Chapter 14 Chapter 14

Coral ReefsCoral Reefs

CoralsCorals• Corals are _____, a Corals are _____, a

phylum of animals phylum of animals characterized by characterized by the presence of the presence of _____ on their _____ on their tentaclestentacles

• These _____ are These _____ are used to capture used to capture prey primarily, but prey primarily, but can also be can also be defensivedefensive

• The soft _____ is The soft _____ is enclosed in a hard enclosed in a hard shell of calcium shell of calcium carbonate created carbonate created by the coral.by the coral.

Basic Types of CoralsBasic Types of Corals• Hermatypic corals-Hermatypic corals-

– Reef builders-mainly scleractinian coralsReef builders-mainly scleractinian corals– Normally contain zooxanthellae, symbiotic Normally contain zooxanthellae, symbiotic

photosynthetic dinoflagellatesphotosynthetic dinoflagellates– Restricted in distribution by water temperature and Restricted in distribution by water temperature and

water qualitywater quality

• Ahermatypic corals-mainly gorgonians (sea fans and sea Ahermatypic corals-mainly gorgonians (sea fans and sea whips)whips)– Do not build reefsDo not build reefs– May not contain zooxanthellaeMay not contain zooxanthellae– Not as restricted by water temperature and water qualityNot as restricted by water temperature and water quality

Anatomy of a Coral ReefAnatomy of a Coral Reef• Coral reef polyps Coral reef polyps

are are interconnected interconnected by thin layers of by thin layers of tissuetissue

• They share a They share a nervous system nervous system and digestive and digestive connection – this connection – this is due to the fact is due to the fact that the entire that the entire colony came from colony came from one original polyp one original polyp that reproduced that reproduced asexually by asexually by _____to produce _____to produce the colonythe colony

Anatomy of a Coral ReefAnatomy of a Coral Reef• Each polyp Each polyp

continues to lay continues to lay down new CaCOdown new CaCO33 beneath the polyp beneath the polyp body so the entire body so the entire coral colony coral colony continues to grow continues to grow upwardupward

• Each polyp is Each polyp is normally 1 to 3 normally 1 to 3 mm in widthmm in width

• However, the However, the collection of collection of polyps together polyps together can collectively can collectively grow to extremely grow to extremely large sizes and large sizes and weigh several tonsweigh several tons

Coral NutritionCoral Nutrition

• The zooxanthellae housed inside the The zooxanthellae housed inside the coral produce _____ through coral produce _____ through photosynthesisphotosynthesis

• Some of this _____ is passed to the Some of this _____ is passed to the coralcoral

• This mutualism aids the coral and This mutualism aids the coral and helps the entire reef grow fasterhelps the entire reef grow faster

• In fact, if zooxanthellae are supplied In fact, if zooxanthellae are supplied with enough light, they can feed the with enough light, they can feed the coral completelycoral completely

Coral NutritionCoral Nutrition

• Of course, coral also possess ____ that Of course, coral also possess ____ that contain nematocysts to sting potential contain nematocysts to sting potential prey that come within reach of the prey that come within reach of the tentaclestentacles

• These cells allow the coral to feed on These cells allow the coral to feed on small organisms in the water, mostly small organisms in the water, mostly zooplanktonzooplankton

• Some corals also produce sheets of Some corals also produce sheets of mucous to capture zooxanthellae or mucous to capture zooxanthellae or detritus detritus

Coral NutritionCoral Nutrition• Coral can also feed Coral can also feed

using extensions of using extensions of the gut wall called the gut wall called mesenterial mesenterial filamentsfilaments

• These filaments These filaments secrete digestive secrete digestive enzymesenzymes

• The coral can extend The coral can extend the filaments with its the filaments with its enzymes allowing enzymes allowing the coral to feed and the coral to feed and digest food outside digest food outside the bodythe body

Conditions Required for Reef Conditions Required for Reef GrowthGrowth

• Hard substrateHard substrate

• Optimal lightOptimal light

• Narrow temperature rangeNarrow temperature range

• Narrow _____ range = stenohalineNarrow _____ range = stenohaline

• Low sediment load in waterLow sediment load in water

• Low pollutionLow pollution

• Narrow range of pHNarrow range of pH

Reproduction and Growth of Reproduction and Growth of New CoralNew Coral

• Corals reproduce both sexually and Corals reproduce both sexually and asexuallyasexually

• Asexual reproduction allows for the growth Asexual reproduction allows for the growth of buds from a single polypof buds from a single polyp

• Sexual reproduction results in the creation Sexual reproduction results in the creation of a planula larvae that is able to ride on of a planula larvae that is able to ride on the water currents to a new environmentthe water currents to a new environment

• These larvae will settle out of the currents These larvae will settle out of the currents to begin growth on existing reefs or other to begin growth on existing reefs or other hard substratehard substrate

Light RequirementsLight Requirements

• Light must be able to penetrate the water Light must be able to penetrate the water to the depth of a coral to allow the to the depth of a coral to allow the symbiotic zooxanthellae to symbiotic zooxanthellae to photosynthesizephotosynthesize

• Due to this, corals are normally found in Due to this, corals are normally found in shallow watersshallow waters

• Some may be found to depths over 150 Some may be found to depths over 150 feet if water clarity permits light feet if water clarity permits light penetration to that depthpenetration to that depth

• Corals are mostly found on continental Corals are mostly found on continental shelves, near islands or on seamounts due shelves, near islands or on seamounts due to this light/depth dependencyto this light/depth dependency

Temperature RequirementsTemperature Requirements• Hermatypic (_____ building) corals tolerate a Hermatypic (_____ building) corals tolerate a

narrow temperature range compared with narrow temperature range compared with most marine organismsmost marine organisms

• Corals can only reproduce and grow if water Corals can only reproduce and grow if water temperatures are over 68 degrees Ftemperatures are over 68 degrees F

• Higher water temperatures are preferred for Higher water temperatures are preferred for optimal growthoptimal growth

• However, water temperatures above 86 However, water temperatures above 86 degrees (or higher in some species) causes degrees (or higher in some species) causes coral stresscoral stress

• Coral stress can lead to “bleaching,” an Coral stress can lead to “bleaching,” an expulsion of zooxanthellae from the coral polypexpulsion of zooxanthellae from the coral polyp

Coral Stress and BleachingCoral Stress and Bleaching• The expulsion of The expulsion of

zooxanthellae is zooxanthellae is termed bleaching termed bleaching because the because the normally colored normally colored coral is bleach white coral is bleach white after this expulsionafter this expulsion

• The zooxanthellae The zooxanthellae give the corals their give the corals their distinctive color distinctive color

Coral Stress and BleachingCoral Stress and Bleaching

• Other events besides high Other events besides high temperatures can cause coral bleachingtemperatures can cause coral bleaching

• Poor water quality, increased sediment Poor water quality, increased sediment in water column, wave stress or disease in water column, wave stress or disease can also cause bleachingcan also cause bleaching

• If corals remain bleached for too long, it If corals remain bleached for too long, it can result in death of the coralcan result in death of the coral

• Widespread bleaching often occurs as a Widespread bleaching often occurs as a result of extreme weather events such result of extreme weather events such as hurricanesas hurricanes

Salinity RequirementsSalinity Requirements

• Normal sea salinity is around ___ parts per Normal sea salinity is around ___ parts per thousandthousand

• Corals maintain good health in an Corals maintain good health in an environment around 35 pptenvironment around 35 ppt

• Corals normally do not exist near areas Corals normally do not exist near areas where rivers deliver large amounts of where rivers deliver large amounts of freshwater to the marine environment freshwater to the marine environment because of this salinity requirementbecause of this salinity requirement

• Low salinity can result in coral bleaching Low salinity can result in coral bleaching as wellas well

Sediments and Wave ActionSediments and Wave Action

• Corals do not normally grow in areas Corals do not normally grow in areas where wave action is highwhere wave action is high

• This is due to the fact that heavy This is due to the fact that heavy wave action results in the suspension wave action results in the suspension of sediments in the water columnof sediments in the water column

• Suspended sediments can settle on Suspended sediments can settle on corals causing damage as well as corals causing damage as well as reduce water clarityreduce water clarity

• Reduced water clarity means that Reduced water clarity means that zooxanthellae photosynthesis will be zooxanthellae photosynthesis will be reduced or stoppedreduced or stopped

Coral Sensitivity to PollutionCoral Sensitivity to Pollution

• Even low levels of pollution can kill Even low levels of pollution can kill coral polypscoral polyps

• High nutrient levels (eutrophication) High nutrient levels (eutrophication) can also allow algae levels to risecan also allow algae levels to rise

• Increased algal growth can shade Increased algal growth can shade light-sensitive corals and their light-sensitive corals and their zooxanthellaezooxanthellae

Types of Coral Growth Types of Coral Growth FormsForms

• Coral colonies can come in a variety Coral colonies can come in a variety of shapes and sizesof shapes and sizes

• Taller, more branching tend to be Taller, more branching tend to be more common in shallower areas of more common in shallower areas of the reef due to intense competition the reef due to intense competition for space and lightfor space and light

• Flatter forms tend to be more Flatter forms tend to be more common in deeper areas of the reef; common in deeper areas of the reef; this probably assists with capturing this probably assists with capturing all possible light available.all possible light available.

Types of Coral Growth Types of Coral Growth FormsForms

Branched and massive respectively. (Photographs by Dr. Donald Keith)

Foliaceous. Photograph by Dr. Donald Keith

Brain, lettuce leaf, and staghorn would be a few of the corals seen in the Flower Gardens Coral Reef off the Galveston, Texas Coast

Other Organisms that Other Organisms that Contribute to Reef Structure Contribute to Reef Structure

Besides Stony CoralsBesides Stony Corals• Coralline algae (also produce calcium Coralline algae (also produce calcium

carbonate)carbonate)• Soft corals such as sea whips and sea fans Soft corals such as sea whips and sea fans

(gorgonians) (produce a soft protein)(gorgonians) (produce a soft protein)• Other cnidarians such as hydrozoans or Other cnidarians such as hydrozoans or

anemonesanemones• SpongesSponges

Types of Coral ReefsTypes of Coral Reefs

1.1.Fringing ReefsFringing Reefs

2.2.Barrier reefsBarrier reefs

3.3.AtollsAtolls

Fringing ReefsFringing Reefs

• Simplest and most common form of reefsSimplest and most common form of reefs• These reefs develop near the shore in These reefs develop near the shore in

tropical waterstropical waters• These reefs develop as narrow strips along These reefs develop as narrow strips along

the shorethe shore• The fringing reef shown in the next The fringing reef shown in the next

photograph encircles the island it has photograph encircles the island it has formed nearformed near

Fringing ReefsFringing Reefs• Fringing reefs Fringing reefs

consist of an inner consist of an inner reef flat and an reef flat and an outer reef slope outer reef slope

• The reef flat is The reef flat is wide, gently wide, gently sloping and may sloping and may be exposed in be exposed in places at low tideplaces at low tide

• The reef slope is The reef slope is much more steep much more steep and is not exposed and is not exposed to airto air

• The reef crest is The reef crest is the shallow, upper the shallow, upper edge of the reef edge of the reef slopeslope

• Growth is normally Growth is normally high in this areahigh in this area

Barrier ReefsBarrier Reefs

• Occur along shore also Occur along shore also but separated by a but separated by a lagoon.lagoon.

• Typical construction of a Typical construction of a barrier reef: a back-reef barrier reef: a back-reef slope, a reef flat and slope, a reef flat and crest and a fore-reef crest and a fore-reef slopeslope

• The fore-reef slope is The fore-reef slope is steeper and upper parts steeper and upper parts and crest are most and crest are most diversediverse

• The depth of the water The depth of the water drops quickly in this drops quickly in this area!area!

• The slope of the back-The slope of the back-reef slope may also be reef slope may also be steep, but not to the steep, but not to the degree of the fore-reef degree of the fore-reef slopeslope

Barrier ReefsBarrier Reefs

• Waves often wash sediment onto the back Waves often wash sediment onto the back reef slope or reef flat which causes reef slope or reef flat which causes reduced coral growth in this areareduced coral growth in this area

• Enough sediment may accrue to form Enough sediment may accrue to form small islands on the reef small islands on the reef

• These islands are called keys in the US or These islands are called keys in the US or cayes elsewhere. Florida has many keys. cayes elsewhere. Florida has many keys.

A rich back-reef slope of a Pacific barrier reef not covered by sand

Great Barrier Reef, AustraliaGreat Barrier Reef, Australia

• The Great Barrier Reef is considered the The Great Barrier Reef is considered the largest reef structure in the world with largest reef structure in the world with regards to total coral arearegards to total coral area

• The length of the reef systems exceeds The length of the reef systems exceeds 1200 miles1200 miles

• The width varies from 10 – 200 milesThe width varies from 10 – 200 miles

• Many cayes exist along the reef systemMany cayes exist along the reef system

AtollsAtolls

• An atoll is a circular An atoll is a circular reef structure reef structure surrounding a surrounding a central lagooncentral lagoon

• Sand cayes may be Sand cayes may be a part of the atoll a part of the atoll structurestructure

• Width varies from Width varies from less than one mile less than one mile to over 20 milesto over 20 miles

Atoll FormationAtoll Formation• Atolls form when a Atolls form when a

volcanic island is formedvolcanic island is formed• The coral begins as a The coral begins as a

fringing reef around the fringing reef around the newly formed islandnewly formed island

• Over time, weathering Over time, weathering and geologic activity and geologic activity lowers the level of the lowers the level of the islandisland

• Eventually, the island Eventually, the island disappears below the disappears below the water’s surface, leaving water’s surface, leaving only the outer band of only the outer band of coral surrounding a coral surrounding a lagoonlagoon

AtollsAtolls

• Atolls possess a reef flat and inner and Atolls possess a reef flat and inner and outer reef slopesouter reef slopes

• The inner slope has a more gradual slope The inner slope has a more gradual slope than the outer slope and the lagoon than the outer slope and the lagoon normally has a depth of 200 feet or less normally has a depth of 200 feet or less

• The two largest atolls are in the Maldives The two largest atolls are in the Maldives (Indian Ocean) and the Marshall Islands (Indian Ocean) and the Marshall Islands (Pacific)(Pacific)

Coral Reef EcologyCoral Reef Ecology

• Like any marine environment, the coral reef Like any marine environment, the coral reef community is structured due to limiting community is structured due to limiting resources, competition, and predationresources, competition, and predation

• The water column surrounding the reef is The water column surrounding the reef is normally very low in nutrientsnormally very low in nutrients

• This is due to the position of reefs away This is due to the position of reefs away from rivers and other sources of nutrients from rivers and other sources of nutrients (remember corals form best away from (remember corals form best away from possible sedimentation sources)possible sedimentation sources)

Coral Reef EcologyCoral Reef Ecology• Due to this low nutrient Due to this low nutrient

content, the water content, the water surrounding the reef does surrounding the reef does not support the degree of not support the degree of primary production by primary production by phytoplankton seen in phytoplankton seen in other marine communitiesother marine communities

• The food chain here is The food chain here is based around the primary based around the primary production of production of zooxanthellae located in zooxanthellae located in the body of the corals and the body of the corals and other organisms other organisms

• The coral benefits the The coral benefits the zooxanthellae by zooxanthellae by providing carbon dioxide, providing carbon dioxide, nitrogen and phosphorus nitrogen and phosphorus (by-products), and shelter(by-products), and shelter

• The zooxanthellae provide The zooxanthellae provide sugars to the corals (and sugars to the corals (and any organism that feeds any organism that feeds on the corals or their on the corals or their partsparts

Coral Reef EcologyCoral Reef Ecology

• Cyanobacteria present on the reef Cyanobacteria present on the reef are able to fix nitrogen which is then are able to fix nitrogen which is then available to other reef inhabitants available to other reef inhabitants who feed on the cyanobacteriawho feed on the cyanobacteria

• Seagrasses and algae can also feed Seagrasses and algae can also feed reef inhabitantsreef inhabitants

• Water currents may bring some Water currents may bring some phyto- and zooplankton to the reef phyto- and zooplankton to the reef communitycommunity

Typical Coral Reef Food Typical Coral Reef Food WebWeb

Many Organisms Feed Directly on Many Organisms Feed Directly on Corals or their mucus, eggs, & Corals or their mucus, eggs, &

larvaelarvae

Deterring PredationDeterring Predation

• Many coral reef inhabitants can deter predation Many coral reef inhabitants can deter predation by producing toxic or foul tasting chemicals – this by producing toxic or foul tasting chemicals – this is common in soft corals and spongesis common in soft corals and sponges

• These chemicals can be released into the water These chemicals can be released into the water to kill organisms or limit growth surrounding to kill organisms or limit growth surrounding these organismsthese organisms

• Other reef dwellers may produce hard structures Other reef dwellers may produce hard structures to deter predation – sponges produce spicules of to deter predation – sponges produce spicules of calcium carbonate or silica while soft corals calcium carbonate or silica while soft corals produce calcium carbonate needles known as produce calcium carbonate needles known as scleritessclerites

• Coralline algaes as well as other organisms grow Coralline algaes as well as other organisms grow in an encrusting form that is difficult to feed upon in an encrusting form that is difficult to feed upon because of its thinness and hardnessbecause of its thinness and hardness

Competition on the ReefCompetition on the Reef

• There is high competition for There is high competition for space and light on the reefspace and light on the reef

• To limit competition, some corals To limit competition, some corals grow fast and upright to grow fast and upright to maximize their ability to maximize their ability to reproduce quicklyreproduce quickly

• Others grow slow but massive to Others grow slow but massive to out-compete other organisms in out-compete other organisms in the long runthe long run

• Corals may feed directly or sting Corals may feed directly or sting one another if they contact each one another if they contact each other other

• Shown at right is interspecific Shown at right is interspecific coral competition (and some coral competition (and some coral death in the pink area)coral death in the pink area)

Competition on the ReefCompetition on the Reef

• Other organisms Other organisms such as fish and such as fish and crustaceans must crustaceans must also compete for also compete for limited resourceslimited resources

• They normally do They normally do this by feeding in this by feeding in slightly different slightly different areas or in slightly areas or in slightly different waysdifferent ways

• Each has its own Each has its own ecological nicheecological niche