However, the soil. Cover the box with clear the sense organs are best developed in free-living marine plastic wrap, and put a few air holes annelids. Many of these species have a variety of adaptations for in the plastic wrap. Have students detecting stimuli: sensory tentacles,chemical receptors, stato- observe the earthworms for several cysts that help detect gravity, and two or more pairs of eyes. days. Ask them to make labeled dia- Movement Annelids have two major groups of body muscles grams of the animals. Advise them to ̆ Figure 27–14 These feather- that function as part of a hydrostatic skeleton. Longitudinal note especially how earthworms duster worms exchange gases muscles run from the front of the worm to the rear and can move through the soil. underwater using feathery gills. contract to make the worm shorter and fatter. Circular muscles Applying Concepts How do wrap around each body segment and can contract to make the land-dwelling annelids exchange Use Community Resources gases? worm longer and thinner. The earthworm moves by alternately Contact a local environmental group, contracting these two sets of muscles, using its setae to prevent slipping. Burrowing annelids use their muscles to force their way garden center, farming association, through heavy sediment. Marine annelids have paddlelike or garden club for a reference to a appendages, or parapodia (singular: parapodium), on each person in your area who uses earth- segment, which they use for swimming and crawling. worms to compost household organic garbage. Ask this person to Reproduction Most annelids reproduce sexually. Some species speak to the class about how to set use external fertilization and have separate sexes. Other up such a system and what foods can annelids are hermaphrodites. Individuals rarely fertilize their and cannot be added to the earth- own eggs. Instead, two worms attach to each other, as shown in worm habitat. Students might be Figure 27–15, exchange sperm, and then store the sperm in surprised to learn that such a system special sacs. When eggs are ready for fertilization, a clitellum can be clean and without unpleasant (kly-TEL-um), or band of thickened, specialized segments, odors. It can not only dispose of secretes a mucus ring into which eggs and sperm are released. organic wastes responsibly but also Fertilization takes place within this ring. The ring then slips off provide great soil for a garden. the worm’s body and forms a protective cocoon.Young worms hatch weeks later.

̈ Figure 27–15 Some annelids, including these earthworms, are her- maphrodites. Each worm produces both eggs and sperm. During mating, the worms exchange sperm, which will eventually be used to fertilize egg cells. Applying Concepts When are the eggs fertilized?

696 Ch 27

FACTS AND FIGURES

Slithering through the soil muscles contract and make the worm longer, Annelids move through soil and sediment by each segment must become narrower. When the using the power of their muscles and the liquid earthworm moves forward, its first few body seg- inside their body segments—their hydrostatic ments elongate while the segments just behind skeleton. Each body segment is sealed off from them hold their position. Then, the first few seg- the segment next to it, which means body fluids ments shorten and widen. Alternating can’t move from one segment to another. When contractions and elongations continue along the the longitudinal muscles contract and make the length of the worm’s body, enabling it to move worm shorter, each segment has to become through soil or sediment. wider. In a similar manner, when the circular

696 Chapter 27 Anus Groups of Annelids Use Visuals Figure 27–16 Have students study Setae the diagram of the oligochaete. Point out that this worm has body seg- ments—annelids are worms with segmented bodies. Then, as you point out different labeled parts, call on students to describe the feeding, circulation, respiration, and response Body segments of an oligochaete. Ask: Where does Gizzard Crop Dorsal blood vessel fertilization take place in an oligochaete? (In the clitellum) Clitellum Mouth Build Science Skills Brain Inferring Divide the class into small Ganglion groups, and provide each group with Circular Ventral an earthworm, a type of oligochaete. muscle blood vessel Students should examine the worm Longitudinal Nephridia Ganglia Ring Reproductive with a hand lens and try to locate the muscle vessels organs setae. Have groups formulate a hypothesis to explain the function of ̆ Figure 27–16 Earthworms are Groups of Annelids oligochaetes that live in soil. Earth- the setae. Then, have students place worms carry out essential functions the earthworm on a smooth surface, Because of their visible segmentation, all annelids show a basic using digestive, circulatory, excretory, such as a glass dish or the shiny side similarity. Annelids are divided into three classes—oligochaetes, nervous, and reproductive systems. of a piece of aluminum foil, as well as leeches, and . Many organs, including nephridia and blood vessels, repeat in nearly every on a rougher surface, such as a damp Oligochaetes The class Oligochaeta, or oligochaetes body segment. paper towel. Students should com- (AHL-ih-goh-keets), contains earthworms and their relatives. pare the worm’s movements on the Oligochaetes are annelids that typically have stream- two surfaces and infer the function of lined bodies and relatively few setae compared to poly- the setae. (Students should infer that chaetes. Most oligochaetes live in soil or fresh water. setae enable an earthworm to grip a Earthworms, such as the one shown in are long, Figure 27–16, surface as it moves.) pinkish-brown worms that are common in woods, fields, and gardens. Tubifex worms—another common oligochaete— are red, threadlike aquatic worms that are sold in pet stores as food for tropical fish. Although earthworms spend most of their lives hidden under- The annelids of the class Polychaeta ground, you may find evidence of their presence above ground in have “many .” the form of squiggles of mud known as castings. Recall that an earthworm—which swallows just about anything it can get into its mouth—uses its pharynx to suck a mixture of detritus and soil particles into its mouth. As the mixture of food and soil passes through the intestine, part of it is digested and absorbed. Sand Oligochaete comes from the grains,clay particles, and indigestible organic matter pass out Greek words oligos, meaning “few” through the anus in large quantities, producing castings. Some or “small,” and chaite, meaning tropical earthworms produce enormous castings—as large as 18 “.” If poly- means “many,” centimeters long and 2 centimeters in diameter! what is a characteristic of the group of annelids known as What are earthworm castings? polychaetes?

TEACHER TO TEACHER Answers to . . . Earthworm castings are Have students in groups of four observe live Students can use different tuning forks to see a mixture of sand, clay, and undigested earthworm movements under different condi- how earthworms react to different vibrations. food that an earthworm expels from its tions, such as dry, wet, cold, or warm, and on Have groups decide how this information could anus. different surfaces, such as felt, glass, wood, plas- be used to “hunt” for earthworms. tic wrap, or gauze. Students record their Figure 27–14 Land-dwelling observations, and each group reports to the —Keith Orgeron annelids exchange gases through their class. Teacher moist skin. Carenero High School Figure 27–15 Eggs are fertilized after Lafayette, LA eggs and sperm are released into the mucus ring secreted by the clitellum.

Worms and Mollusks 697 27–3 (continued) Figure 27–17 Most leeches are external parasites. Medicinal leeches, Make Connections such as the one below, were once used routinely to attempt to treat con- Health Science Encourage inter- ditions ranging from headaches to ested students to contact a large, mental illness to obesity. Doctors local teaching hospital and inquire believed that diseases were caused by about whether leeches are used in an excess of blood, so they applied leeches to the patient’s skin to remove any surgical procedures at that hospi- blood from the body. Here, a man tal. If so, have students find out how who lived in the Middle Ages has the leeches are collected and the become so fat that he has been con- conditions in which they are kept fined to a room and covered with leeches. while waiting for use in surgery. In addition, have students find out about the natural environment in which such leeches live and describe an artificial setup in which leeches could be kept for use in medical pro- cedures. Leeches The class Hirudinea (hir-yoo-DIN-ee-uh) contains the leeches, most of which live in moist habitats in tropical coun- Build Science Skills tries. Leeches are typically external parasites that Classifying Divide the class into suck the blood and body fluids of their host. Roughly one small groups, and ask each group to fourth of all leeches are carnivores that feed on soft-bodied make up 10 to 20 questions for a invertebrates such as snails, worms, and insect larvae. game called Annelid Worms—Which Leeches have powerful suckers at both ends of their bodies Class? The idea of the game is for the that help them cling to their hosts. The posterior sucker can also question to be a statement about one anchor a leech to rocks or leaves as it waits for a host to pass. group of annelids. The question is Some leeches force a muscular extension called a proboscis ̄ Figure 27–18 Polychaetes answered by classifying the statement (proh-BAHS-is) into the tissue of their host. Others slice into the are marine annelids. The bearded skin with a razor-sharp pair of jaws. Once a wound has been in the annelid class to which it applies. fireworm is a that lives in For example: “These worms spend coral reefs. It is best known for its made, the leech uses its pharynx to suck blood from the area. Some leeches also release a substance that anesthetizes the most of their lives underground— method of defense—its setae, or bristles, break off when touched and wound—keeping the host from knowing it has been bitten! which class?” The answer is cause irritation and burning. Leeches were once commonly used to treat medical condi- Oligochaeta. When all groups have fin- tions.Today the use of medicinal leeches is undergoing a revival ished writing, pool the questions and of sorts. Doctors are finding that leeches can reduce swelling eliminate any duplicates. Place the after surgery. After surgeries in which a body part is reattached, remaining questions in a box for stu- hungry leeches are applied to the area. These leeches can suck dents to draw at random. several milliliters of blood at a time—up to five times their own weight! They also secrete a fluid that prevents blood from clotting. This anti-clotting mechanism helps relieve pressure and congestion in the healing tissues.

Polychaetes The class Polychaeta, or polychaetes (PAHL-ih-keets), contains sandworms, bloodworms, and their relatives. Polychaetes are marine annelids that have paired, paddlelike appendages tipped with setae. The setae are the brushlike structures on the worm shown in Figure 27–18. Polychaetes live in cracks and crevices in coral reefs; in sand, mud, and piles of rocks; or even out in the open water. Some burrow through or crawl over sediment.

HISTORY OF SCIENCE

Leeching to health blood, prevents blood from clotting, and acts as For centuries, the medicinal leech Hirudo medici- an anesthetic. A leech may eat up to five times its nalis was used for bloodletting—a treatment own body mass in blood before it drops off, and thought to cure a wide range of illnesses. Shortly it may not need to feed again for 30 weeks. The after the Civil War, more than 1.5 million leeches leech is now scarce due to overcollecting and per year were used in the United States alone. habitat destruction. It is still in demand, though, When in contact with a host, the leech attaches for hirudin, which is used as an anticoagulant itself and draws out blood by a pumping action. for some heart patients and in some surgical At the same time, the leech’s salivary glands procedures. secrete hirudin, a substance that dilutes the host’s

698 Chapter 27 Ecology of Annelids Ecology of Annelids Make Connections The importance of earthworms in nature was noted as far Earth Science Explain that good soil back as ancient Greece, when Aristotle called them “the contains sediment particles, humus intestines of the earth.” Charles Darwin was impressed (organic matter), water, and air. The enough with earthworms that he devoted years—and an air is especially important, because if entire book—to their study. Earthworms, like the one shown the soil is too compact, plants have a in Figure 27–19, and many other annelids spend their lives difficult time growing. Ask students: burrowing through soil, aerating it, and mixing it to depths How do earthworms help to mix air of 2 meters or more. Their tunnels provide passageways for into soil? (By moving through the soil, plant roots and water and allow the growth of beneficial, oxygen-requiring soil bacteria. Earthworms pull plant they break up particles and make tun- matter down into the soil and pass it through the gut. nels. These actions provide space for air There, they grind it, partially digest it, and mix it with to move throughout the soil. The move- bacteria that help the plant matter decompose.Worms ment of earthworms aerates the soil.) also “mine” minerals from deeper soil layers, bringing them up to the surface. Earthworm feces (castings) are rich in nitrogen, phos- phorus, potassium, micronutrients, and 3 ASSESS beneficial bacteria. You’ve probably seen a bird struggling to pull an earth- Evaluate Understanding worm out of the ground. Earthworms are an important part Call on students at random to provide of the diet of many birds, such as robins. Moles, skunks, characteristics and examples of the toads, and snakes also prey on earthworms. ̆ Figure 27–19 Some annelids, three classes of annelids: oligochaetes, In the sea, annelids participate in a wide range of food including this earthworm, burrow leeches, and polychaetes. chains. Many marine annelids have free-swimming larvae that through soil, mixing it as they go. Predicting What might happen to a are part of the animal plankton that is consumed by fishes and garden if all the annelids in the soil were Reteach other plankton feeders. As adults, some marine annelids are killed? mud-dwelling filter feeders that are common in areas where To help students understand and sediment is disturbed or large amounts of organic material are remember how annelids’ bodies carry present. These worms are especially numerous where pollution out the essential life functions, refer to from sewage promotes the growth of bacteria and algae. As any Figure 27–16. As you review each fisher knows, many bottom-dwelling polychaetes are important function as described in the text, have in the diets of fishes. , such as crabs and lobsters, students point out the organs also include annelids in their diets. involved in performing that function.

27–3 Section Assessment Students should explain that a Food Chains and Annelids food chain is a series of steps in an Review what you learned ecosystem in which organisms 1. Key Concept What 4. Critical Thinking Inferring about food chains in transfer energy by eating and features distinguish annelids from An earthworm has more light- Chapter 3. Then, draw a roundworms? sensitive cells in its anterior and possible food chain involving being eaten. The details of the 2. Key Concept List the posterior segments than in other an annelid. The food chain food chain students draw will vary. defining characteristics for each parts of its body. Explain how this should include at least three A typical food chain might include class of annelid. is advantageous for the worm. levels. plant matter eaten by an earth- 3. Describe the feeding strategies of worm, which is then eaten by a earthworms and leeches. bird.

If your class subscribes to the iText, use it to review the Key Concepts in 27–3 Section Assessment Section 27–3. 1. Unlike roundworms, annelids have seg- 3. Earthworms use their pharynxes to suck soil mented bodies and a true coelom that is and detritus into their esophagus. Leeches Answer to . . . lined with mesoderm. suck the blood and body fluids of their host. Figure 27–19 If all the annelids 2. Oligochaetes typically have fewer setae than 4. Having more light-sensitive cells in the front were killed, the soil would not be mixed polychaetes and live in soil or fresh water. and back ends is advantageous, because the and aerated, and it would lack pas- Leeches are typically external parasites that animal moves forward and may be attacked sageways for roots, decomposed plant suck the blood and body fluids of their host. by a predator from the rear. matter, soil bacteria, and minerals Polychaetes are marine annelids that have from deeper soil layers. As a result, the paired, paddlelike appendages tipped with flowers and vegetables planted in the setae. garden would fail to thrive.

Worms and Mollusks 699 What Can Be Done About the Zebra Mussel? After students have read the feature, lead a class discussion of the prob- lems caused by zebra mussels and ebra mussels (Dreissena polymorpha) were intro- other exotic species. Then, encour- Zduced into the United States from Eastern age students to further investigate Europe and Asia when ships from the areas emp- the zebra mussel problem and pre- tied their ballast tanks. They were first spotted in pare a report to the class. You might the Great Lakes in the mid-1980s. Zebra mussels divide the class into small groups and have few natural enemies here and reproduce very assign each group one aspect of the rapidly. They have already colonized the entire problem, including zebra mussels’ life Great Lakes region and have spread to rivers in cycle and feeding habits, problems more than ten states. caused by zebra mussels to power Zebra mussels live attached to almost any sur- plants and other facilities, the threat face—from shopping carts to fiberglass boats—and to native mussel populations and can form layers up to 20 centimeters thick. They other species, and methods previous- have caused serious structural damage and have ly tried to eradicate the zebra clogged water supply lines to power plants and water treatment facilities. One paper company, for mussels. After groups have reported example, spent over a million dollars to remove their findings, have students discuss zebra mussels that were clogging its cooling pipes. what they think should be done to Zebra mussels also threaten the ecology of address this problem. Eradication aquatic communities. They can tolerate a wide Other groups contend that zebra mussels should range of temperatures and light intensities. In be eradicated. Engineers, for example, are devel- Research and Decide some habitats, they have displaced native mol- oping robotic submarines that can remove mussels lusks, almost making them extinct. Zebra mussels from pipelines. Chemists are testing chemicals for have also depleted the food of many fish species. the potential to destroy or disrupt the life cycle of 1. Most students will suggest that the What can be done to control zebra mussels and zebra mussels. Other scientists are adding chemi- advantage of eradication would be other exotic (nonnative) species and prevent new cals to paints and plastics to prevent mussels from that the zebra mussels would no ones from arriving? attaching to new surfaces. longer cause problems. The disad- The Viewpoints vantages would include cost and Research and Decide the possibility of harming other Control and Prevention 1. Analyzing the Viewpoints To make an species by the methods used to Many scientists believe that there is no way to informed decision, learn more about this issue destroy the mussels. An advantage remove zebra mussels and many other estab- by consulting library or Internet resources. Then, of control and prevention might lished exotic species. Instead, these scientists determine the advantages and disadvantages of be a lower cost. A disadvantage attempt to control the growth of populations and each proposed solution to the problems caused might be a lack of effectiveness prevent the transfer of exotic species to new areas. by zebra mussels. and continued damage caused by One regulation, for example, could require boaters 2. Forming Your Opinion What measures do zebra mussels. to filter and chemically clean all ballast water. you think would be most effective in dealing 2. A thoughtful response will reflect Another approach would be to find beneficial uses with exotic species? a realistic assessment of the prob- for zebra mussels. Scientists are already explor- lem and the difficulty in finding a ing the ability of zebra mussels to filter large vol- solution. The measures chosen by umes of waste water. students will vary. Whichever For: Links from the authors measure is judged most effective, Visit: PHSchool.com it should be described in detail Web Code: cbe-8273 with a reasoned assessment of its chances for success.

BACKGROUND

Clogged pipes cause problems water-intake pipes. A zebra mussel is only about A mussel is a bivalve that permanently attaches 2 centimeters in length, and a few of them in a Students can research population itself to an underwater surface by means of pipe doesn’t cause concern. But Detroit Edison growth and control of zebra mus- anchor lines called byssal threads. These threads reported that on a single water-intake screen, sels on the site developed by authors Ken Miller and Joe Levine. are secreted as a liquid by the byssal gland in the there were 700,000 mussels per square meter. animal’s foot. The liquid flows down a groove in In the winter of 1988, ice combined with mussels the foot, sticks to the rock or other surface, and blocked water intake to Detroit Edison, resulting hardens. Then, the foot is withdrawn, with the in power outages. During the 1990s, billions of thread permanently in place. In this way, zebra dollars were spent around the Great Lakes clean- mussels attach themselves to the insides of ing and refitting pipes.

700 Chapter 27