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Coral Gardens Teacher’s Guide Life on the Coral Reef

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LIFE ON THE CORAL REEFTHE CORAL REEF ECOSYSTEM

Coral reefs provide the basis for the mostproductive shallow water ecosystem in the world.An ecosystem is a community of organismsinteracting with one another and with the chemicaland physical factors making up their environment.All parts are necessary for the successfulmaintenance of life. If one part of the ecosystem isthrown out of balance by either natural or human-made causes, then the survival of the others isthreatened.

DID YOU KNOW? All of the Earth’s ecosystemsare interrelated, forming a shell of life that coversthe entire planet – the biosphere. For instance, iftoo many mangroves are cut down, soil and silt arewashed by heavy rain into the sea. The muddy siltdrifts out to the nearby seagrass beds and coralreefs, covering them. Some corals can remove thesilt, but many cannot. If the silt is not washed offwithin a short period of time by the current, thepolyps suffocate and die.

Not only are the mangroves destroyed, but also theneighbouring coral reef.

REEF ZONES

Coral reefs are not uniform, but are shaped by theforces of the sea and the structure of the sea floorinto a series of different parts or reef zones.Understanding these zones is useful inunderstanding the ecology of coral reefs. Keep inmind that these zones can blend gradually into oneanother, and that sometimes a zone will be missingentirely. Coral reef ecosystems are alsointerrelated with nearby terrestrial ecosystems.

The main reef zones are: the lagoon (or sound)including the beach, mangroves, seagrass beds,and patch reefs; the reef crest or fringing reef;and the reef face, including in Cayman the Upperand Lower terraces and the Cayman Wall (Figure2-1).

Figure 2-1. Reef zones: showing the lagoon, reef crest, and reef face. (Illustration: Wendy Weir).


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Lagoon or Sound

Between the beach and the reef crest lie theprotected shallow waters of the lagoon or sound.This is a very rich, diverse zone and home to avariety of plant and animal life, includingmangroves, seagrasses, sponges, conchs, seaurchins, fish, spiny lobsters, sea turtles, and smallsharks.

Beach: On the land side of the lagoon a sandybeach is often found, formed by waves washing

ground-up coral skeletons, animals and plants ontothe shore. Some animals use the beach habitat forreproduction. Sea turtles climb up the beach atnight to lay up to 100 eggs in the warm sand

(Figure 2-2a), and some of the shore birds, such asCrested Terns, use the beach for their nestingground (Figure 2-2b). Other birds rest on thebeach during their migrations north and south,while some feed there.

Mangroves: The Cayman Islands have threespecies of mangroves found in the transitional zonebetween land and water. Their pattern ofdistribution generally correlates with the toleranceof each species for seawater immersion, althoughthe classic zonation pattern described below doesnot always apply to Cayman's mangrove wetlands.Outermost are the most salt-tolerant, the redmangroves, which blend shoreward into the blackmangroves, and innermost are the whitemangroves. As pioneering red mangrovescontinue to colonise the outer edges, sediment istrapped and consolidated among the dense rootsystems and the land gradually rises (Figure 2-3).This allows black mangroves to expand their

seaward range by moving in on red mangroves,pushing them further outward. White mangrovesare joined by Buttonwood, Swamp Fern and avariety of dry land trees such as Mahogany andwild Fig at the landward extent of the mangroveforest. Here, leaf litter and dead wood accumulate,decompose and gradually increase the land height.

Mangrove trees are unique because they areextremely well adapted to tropical coastalenvironments characterised by high temperatures,

(b)

(a)

Figure 2-2. (a) Loggerhead turtle laying eggs in thesand (Illustration: Wendy Weir), and (b) CrestedTerns nesting on the beach (Illustration: Wendy Weir)

Figure 2-3. Red mangrove in North Sound. (Photo fromDOE collection.)


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FACT: Since the last Ice Age 10,000 years ago, sea level in the Caribbean has been rising at a rate of 1foot every 100 years. Mangroves are able to keep pace with sea level rise as layers of dead wood andleaf litter that forms carbon-rich peat are regularly deposited on the continuously thickening wetlandfloor. It is estimated that Grand Cayman’s Central Mangrove Wetland lays down approximately 3 millioncubic feet of peat each year. Mangrove wetlands function as a carbon sink by removing carbon dioxide- a greenhouse gas that contributes to global warming - from the atmosphere, through the process ofphotosynthesis. Cayman’s wetlands are doing their small part in combating the problem of globalwarming by offsetting our fast increasing carbon dioxide emissions from vehicles, electricity generationand deforestation.

thick muddy sediments, low oxygen content ofwater and widely fluctuating salinities. Adaptationsto this environment include the possession of saltglands in the leaves which function as an excretingmechanism to remove excess salt, and highlydeveloped aerial and below ground root systems ofthe red and black mangroves. Red mangroves,which can grow to a height of 25m (80ft), have asystem of prop roots that extend 1m (3ft) or moreabove the surface of the soil and contain manysmall pores called lenticels. At low tide thelenticels allow oxygen to diffuse into the plant anddown to the underground roots by means of smallopen passages. Black mangroves, which can growto 20m (64ft), have small air roots calledpneumatophores (pronounced “nyoomatofors”)which extend vertically upward from theunderground roots to a height of 20-30cm (8-12inches) above the soil. These pneumatophoresresemble hundreds of tiny fingers sticking out ofthe mud underneath the tree canopy. At low tideoxygen travels to all living root tissue through thepneumatophores. White mangroves, having neitherof these types of roots, use lenticels in the lowerportion of the trunk to obtain oxygen.

Mangroves provide many important functions:

• They help in the process of shorelinestabilisation by trapping and consolidatingsediments in their roots that would otherwisebe washed back to sea by waves. Mangrovesassist in protecting the coastline as their rootsystems absorb and disperse wave energy incalm and stormy conditions, thereby reducingextensive erosion and flooding. The CaymanIslands are topographically low and flat and aresusceptible to flooding from the sea. Ourmangroves are vital for maintaining the stabilityof our coastlines and protecting coastalsettlements in the event of storms andhurricanes.

• The complex tangle of roots and branches

provide excellent habitat for many differentmarine species to feed, rest, mate and givebirth. The prop roots serve as nursery groundsfor young spiny lobsters (which can spend upto two years in this environment) and crabs.These animals help to recycle minerals andorganic matter in the mangrove forest byfeeding on leaf litter or nutrients from theretreating tide. Commercially important fishsuch as snapper and grunt also seek theprotection of the roots at the juvenile stage untilthey grow large enough to safely occupy thereef. Stingrays and small nurse sharks hideamong the roots where they rest and searchfor food. Oysters and mussels, used for foodby many coastal populations elsewhere in theworld, grow on the mangrove roots.

• Due to their diverse structural habitat,mangroves are home to a variety of threatenedor endangered birdlife. Herons, egrets, ducksand gallinules, as well as the endangeredCayman parrot, are common sights inCayman’s mangroves (Figure 2-4). Ourmangroves also play a vital role in providing awintering habitat for migrant birds from North

Figure 2-4. Cayman parrots nesting in CentralMangrove. (Photo from DOE collection).


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America. The birds are part of the complexmangrove food web and some contributesignificantly to nutrient cycling in the coastalzone. For instance, droppings from residentbirds and falling plant material enrich the soilbelow, increasing the mangrove’s capacity tosupport other plant life.

Mangroves are one of the most productive naturalsystems on earth. However, human activities canseverely reduce the net primary productivity uponwhich tropical fisheries depend. Unfortunately, it isonly recently that the need to protect mangrovehabitats has been recognised. Mangroves maynow be one of the Earth’s most endangeredhabitats with an estimated 50% of all mangrovewetlands already cleared or altered worldwide. In

Cayman mangroves are cut down and filled tocreate land for resorts and waterfront homes, whilein other parts of the world they are used forcharcoal and to make room for mariculture pondsand development.

DID YOU KNOW? Little Cayman’s Booby PondNature Reserve is regionally significant as one ofthe largest breeding colonies of the Red-footedBooby bird in the Western Hemisphere (5,000breeding pairs or 10,000 birds) (Figure 2-5). For

this reason, this National Trust site has beendesignated as a wetland of internationalimportance under the Ramsar Convention, and isprotected as an Animal Sanctuary under locallegislation through the Animals Law 1976.

Seagrass, or “black grass” as it is known inCayman, is the term for group of flowering marineplants that grow on the sandy floor in clear shallowlagoons and bays found inside the fringing reef.While four species of seagrass exist in the CaymanIslands, the most common species is turtle grass,which covers some 60% of the North Soundseabed (Figure 2-6).

Seagrasses are unique plants that take in nutrientssuch as nitrogen and phosphorous through theirroots and leaves. Seagrasses are limited indistribution by the availability of light, so when thereis a high suspension of silt in the water column,they may not receive sufficient light and willeventually be replaced by species tolerant to low

Figure 2-5. Young Booby bird in a nest in themangroves. (Photograph from DOE collection)

FACT: The Central Mangrove Wetland(CMW) is the ecological heart of GrandCayman providing a range of criticalenvironmental services important to theisland as a whole. Little Sound and greaterNorth Sound are provided with vitalnutrients from the entire CMW (8,500acres) through tidal flushing and occasionalmassive overflows of accumulatedrainwater. Due to the local convectionsystem, the CMW contributes a large partof the rainfall in West Bay and GeorgeTown. West Indian Whistling Ducks, GrandCayman Parrots, Snowy Egrets, Herons anda variety of other native birds depend onthe CMW for food, shelter and nestingsites. Various crab species, smallercrustaceans, countless species of insectsand other invertebrates inhabit theWetland, along with fish, Higatees, agoutis,and an array of other animal life.Approximately 1,500 acres of the CMWwhich fringes Little Sound is protected aspart of the Marine Parks’ EnvironmentalZone under the Marine ParksRegulations, which have been in effectsince 1986. Due to its internationalecological importance, there are plans todeclare Little Sound and its fringingmangrove a Ramsar site. Efforts are alsounder way to increase the total area of theCMW under protection, through theacquisition of conservation land and othermeans. The National Trust currently owns700 acres in the CMW.


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light conditions. Due to the ability of their root andrhizome systems to spread laterally, they are ableto colonise extensive areas of thick sand and mudsubstrates.Seagrasses provide many important functions:

• They serve both as a protective nursery fornumerous species of reef fish and as a richsource of food for adult fish that hide on thecoral reef during the day and venture into theseagrass beds at night to feed. Only a fewanimals, such as the sea turtle, trunkfish andmanatees, can be seen cruising between theseagrass and the coral reef during the day.

• Seagrass beds provide protection and shelterfor juvenile forms of many reef species.Conch, lobster and other commercial speciesare afforded protection within seagrass beds inthe sounds of Grand Cayman and the SisterIslands, and are of enormous importance to theIslands’ local fishing and recreationalindustries.

• Seagrasses possess an extensive rootstructure that function to stabilise the seabedby trapping sediments and binding themtogether.

• Seagrass blades dampen wave action and inturn provide an enormous economic benefit byhelping to control erosion of beaches andshorelines, especially in high wave energyareas associated with windward or ‘weather’shores.

• They provide an aesthetic benefit by helping tomaintain clear water by subduing particles fromturbid water and keeping them bound in place.The Cayman Islands’ tourism industry depends

heavily on aesthetically clear water that in turndepends on seagrasses.

Managing seagrass beds is an important part ofmanaging fisheries. This notion is not widelyunderstood and there is little inclination to protectthis ecosystem. Thus many areas of seagrassesand their associated fisheries are now threatened.

Patch Reefs: Some lagoons or sounds also containfairly flat, circular or oval islands of coral, calledpatch reefs (or “panshoals” by Caymanians),which are surrounded by sand and seagrass. Theyvary greatly in size, from that of a small car to

larger than a football field. The diversity of marinelife also varies greatly depending upon the size ofthe reef. Usually, the larger the patch reef, thegreater the diversity.

Figure 2-6. The seagrass beds of North Sound. (Photofrom DOE collection).

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(b)Figure 2-7. (a) Sea stars, and (b) sea cucumber.(Illustrations: Wendy Weir)


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(a)

(b)Figure 2-8. (a) Crab, and (b) sea urchin.(Illustrations: Wendy Weir)

Some of the marine life that exists on or aroundpatch reefs are fish, sea stars, sea turtles, sea

cucumbers, and molluscs, such as conchs (Figure2-7). Sea cucumbers, which resemble giant slugs,move slowly across the sea floor, extracting foodfrom the sediment in the sand and processing largequantities of it through long tubular bodies.

DID YOU KNOW? The green turtle is a vegetarianand feeds mainly on turtle grass. Unfortunately,like other sea turtles around the world, it isendangered. Each year, large numbers of turtlesare caught and drowned in fishing nets, their eggsand flesh hunted for food, and their shells and skinused for ornament.Reef Crest

The reef crest is the highest (shallowest) part ofthe entire reef, and the most easily visible fromabove the surface of the sea. It can be identifiedfrom the shore or air as a brownish bandhighlighted by a line of white breaking waves alongits outer edge. Low tides and waves often exposeportions of the reef crest, and storms crash againstit, breaking off coral branches and plates. For thisreason, fewer species of coral are present herecompared to further down on the reef face. Still,the reef crest is home to many plants and animals,including parrotfish, barnacles, and coralline algae.

DID YOU KNOW? Crabs and sea urchins (Figure2-8) are like janitors. They work to keep the reefclean. The crabs eat dead fish debris and the seaurchins eat the vegetation, such as algae, thatgrows on dead coral, keeping the surface clean sonew coral can settle and grow.

Reef Face

The seaward-facing slope of the reef is called thereef face and may be divided into two parts, upperand lower.

The upper zone (or fore reef terrace in Cayman)is more brightly lit by the sun and is home to atremendous diversity of reef life, such as hard andsoft corals, sponges (Figure 2-9), and seaanemones, lobster (Figure 2-10), butterflyfish,angelfish (2-11), trunkfish, damselfish (Figure 2-12), sea stars, clams, surgeonfish, parrotfish,

Figure 2-9. Sponge. (Illustration: Wendy Weir)


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molluscs, sea turtles and much more.Zooplankton (microscopic drifting animals) andphytoplankton (microscopic drifting plants) (Figure2-13) are swept into the reef face by upwellingcurrents from deeper parts of the ocean, supplyingthe reef life with easy access to food. This zoneconsists of gentle slopes which extend from about10 to 15 feet (3 to 5m) to around 60 to 70 feet (18to 22m) in depth.

The lower zone (or deep reef terrace in Cayman)begins where the reef face starts to drop off. Onsome reefs, this area is characterised by spur-and-

groove formations (high ridges of coral divided bysand channels or grooves) that run perpendicularto the shore. On Cayman’s 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. This feature is knownas the Cayman Wall (Figure 2-14). Some of the

marine life found living in this area are soft corals,moray eels (Figure 2-15), sharks, manta rays, andeagle rays (Figure 2-16).

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 orderto withstand constant wave action and all but the

heaviest of storms. Further down the face wherethe water is more calm, these same corals developthinner branches of large plate-like forms in orderto receive as much light as possible for theirendosymbionts.

Figure 2-10. Spiny lobster. (Photo from DOEcollection.)

Figure 2-11. Grey angelfish. (Photo from DOEcollection.)

Figure 2-12. Damselfish amid algae garden.(Photograph from DOE collection.)

Figure 2-13. Plankton: zooplankton andphytoplanktion. (Illustration: Wendy Weir)


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VALUABLE INTER-RELATIONSHIPS

The beach, mangroves, seagrass beds, patch reefand coral reef all work together to form a healthyecosystem. The mangroves and seagrass bedsserve as nurseries for young marine organisms.They protect the reef from being smothered bynatural siltation by trapping soil runoff and silt fromthe land, which helps to clear the water andenhances coral growth. Mangroves provide a richsource of nutrients for growth of plankton on whichcoral polyps feed, and they protect coastlines fromerosion and regulate the land drainage to thelagoons. The reef protects the seagrass beds fromheavy wave action, and also protects the beachand mangroves from erosion by waves. Reefsprovide sand for the beach and seagrass beds andthey contribute some of the sediment for themangroves. Harm or destruction to one or more of

these elements in the ecosystem seriously affectsthe health and survival of the others.

THE FOOD CHAIN

Nutrients 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 ofphotosynthesis. Algae occur in all sizes, from themicroscopic endosymbionts that live inside coral totiny drifting plants called phytoplankton to largeseaweeds. The primary producers then passenergy and nutrients along the food chain to theprimary consumers, animals that feed on plant

Figure 2-14. The Cayman Wall. (Photo fromDOE collection.)

Figure 2-15. Moray eel. (Photo from DOEcollection).

Figure 2-16. Spotted eagle ray. (Photo from DOEcollection.)


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material and are therefore called herbivores. Inturn, these primary producers are eaten by otheranimals, the secondary consumers, calledomnivores, if they eat both plants and animals,and carnivores, if they eat only animals.Decomposers, mainly fungi and bacteria, throughthe process of decomposition, produce detritus andnutrients which enter the food chain again.

Energy and nutritional exchange within the reefecosystem is an ongoing process, with some formsof reef life feeding mainly during the day (diurnal),some during the night (nocturnal), and someduring the hours of dawn and dusk (crepuscular).

Scientists now realise that the reef is able toproduce and retain much of its own energy throughthis complex process of recycling food (Figure 2-17).

Herbivores

Herbivores graze on plant material. Mostherbivores live in shallow, well-lit water becausethat is where the plants grow best. They range insize from very small, like some zooplankton thatfeed on phytoplankton, to very large, like the giantclam found in Australia that can reach more than

Figure 2-17. The food chain. (Illustration: Wendy Weir)


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40 inches (1m) in length and weigh more than1,000 pounds (453kg).

Like coral, the giant clam (Figure 2-18a) has asymbiotic relationship with its microscopic plant

partners, the endosymbionts. The clam grows itsown food source, the endosymbionts, in its fleshymantle. The endosymbionts give the mantle itsblue, green and brown colour. Some of thecarbohydrates from the endosymbionts areabsorbed by the mantle’s tissue while otherendosymbionts are harvested by its blood cells anddigested. Humans have devastated populations ofthese clams in many parts of the Pacific, harvesting

them for food and decoration. Fortunately, projectsare underway to farm these giant clams and returnthem to the coral reefs.

Among the reef fish that eat plants, there are threebasic families: damselfish, parrotfish, andsurgeonfish. The damselfish (Figure 2-18b) isconsidered a reef “farmer” because it activelyguards and grows small patches of algae on anarea of coral to serve as its food source. Thebrightly-coloured parrotfish has fused teeth thatresemble a parrot’s beak, which it uses to bite andscrape algae off the hard coral. After ingesting bitsof coral, it crushes them in its digestive system,and excretes them as sand. The surgeonfishearned this name because of a sharp, knife-likespine located on each side of its body near thebase of its tail. When threatened by a predator, itraises its tail making it difficult and painful for thepredator to swallow. They are one of the morecommon herbivores found on the reefs around theworld.

Other reef herbivores are conch, snails, sea urchin,herring and some sea turtles.

Omnivores and Carnivores

A number of animals on the coral reef areomnivores, feeding on both plants and animals.Among the most common of these are the gracefuland beautifully coloured angelfish. Along withherbivores, omnivores are the most visible marinelife on the reef during the day.

The carnivores on the reef feed only on animals,using many different strategies to capture theirprey. The amount of food eaten by carnivoresvaries greatly, from certain sharks who eat 10% oftheir body weight in food per week to the smalllizardfish who eats almost 80% of its total bodyweight in small fish each week.

The best known carnivore is the shark (Figure 2-19), although a few species of shark do not eatmeat. Sharks have been present in the ocean for300 million years and can be found at all depths.Many reef sharks, such as the nurse shark, aremost active at night, resting amid the coral duringthe day. Various experiments have shown thatsharks rarely bother divers who remain passive;sharks only become aggressive when provoked.Sharks are now being over-fished for their meat,their fins (to make shark’s fins soup), and theircartilage (for use as an anti-cancer agent).

(a)

(b)Figure 2-18. (a) Giant clam, and (b) damselfish onreef. (Illustrations: Wendy Weir)


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Because they are at the top of the reef food chain,their loss could greatly impact the reef community.

Other carnivores are barracuda, jacks, andgrouper.

DID YOU KNOW? There are cleaning stationsalong the reef. At special locations, small fish,cleaner fish and shrimp, can be found that willclean debris and parasites off of larger fish. Thefish being cleaned will allow them to move about intheir gills and mouths, and will not eat them. Insome places, fish actually line up for this service.Cleaner fish are very important in keeping fish, andtherefore the reef, healthy and strong.

Decomposition

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, nutrientsnecessary for new plant growth are released backinto the reef environment.

Decomposition also plays a role in shaping theoverall reef environment. For instance, when coraldies, its limestone skeleton is overgrown byseaweeds and algae, and is invaded by animals,such as worms that bore tubes into its surface.Other animals follow, using these tubes for theirhomes. Some species of sponges also bore intothe dead coral by releasing chemicals to breakdown the limestone skeleton into tiny pieces.

These pieces then drift down towards the bottom,forming part of the sediment on the reef floor.

DID YOU KNOW? It has been reported thatworms and sponges are able to make a deadstaghorn coral branch completely disappear withinone year by boring into its surface.

PREDATION AND PROTECTION

Methods of Predation

Many reef animals feed on plankton. Planktonconsists of both animals (zooplankton) and plants(phytoplankton), and some reef animals feed onboth while others just feed on animals. Forinstance, coral polyps and Christmas treeworms extend their tentacles to catch plankton asit floats by. Like its relative the coral polyp, the seaanemone, a carnivore, attaches itself to a piece oflimestone rock and extends its poisonous tentaclesto catch the zooplankton and tiny fish that float byin the ocean current.

Figure 2-19. Grey reef shark. (Illustration: Wendy Weir)


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The scorpionfish (Figure 2-20a) uses camouflageto hide itself among the coral and then ambushesunsuspecting prey as it passes. Some sharks andbarracudas use speed to chase down their prey.

The trumpetfish uses stealth, hiding behindanother fish or within a waving gorgonian coral inorder to sneak up on its prey.

The queen triggerfish (known to Caymanians asold wife) (Figure 20b) and trunkfish catch smallcrustaceans and fish that hide in the sand byblowing streams of water out of their mouth tomove the sand, or by removing the sand with theirfins or snout.

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 theability to enter difficult spaces to attack its prey.Some moray eels are even able to wiggle out of thewater and up on the beach to catch crabs.

Sea stars live on and around the coral reef, eatingcoral polyps and buried molluscs, such as clamsand scallops, by enveloping their stomach aroundthe food item. All sea stars are characterised byradial symmetry wherein the body parts arerepeated around a centre, like the spokes of awheel.

One type of sea star, the crown-of-thorns (Figure2-21a) is very threatening to coral reefs because iteats coral polyps, killing the coral. Entire reefecosystems along the Great Barrier Reef ofAustralia and parts of the Philippines have beentemporarily devastated due to populationexplosions of the crown-of-thorns. There is quite a

controversy regarding whether the crown-of-thornspopulation explosion is a natural, cyclicaloccurrence or the result of reef disturbances byhumans. Some scientists believe populationexplosions are caused by nutrient overload fromsewage, which supplies a rich source of food forbaby crown-of-thorns. The only natural predator onthe crown-of-thorns is the giant triton (Figure 2-21b), a large marine mollusc which has been

(a)

(b)Figure 2-20. (a) Scorpionfish, and (b) queentriggerfish. (Photos from the DOE collection).

(a)

(b)

Figure 2-21. (a) Crown of thorns starfish (Photofrom DOE collection) and (b) Giant Triton (Photoby Terry Brown).


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overcollected for its beautiful shell. Somescientists believe that this is another reason thecrown-of-thorns has been able to multiplyunchecked in certain areas.

DID YOU KNOW? If part of a sea star is brokenoff, the sea star will regenerate (grow back) themissing part. The part that is missing will also growinto a new sea star.

Methods of Protection

Coral reef animals use many different forms ofprotection. Some hide in the sand, others hideamong the coral. Some swim into the lagoon andhide among the mangrove roots while others swimin schools along the reef face. Some eject poisonwhile others inflate themselves to a much larger

size. Some of the same methods that are used forpredation are also used for protection.

Certain fish use their body colour as camouflageto blend in with the surrounding environment,making it difficult for predators to see them. Thebright yellow four-eye butterflyfish has a roundblack spot near its tail that looks like an eye. Whena predator aims for what it thinks is the head (but isactually the tail), the butterflyfish is able to escapein the opposite direction. Its thin, pancake-likesides enable it to swim quickly between narrowcoral formations and hide in places where itspredator cannot fit (Figure 2-22a). Butterflyfish areactive during the day, showing their bright, strikingcolour patterns. At night, they seek shelter close tothe reef’s surface, often taking on a dull, darkcolour pattern to blend better with the reef.

The sea anemone has a unique, synergistic(a)

(b)

Figure 2-22. (a) Four-eye butterflyfish, and (b)Banded clinging anemone crab. (Photos from DOEcollection).

Figure 2-23. (a) Nudibranch, and (b) Smoothtrunkfish. (Photos from DOE collection).

(a)

(b)


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relationship with the diamondback blenny, whichhides among the anemone’s poisonous tentacles,safely protected from predators. It is believed thatthe mucus coating on the blenny protects it fromthe stinging cells in the sea anemone’s tentacles(Figure 2-22b). In return, the blenny, being veryterritorial, drives off any fish that try to prey on theanemone. It also drops bits of food for theanemone 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 not

trigger the stinging cells. Instead, the cells migrateto the nudibranch’s exposed gills and serve thesnail for protection. Nudibranchs come in a widevariety of shapes and colours (Figure 2-23a). Theirbright colours warn predators of this poison.

The trunkfish (Figure 2-23b), which is sometimescalled a boxfish, has a hard, boney body orcarapace similar to that of a trunk, which makes ithard for predators to eat. Some secrete poison

from their skin when under stress. Trunkfish swimslowly, eating small animals, algae, and sponges.

The stonefish uses more than one method forprotection: camouflage to blend in with itsenvironment, and lethal poison in its dorsal spine toavoid being eaten. If touched, a small amount ofits poison can seriously harm a diver.

The octopus (Figure 2-24b) and squid, and theirrelative the cuttlefish, are also excellent mastersat disguise, able to change their body colour rapidlyto match their surroundings or to eject a thick cloudof black ink which acts as a “smoke-screen” to hidetheir escape. They have a highly developednervous system with a relatively large brain andlarge eyes. These large eyes give them a greaterfield of vision and ability to see at night, thushelping them to evade predators. They also usecamouflage 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, manyparrotfish hide themselves among coral andexcrete mucus from their mouth to form a soft,protective envelope around their body. Thetransparent mucus traps their scent so thatpredators cannot find them.

The hermit crab (Figure 2-24a) and the smallyellow gobie protect themselves from largerpredators by hiding inside holes. The hermit crabmakes its home inside an empty shell and thegobie hides in a piece of hollow coral. At theslightest sign of a threat, they duck inside where itis safe. Gobies are among the smallest of reef fish,some measuring less than half an inch (1cm) inlength.

When attacked, porcupinefish and pufferfishprotect themselves by drawing water into theirabdomen to inflate themselves to more than twicetheir normal size, thereby making it difficult forpredators to swallow them. They also have largeprotruding eyes that can see in all directions,enabling them to spot predators quickly. Pufferfishproduce a powerful poison, tetrodotoxin, whichcan cause serious illness and even death inhumans who eat one that is prepared improperly.Despite this problem, pufferfish are considered adelicacy in Japan.

Figure 2-24. (a) Speckled hermit crab, and (b)octopus (Photos from DOE collection).

(a)

(b)


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THE CORAL REEF ECOSYSTEM.....................................................................................................................................1REEF ZONES........................................................................................................................................................................1

Lagoon or Sound ...............................................................................................................................................................2Reef Crest ..........................................................................................................................................................................6Reef Face ...........................................................................................................................................................................6

VALUABLE INTER-RELATIONSHIPS...........................................................................................................................8THE FOOD CHAIN..............................................................................................................................................................8

Herbivores..........................................................................................................................................................................9Omnivores and Carnivores .............................................................................................................................................10Decomposition.................................................................................................................................................................11

PREDATION AND PROTECTION ..................................................................................................................................11Methods of Predation......................................................................................................................................................11Methods of Protection.....................................................................................................................................................13

life on the coral reefdoe.ky/wp-content/uploads/2015/01/chapter-2.pdfcoral gardens teacher’s guide...

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