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Cora l Ree f Teacher ’s Gu ide

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Li f e on the Cora l Ree f

Life on the Coral Reef

THE CORAL REEF ECOSYSTEMCoral reefs provide the basis for the most productiveshallow water ecosystem in the world. An ecosystemis a group of living things, such as coral, algae andfishes, along with their non-living environment, suchas rocks, water, and sand. Each influences the other,and both are necessary for the successful maintenanceof life. If one is thrown out of balance by either naturalor human-made causes, then the survival of the otheris seriously threatened.

DID YOU KNOW? All of the Earth’s ecosystems areinterrelated, forming a shell of life that covers theentire planet – the biosphere. For instance, if too manytrees are cut down in the rainforest, soil from the for-est is washed by rain into rivers that run to the ocean.

The muddy silt drifts out to sea, covering the nearbycoral reefs. Some corals can remove the silt, but manycannot. If the silt is not washed off within a short pe-riod of time by the current, the polyps suffocate anddie. Not only the rainforest is destroyed, but also theneighboring coral reef.

Reef ZonesCoral reefs are not uniform, but are shaped by theforces of the sea and the structure of the sea floor intoa series of different parts or reef zones. Understand-ing these zones is useful in understanding the ecol-ogy of coral reefs. Keep in mind that these zones canblend gradually into one another, and that sometimesa zone will be missing entirely. Coral reef ecosys-

Figure 2-1. Reef zones: (a) lagoon, (b) reef crest, and (c) reef face. (Illustration: Wendy Weir)

BEACH MANGROVE SEAGRASS PATCH REEF

LAGOON

REEF CREST REEF FACE


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conchs (pronounced “kongk”), sea urchins, fish, spinylobsters, sea turtles, and small sharks.

Beach: On the land side of the lagoon a sandy beachis often found, formed by waves washing ground-upcoral skeletons, animals and plants onto the shore.Some animals use the beach habitat for reproduction.Sea turtles climb up the beach at night to lay up to100 eggs in the warm sand (Figure 2-2a), and someof the shore birds, such as crested terns, use the beachfor their nesting ground (Figure 2-2b). Other birdsrest on the beach during their migrations north andsouth, while some feed there.

Mangroves: Mangroves are often found in the tran-sitional area between land and water. These treesare fairly unique because they thrive in seawater.Growing quickly, about 2 feet (60cm) in the first year,some mangroves have specialized seed pods, calledpropagules, that grow on the end of roots that dropdown from the canopy of the tree. When the podsripen, they fall into the mud and send out roots tostart a new tree. Mangroves also have prop rootsthat stick out from the water, absorbing oxygen andpreventing the underwater roots from suffocating (Fig-ure 2-3a).

Mangroves are very important for several reasons:

• They help in the process of claiming land from thesea and in protecting the coastline because theirroots trap and consolidate sediments that wouldotherwise be washed back out by waves.

• Droppings from resident birds and falling plantmaterial enrich the soil below, increasing its capac-ity to support other plant life.

• They serve as an important habitat for many differ-ent species of animals. Their complex tangle of rootsand branches provides an excellent place for ani-mals to feed, rest, mate, and give birth. For in-stance, large fruit-eating bats roost in the mangrovebranches until nightfall when they fly out to feed.Lobsters and fiddler crabs feed on nutrients fromthe retreating tide, recycling minerals and organicmatter into the mangrove forest. Other crabs eatthe mangrove leaves. Juvenile fish seek the pro-tection of the roots until they grow large enough tosafely occupy the reef. Stingrays, small sharks, andcrocodiles (Figure 2-3b) hide among the rootswhere they rest and search for food. Oysters and

tems are also interrelated with nearby terrestrial eco-systems, that may consist of tropical rainforests,coastal lagoons, or small islands.

The main reef zones are: the lagoon, including thebeach, mangroves, seagrass beds, and patch reefs;the reef crest or flat; and the reef face (Figure 2-1).

LAGOON

Between the beach and the reef crest lie the protected,shallow waters of the lagoon. This is a very rich, di-verse zone and home to a variety of plant and animallife, including mangroves, seagrasses, sponges,

(a)

(b)Figure 2-2. (a) Loggerhead turtle laying eggs in the sand,and (b) crested terns nesting on the beach. (Illustrations:Wendy Weir)


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mussels grow on the mangrove roots and are usedfor food by native coastal people.

Unfortunately, it is only recently that the need to pro-tect mangrove habitats has been recognized. Themangrove forest may be one of the Earth’s most en-dangered habitats with an estimated 50% of all man-grove swamps already cleared or altered worldwide.They are cut down for use as coal, and to make roomfor mariculture ponds and development, such as re-sorts and waterfront homes.

DID YOU KNOW? Madagascar once had large man-grove forests along its coast which served as a nursery

for many species of reef fish. Now these forests havealmost been entirely destroyed by logging for charcoal.

Seagrass: Seagrasses are a group of marine plantsthat grow on the sandy floor of the lagoon, most of-ten amid the patch reefs. They serve both as a pro-tective nursery for numerous species of reef fish andas a rich source of food for adult fish who hide on thecoral reef during the day and venture into the sea-grass beds at night to feed. Only a few animals, suchas the sea turtle, trunkfish, and sea cow (called a dug-ong in the Indian and Pacific Oceans and a manateein the Atlantic and Caribbean), can be seen cruisingbetween the seagrass and the coral reef during theday (Figure 2-4).

Figure 2-3. (a) Mangroves along shore, and (b) saltwatercrocodile. (Photos: Wendy Weir)

(b)

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Figure 2-4. Dugong and calf grazing on seagrass.(Illustration: Wendy Weir)

Patch Reef: Some lagoons also contain fairly flat,circular or oval islands of coral, called patch reefs,which are surrounded by sand and seagrass. Theyvary greatly in size, from that of a small car to largerthan a football field. The diversity of marine life alsovaries greatly depending upon the size of the reef.Usually, the larger the patch reef, the greater thediversity.

Some of the marine life that exists on or around thepatch reef are fish, sea stars, sea turtles, sea cucum-bers, and mollusks, such as clams and conchs (Figure2-5). Sea cucumbers, which resemble giant slugs,move slowly across the sea floor, extracting food fromthe sediment in the sand and processing large quan-tities of it through their long tubular bodies.

DID YOU KNOW? The green turtle is a vegetarianand feeds only on seagrass. Unfortunately, like othersea turtles around the world, it is endangered. Eachyear, large numbers of sea turtles are caught and


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pools a few inches above sea level are sometimesformed on the reef crest. These pools are constantlyreplenished with seawater and are home to crabs,small fish, sea urchins, worms, sea stars, and marinesnails, among other creatures. These animals arealso found on other parts of the reef.

DID YOU KNOW? Crabs and sea urchins (Figure 2-6) are like janitors. They work to keep the reef clean.The crabs eat dead fish debris and the sea urchins eatthe vegetation, such as algae, that grows on deadcoral, keeping the surface clean so new coral can settleand grow.

Figure 2-5. (a) Sea stars, and (b) sea cucumber.(Illustrations: Wendy Weir)

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Figure 2-6. (a) Crab, and (b) sea urchin.(Illustrations: Wendy Weir)

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drowned in fishing nets, their eggs and flesh huntedfor food, and their shells and skin used for ornament.

REE F CREST

The reef crest is the highest (shallowest) part of theentire reef, and the most easily visible from abovethe surface of the sea. It can be identified from theshore or air as a brownish band highlighted by a lineof white breaking waves along its outer edge. Lowtides and waves often expose portions of the reefcrest, and storms crash against it, breaking off coralbranches and plates. For this reason, fewer species ofcoral are present here compared to further down onthe reef face. Still, the reef crest is home to manyplants and animals, including parrotfish, barnacles, andcoralline algae. As the tide changes, rimmed tide


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REE F FACE

The seaward facing slope of the reef is called the reefface and may be divided into two parts, upper andlower.

The upper zone is more brightly lit by the sun and ishome to a tremendous diversity of reef life, such ashard and soft corals, sponges (Figure 2-7), clownfishand sea anemones, lobster (Figure 2-8), butterflyfish,angelfish (Figure 2-9), trunkfish, damselfish (Figure2-10), sea stars, clams, lionfish, parrotfish, mollusks,sea turtles and much more. Zooplankton (micro-scopic drifting animals) and phytoplankton (micro-scopic drifting plants) (Figure 2-11) are swept intothe reef face by upwelling currents from deeper partsof the ocean, supplying the reef life with easy accessto food. This zone consists of gentle slopes whichextend from about 10 to 15 feet (3 to 5m) to around60 to 70 feet (18 to 22m) in depth.

Figure 2-7. Sponge. (Illustration: Wendy Weir)

Figure 2-8. Spiny lobster. (Photo: Milton Beral)

Figure 2-10. Damselfish amid soft coral. (Photo: TerryBrown)

Figure 2-9. Gray angelfish. (Photo: Larry Benvenuti)


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Figure 2-11. Plankton: zooplanktonand phytoplankton. (Illustration:Wendy Weir)

Figure 2-12. Moray eel. (Photo: Terry Brown)

PHYTOPLANKTONZOOPLANKTON

Figure 2-13. Spotted eagle ray. (Photos: Terry Brown)

The lower zone begins where the reef face starts todrop off. On some reefs, this area is characterized byspur-and-groove formations (high ridges of coral di-vided by sand channels or grooves) that run perpen-dicular to the shore. On other reefs, the face dropsoff sharply, plunging vertically hundreds of feet intothe depths of the sea where it becomes too dark forreef-building coral to grow. Some of the marine lifefound living in this area are soft corals, moray eels(Figure 2-12), sharks, manta rays, and eagle rays (Fig-ure 2-13).

DID YOU KNOW? Some coral species can take ondifferent shapes in different areas. Near the top ofthe reef face, they can grow short and fat in order towithstand constant wave action and all but the heavi-est of storms. Further down the face where the wateris more calm, these same corals develop thinnerbranches or large plate-like forms in order to receiveas much light as possible for their zooxanthellae.

VALUABLE RELAT IONSH I P

The beach, mangroves, seagrass bed, patch reef andcoral reef all work together to form a healthy ecosys-tem. The mangroves and seagrass beds protect thereef from being smothered by natural siltation by trap-ping soil runoff and silt from the land. They also serveas nurseries and a rich source of nutrients for reef life.In return, the reef helps to protect the beach, man-groves, and seagrass beds from erosion by waves; itprovides sand for the beach and seagrass beds; and,it contributes some of the sediment for the mangroves.Harm or destruction to one or more of these elementsin the ecosystem seriously affects the survival of theothers.


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The Food ChainNutrients and energy move through the coral reefecosystem along numerous pathways. At the baseof the food chain are the primary producers: theplants, including phytoplankton, algae, seagrasses,and mangroves, that are capable of photosynthesis.

Figure 2-14. The food chain. (Illustration: Wendy Weir)

Algae occurs in all sizes, from the microscopic zoox-anthellae that live inside coral to tiny drifting plantscalled phytoplankton to large seaweeds. The primaryproducers then pass energy and nutrients along thefood chain to the primary consumers, animals that

PLANTS

▼

HERBIVORES

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CARNIVORES

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DECOMPOSERS

(BACTERIA AND FUNGI)

NUTRIENTS

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▼

KEY

Plants: Seagrass, phyto-plankton, algae

Herbivores: Giant clam, damselfish,parrotfish

Carnivores: Shark, barracuda


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feed on plant material and are therefore called herbi-vores. In turn, these primary consumers are eaten byother animals, the secondary consumers, called om-nivores, if they eat both plants and small animals,and carnivores, if they eat only animals. The foodchain ends with the decomposition of the plants andanimals by decomposers, mainly fungi and bacteria.

Energy and nutritional exchange within the reef eco-system is an ongoing process, with some forms ofreef life feeding mainly during the day (diurnal), someduring the night (nocturnal), and some during thehours of dawn and dusk (crepuscular). Scientistsnow realize that the reef is able to produce and retainmuch of its own energy through this complex pro-cess of recycling food (Figure 2-14).

H E R B I V O R E S

Herbivores graze on plant material. Most herbivoreslive in shallow, well-lit water because that is wherethe plants grow best. They range in size from verysmall, like some zooplankton that feed on phytoplank-ton, to very large, like the giant clam that can reachmore than 40 inches (1m) in length and weigh morethan 1,000 pounds (453kg).

Like coral, the giant clam (Figure 2-15a) has a symbi-otic relationship with its microscopic plant partners,the zooxanthellae. The clam grows its own foodsource, the zooxanthellae, in its fleshy mantle. Thezooxanthellae give the mantle its blue, green, andbrown color. Some of the carbohydrates from thezooxanthellae are absorbed by the mantle’s tissuewhile other zooxanthellae are harvested by its bloodcells and digested. Humans have devastated popu-lations of these clams in many parts of the Pacific,harvesting them for food and decoration. Fortunately,projects are underway to farm these giant clams andreturn them to the coral reefs.

Among the reef fish that eat plants, there are threebasic families: damselfish, parrotfish, and surgeonfish.The damselfish (Figure 2-15b) is considered a reef“farmer” because it actively guards and grows smallpatches of algae on an area of coral to serve as itsfood source. The brightly-colored parrotfish has fusedteeth that resemble a parrot’s beak which it uses tobite and scrape algae off the hard coral. After ingest-ing bits of coral, it crushes them in its digestive sys-

tem, and excretes them as sand. The surgeonfishearned this name because of a sharp, knife-like spinelocated on each side of its body near the base of itstail. When threatened by a predator, it raises its tailmaking it difficult and painful for the predator to swal-low. They are one of the most common herbivoresfound on reefs around the world.

Other reef herbivores are conch, green snails, sea ur-chin, herring, some sea turtles, and sea cows.

Figure 2-15. (a) Giant clam, and (b) damselfish on reef.(Illustrations: Wendy Weir)

(b)

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OMNIVORES AND CARN IVORES

A number of animals on the coral reef are omnivores,feeding on both plants and animals. Among the mostcommon of these are the graceful and beautifully-col-ored angelfish. Along with herbivores, omnivoresare the most visible marine life on the reef during theday.

The carnivores on the reef feed only on animals, us-ing many different strategies to capture their prey.The amount of food eaten by carnivores varies greatly,from certain sharks who eat 10% of their total bodyweight in food per week to the small lizardfish whoeats almost 80% of its total body weight in small fisheach week.

The best known carnivore is the shark (Figure 2-16),although a few species of shark do not eat meat.Sharks have been present in the ocean for 300 mil-lion years and can be found at all depths. Many reefsharks, such as the nurse shark, are most active atnight, resting amid the coral during the day. Variousexperiments have shown that sharks rarely botherdivers who remain passive; sharks only become ag-gressive when provoked. Sharks are now being over-

Figure 2-16. Gray reef shark. (Illustration: Wendy Weir)

fished for their meat, their fins (to make shark’s finsoup), and their cartilage (for use as an anti-canceragent). Because they are at the top of the reef foodchain, their loss could greatly impact the reef com-munity.

Other carnivores are the barracuda, jacks, and grouper.

DID YOU KNOW? There are cleaning stations alongthe reef. At special locations, small cleaner fish andshrimps can be found that will clean debris and para-sites off of larger fish. The fish being cleaned willallow them to move about in their gills and mouths,and will not eat them. In some places, fish actuallyline up for this service. Cleaner fish are very impor-tant in keeping fish, and therefore the reef, healthyand strong.

D E C O M P O S I T I O N

The final stage of the food chain is decomposition.When plants and animals die, their elements arechemically broken down by tiny, simple life forms:fungi and bacteria. In this way, nutrients necessaryfor new plant growth are released back into the reefenvironment.


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Decomposition also plays a role in shaping the over-all reef environment. For instance, when coral dies,its limestone skeleton is overgrown by seaweeds andalgae, and is invaded by animals, such as worms thatbore tubes into its surface. Other animals follow, us-ing these tubes for their homes. Some species ofsponges also bore into the dead coral by releasingchemicals to break down the limestone skeleton intotiny pieces. These pieces then drift down towardsthe bottom, forming part of the sediment on the reeffloor.

DID YOU KNOW? It has been reported that wormsand sponges are able to make a dead staghorn coralbranch completely disappear within one year by bor-ing into its surface.

Predation and ProtectionMETHODS OF PREDAT ION

Many reef animals feed on plankton. Plankton con-

sists of both animals (zooplankton) and plants (phy-toplankton), and some reef animals feed on both whileothers just feed on animals. For instance, coral pol-yps and Christmas tree worms extend their tentaclesto catch plankton as it floats by. Like its relative thecoral polyp, the sea anemone, a carnivore, attachesitself to a piece of limestone rock and extends its poi-sonous tentacles to catch the zooplankton and tinyfish that float by in the ocean current.

The lionfish (Figure 2-17a) uses camouflage to hideitself among the coral and then ambushes unsuspect-ing prey as it passes. Some sharks and barracuda usespeed to chase down their prey. The trumpetfish usesstealth, hiding behind another fish or within a wavinggorgonian coral in order to sneak up on its prey.

The clown triggerfish (Figure 2-17b) and trunkfishcatch small crustaceans and fish that hide in the sandby blowing streams of water out of their mouth tomove the sand, or by removing the sand with theirfins or snout.

Figure 2-17. (a) Lionfish, and (b) clown triggerfish. (Photos: Terry Brown)

(b)(a)


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Figure 2-18. (a) Crown-of-thorns sea star (Photo: JimThompson), and (b) gianttriton. (Photo: Terry Brown)

The moray eel glides along the reef, searching forcrabs and small fish that hide in coral crevices. Itsstrong, narrow head and flexible body give it the abil-ity to enter difficult spaces to attack its prey. Somemoray eels are even able to wiggle out of the waterand up on the beach to catch crabs.

Sea stars live on and around the coral reef, eatingcoral polyps and buried mollusks, such as clams andscallops, by enveloping their stomach around the fooditem. All sea stars are characterized by radial sym-metry wherein the body parts are repeated around acenter, like the spokes of a wheel.

One type of sea star, the crown-of-thorns (Figure2-18a), is very threatening to coral reefs because iteats coral polyps, killing the coral. Entire reef eco-systems along the Great Barrier Reef of Australia andparts of the Philippines have been temporarily devas-

tated due to population explosions of the crown-of-thorns. There is quite a controversy regarding whetherthe crown-of-thorns population explosion is a natu-ral, cyclical occurrence or the result of reef disturbancesby humans. Some scientists believe population ex-plosions are caused by nutrient overload from sew-age which supplies a rich source of food for babycrown-of-thorns. The only natural predator on thecrown-of-thorns is the giant triton (Figure 2-18b), alarge marine mollusk which has been overcollectedfor its beautiful shell. Some scientists believe thatthis is another reason the crown-of-thorns has beenable to multiply unchecked in certain areas.

DID YOU KNOW? If part of a sea star is broken off,the sea star will regenerate (grow back) the missingpart. The part that is missing will also grow into anew sea star.

(b)


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METHODS OF PROTECT ION

Coral reef animals use many different forms of pro-tection. Some hide in the sand, others hide amongthe coral. Some swim into the lagoon and hide amongthe mangrove roots while others swim in schoolsalong the reef face. Some eject poison while othersinflate themselves to a much larger size. Some of thesame methods that are used for predation are alsoused for protection.

Certain fish use their body color as camouflage toblend in with the surrounding environment, makingit difficult for predators to see them. The bright yel-low blue dash butterflyfish has a round black spotnear its tail that looks like an eye. When a predatoraims for what it thinks is the head (but is actually thetail), the butterflyfish is able to escape in the oppositedirection. Its thin, pancake-like sides enable it to swimquickly between narrow coral formations and hide inplaces where its predator cannot fit (Figure 2-19a).Butterflyfish are active during the day, showing theirbright, striking color patterns. At night, they seek

shelter close to the reef’s surface, often taking on adull, dark color pattern to blend better with the reef.

The sea anemone has a unique, synergistic relation-ship with the clownfish which hides among theanemone’s poisonous tentacles, safely protected frompredators. It is believed that the mucus coating onthe clownfish protects it from the stinging cells in thesea anemone’s tentacles (Figure 2-19b). In return,the clownfish, being very territorial, drives off any fishthat try to prey on the anemone. It also drops bits offood for the anemone to eat.

Some marine snails also use the poisonous cells ofsea anemones for protection, but in a different way.When a nudibranch (pronounced “noodebrank”)feeds on the tentacles of the anemone, it does nottrigger the stinging cells. Instead, the cells migrateto the nudibranch’s exposed gills and serve the snailfor protection. Nudibranchs come in a wide varietyof shapes and colors (Figure 2-20a). Their bright col-ors warn predators of this poison.

Figure 2-19. (a) Blue dashbutterflyfish, and (b) clownfishhiding in the tentacles of a seaanemone. (Photos: Terry Brown)

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(b)


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The trunkfish (Figure 2-20b), which is sometimescalled a boxfish, has a hard, boney body or carapacesimilar to that of a trunk, which makes it hard for preda-tors to eat. Some secrete poison from their skin whenunder stress. Trunkfish swim slowly, eating small ani-mals, algae, and sponges.

The stonefish uses more than one method for pro-tection: camouflage to blend in with its environment,and lethal poison in its dorsal spine to avoid beingeaten. It is reported that the stonefish is the most

deadly, venomous fish in the Indo-Pacific region. Iftouched, a small amount of its poison can seriouslyharm a diver.

The octopus and squid, and their relative the cuttle-fish, are also excellent masters at disguise, able tochange their body color rapidly to match their sur-roundings or to eject a thick cloud of black ink whichacts as a “smoke-screen” to hide their escape. Theyhave a highly developed nervous system with a rela-tively large brain and large eyes. These large eyes

(b)

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Figure 2-20. (a) Nudibranch, and(b) spotted trunkfish. (Photos:Terry Brown)


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give them a greater field of vision and ability to see atnight, thus helping them to evade predators. Theyalso use camouflage to sneak up on their prey.

Many wrasses can operate one eye independentlyof the other: one watches where it is going, and theother looks for predators. At dusk, many parrotfishhide themselves among the coral and excrete mucusfrom their mouth to form a soft, protective envelopearound their body. The transparent mucus traps theirscent so that predators cannot find them.

The hermit crab (Figure 2-21a) and the small yellowgobie (Figure 2-21b) protect themselves from largerpredators by hiding inside holes. The hermit crabmakes its home inside an empty shell and the gobiehides in a piece of hollow coral. At the slightest sign

of a threat, they duck inside where it is safe. Gobiesare among the smallest of reef fish, some measuringless than half an inch (1cm) in length.

When attacked, porcupinefish and pufferfishprotect themselves by drawing water into theirabdomen to inflate themselves to more than twicetheir normal size, thereby making it difficult for predatorsto swallow them. They also have large protrudingeyes that can see in all directions, enabling them tospot predators quickly. Pufferfish produce a powerfulpoison, tetrodotoxin, which can cause serious illnessand even death in humans who eat one that isprepared improperly. Despite this problem, pufferfishare considered a delicacy in Japan.

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Figure 2-21. (a) Speckled hermitcrab (Photo: Larry Benvenuti), and(b) yellow gobie hiding in coral(Photo: Terry Brown)

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