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Corals andcoral reefs

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A tiny polyp protecting a giant

Increasing in number

Formation of reefs

Organisms

Adaptation to the environment

Threats to coral reefs

Endangered organisms

you will learnWhat

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A tiny polyp protectinga giant

Have you seen a coral polyp? Is it an animal or a plant?

A coral polyp is a tubular saclike animal. It has a mouth in thecentre. The mouth is surrounded by finger like structures calledtentacles. The tentacles are used for defense and for catchingfood. The polyp cannot move from place toplace as it is attached.

Reef building corals produce an outside skeletalcup of calcium carbonate. This skeletal cupprotects the polyp. During daytime when thepolyps are not feeding the tentacles will bewithdrawn into the skeletal cup. At night, theyextend their tentacles to capture food.

Do you Know?Corals are included in the same groupas jellyfish and sea anemones.

Inside the tissues of the polyp microscopic algae are foundwhich make the corals appear in different colours. Thealgae provides food and oxygen for the coral polyps and inreturn the coral polyps provide protection and access to thelight.

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Polyps: in thedaytime

Polyps: at night

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1 What is a coral polyp?

2 What is the use of the skeletal cup of corals?

3 What is the use of algae that lives inside coral?

4 Why do you think the corals are in differentshapes and sizes?

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Coral shapesShape of corals vary enormously. There arefour main forms.

IdeasK!Coral is an animal whichhas a mouth surrounded bytentacles.

! Inside the coral tissuelives an algae which is ableto make food.

!There are four main typesof corals which are massive,branched, foliaceous andsolitary corals.

Foliaceous Eg. Lettuce leaf coral

Solitarycoral

Eg.Mushroomcorals

Branchedcoral

Eg: Staghorncoral

Massive coral Eg. Brain coral

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There are two types of reproduction in corals. Sexual and asexual reproduction. Sexualreproduction involves joining of male and female eggs. In asexual reproduction, the coralsmay increase in number by budding. Budding occurs when a portion of the parent polyppinches off to form a new individual.

Increasing in number10b

Do you Know?Corals are triggered to releasemillions of sperm or eggs by thephase of the moon on a particularnight. This is known as massspawning.

! !

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Conditions necessary for coral growth

IdeasK!Corals reproduce sexuallyand asexually.

!Corals need warm water,clear water, a suitablesubstrate, salty water, watermovement and light to grow.

? 1 What are the two types of reproduction in corals?

2 Why do corals need clear water to grow?

3 What is the ideal temperature range for coralsto grow?

4 Why do corals need light?

5 Write a brief essay on the ‘life as a coral polyp’.

Warm waterCorals of tropical reefs have a temperaturerange of 18-35ºC. Their most favourabletemperarture is about 28ºC.

Water movementCorals need water movements to removesand and waste material and to bring freshsupplies of food and oxygen. Moving wateralso keeps the salt content the same.

Salty water

Corals will grow when there is just the rightamount of salt in the water. If the water istoo salty corals will not grow.

SubstrateCorals need a firm surface (substrate) toattach themselves and grow.

Clear water

Unclear water will slow or stop the growthof coral. Dirt, silt and other things in thewater will cover the polyp’s mouth andprevent it getting oxygen and food. Dirtywater will also stop light from reaching thealgae that lives inside corals.

Light

Corals need light to grow. The algae thatlives inside the corals need sunlight forphotosynthesis (make food).

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Formation of reefs

Do you Know?Islands range in size from verysmall mud and sand islands ofonly a few square metres, toGreenland, which has an area of2,175,600 km2.

The word “atoll” is derived from“atholhu” a native name inMaldives.

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Coral reefs are found in most tropical areas. Reef building corals secretecalcium carbonate which acts like a cement. These bind together coral, sandand pieces of rubble to form a more solid structure known as the reef.

Types of reefsThere are three main types of reefs.

Fringing reefs - they grow in shallow waterfrom the shoreline to about half a kilometreoffshore.

Atolls- They are ring shaped reefs found inthe middle of the ocean.

Barrier reef - they are found along the edge ofcontinental shelves. The distance from tne reefto the land may range from 0.5 km to 5 km.

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IdeasK!Three main types of reefsare fringing, barrier and atollreefs.

!According to Darwinatolls formed when volcanicislands sank as reefsstarted growing around theisland.As the volcanicisland completely sink acentral lagoon remainedsurrounded by the reefswhich eventually turned intoislands.

1 In which part of the world are coral reefs found?

2 What are the main types of reefs? What type ofreefs do you think are found in Maldives?

3 Make a poster with diagrams showing how atollsare formed according to Darwin’s volcanic theory.

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Darwin’s volcanic theory

4 The island has disappeared. The beltof corals is fully developed.

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3 The island,now almostdisappeared,is surroundedby a ring ofcorals.

5 The combined effects ofthe currents, waves andtides plus the growth ofthe corals changes theemerged part of the atollwhich may rise somemeters above the surfaceforming- sometimes largeislands.

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1 An atoll originates fromfringing reefs surrounding thecoast of a volcanic island.

2 Slowly the islandstarts to sinkand the belt ofcorals becomesmore widerand more compact.

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The modern theory of coral atoll formation was originated by Charles Darwin.

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Organisms10d

Coral reef is the home for many organisms like worms, shrimps, crabs, lobsters, sea anemones,sea urchins, octopuses, squids, clams, skates, some sharks and turtles. Many organisms livingamongst the reef are brightly coloured.

Special body colourings are used by the animals to blend into the colour of their surroundingenvironment so that they can avoid predators. Sometimes body markings on dangerousorganisms act as a warning to others. These markings may be mimicked by other animals asa form of protection from potential predators.

These are some organisms found in reef environment.

Reef

Do you Know?The world’s smallest fish is themarine dwarf goby which has anaverage length of 8.6mm. Thelargest fish is the whale shark, withspecimens recorded at 12.65m.

Star fish Parrot fish Sea anemone

Moray eel Lobster Giant clam

Green sea turtle

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IdeasK!Coral reefs are home tomany different organisms.

!Many organisms havecolours which are similar tothe environment they live in.

? 1 Why do you think that the life forms in coral reefenvironment are so brightly coloured?

2 Name some organisms that are found in the reefother than the ones mentioned here?

Unlike the reef, fish that live inthe lagoon are not very brightlycoloured. Organisms that canburrow in the sand like sting raysand sea cucumbers are found inthe lagoon.

Lagoon

Sting ray

Sea cucumber

Fast moving migratory fish are found in the deep sea. Fish liketuna and shark are generally coloured dark on top and lightunderneath. These colours merge with the dark seabed whenviewed from above and the light sky when viewed from below.

Deep sea

Goby

Sand eel

Shark Tuna

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Adaptation to theenvironment10e

All living things are adapted to their environment.They fit in where they live. Sometimes the environmentchanges and the animal has to change with it.

Do you Know?The eyes of an octopus can adaptto rapid changes in the amount oflight available as it moves quicklyfrom the depths of the ocean tonear the surface.

Camouflage

Octopuses avoid detection bypredators by changing colour quickly.They use their tentacles for walkingand they are also excellent swimmers.

Flat fishes like flounders are able to lie on theirsides and change the upper surface colouringto match the sand they are lying on.

The stone fish is well camouflaged,looking very much like a weed coveredstone. A stone fish in coral rubble isespecially difficult to recognise.

Many butterfly fish and angel fish arestriped and flat. The contrasting lineshelp them to hide in branching corals.

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Other adaptations

Trigger fish have dorsal spine which can belocked into an upright position. It is used to ‘lock’the triggerfish into coral cavities so that predatorscannot pull them out. Most sea slugs are brightly coloured and

patterned. These features suggest that it ispoisonous or dangerous and should be leftalone.

IdeasK!Living things are adaptedto their environment.

!Animals camouflagethemselves to stay alive.

!Some animals havespecial features which areused to protect themselves.

2 What is the use of the dorsal spine in trigger fish?

3 Find out about other marine organisms notmentioned in the text that can camouflage and thathave special features that they use as protection.Make a poster about these organisms.

1 How does the following organisms camouflagethemselves?

a. Butterfly fish

b. Flat fishc. Stone fish

Sting rays havepoison-tippedsting which isused as defenceif somethingstands on it andis also used toattack other rayswhen fightingfor a mate.

Puffer fish or porcupine fish canpuff themselves up to look twice asbig.

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Threats to coral reefs10f

There are both natural and human destructions to corals. Naturally corals are eaten bycrown-of-thorn starfish, coral eating fishes like parrot fish and boring organisms like tubeworms.

When natural disturbances like cyclones, hurricanes, actions of earthquakes and volcanoes arenot frequent, reefs can recover. However, human activities tend to create widespread and longterm disturbances.

Do you Know?Fishing with dynamite, cyanide, orbleach has killed many coral reefsespecially in the Indo-Pacific region.

Human threatsTrash dumped into the water can kill thecoral reef and the organisms that live there.Garbage at the seaside often ends up in thesea and disrupts the coral reef’s delicateenvironment.

Carelessboating,diving, fishing,and otherrecreationaluses of coralreef areas cancause damageto coral reefs.

Parrot fishTube wormCrown of thorns

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Dredging to make harbours covers the coralswith sand.

Due to overfishing, reef fish population greatly decreases in number.The removal of large number of reef fish causes disturbance to theirnatural environment.

Mining corals for building purposes kills thewhole reef area where mining occured. InMaldives mining cannot be carried out onhouse reefs.

IdeasK!Corals are destroyednaturally as well as by humanactivities.

!Corals are threatened bymarine pollution, overfishing,dredging, anchoring,trampeling, coral mining andcollection.

Mined coralsDredging

? 1 What are the organisms that destroys corals?

2 List other ways that are not mentioned in the textof how human activities can destroy coral reefs?

3 Imagine you live on an island and you need toprotect the reef environment. Discuss the waysthat you can protect the reef environment.

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Endangered organisms10g

Endangered organisms are those in danger ofdisappearing from the face of the earth if itssituation is not improved. When its race hasnot been seen in the wild for over fifty years,we say that it is extinct. Those organisms thatmay soon become endangered are calledthreatened species.

Extinction has been occuring naturally for million of years and itcontinues to do so today. The current problem is that organismsare becoming extinct because of humans. One reason is that thereare more people on the earth every day and they kill organismsfor food and sometimes for fun. Another way humans endangeranimals is by changing the environment. Also extinction cancome from pollution such as oil spills and waste.

What causes extinction?

Here are some marine organisms that are endangered in Maldives.Can you think of ways to save them?

Black coral Sea cucumber

Giant clam

Blue whale

Napolean wrasse

Turtles

whale shark

Do you Know?Blue whale is the largest evermammal. There are about 500individuals today in the southernhemisphere, down from anestimated 250,000.Oyster

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1 What are endangered organisms?

2 Why do you think these organisms areendangered? What are the threats to them?

3 Make a poster showing endangered marineorganisms of Maldives.

4 Discuss how you can protect endangered species.

Endangered marine organisms in the world

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IdeasK!Endangered organisms arethose that are in danger ofextinction.

!In Maldives someendangered marineorganisms are:

a whale sharkb mother -of-pearlc giant clamsd black corale turtles

CoralGreen sea turtle (velaa)

Logger head turle (boabodu velaa)

Great white shark

Mako shark

Humpback whale

Bottlenose dolphin

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Born in Canada of Scottish descent, Murray came to Scotland to beeducated and eventually entered Edinburgh University, to studymedicine. Instead he joined a whaler as surgeon for a seven monthvoyage to the Arctic.

Murray is noted today, and particularly in the USA, as the founderof modern oceanography. He studied ocean basins, deep-sea de-posits, and coral-reef formation. As a marine scientist, he took partin the Challenger Expedition (1872-76), the first major ocean graphicexpedition of the world. He was first to observe the Mid-AtlanticRidge and the existence of marine trenches. He attempted withBuchan to construct from temperature and salinity observations aqualitative theory of water movement in the world’s oceans. WithA. Agassiz, he put forward a modified hypothesis for coral reefdevelopment, arguing against Darwin’s hypothesis and suggestingthat subsidence was not always a controlling mechanism. He died in1914, killed by a motor car.

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