Introduction to Corals, Coral Reefs

Dr. Mark A. McGinleyTexas Tech University

BIOL 5311Summer 2011

Great Barrier Reef

• The largest biogenic (made by life) stuctures in the world are coral reefs

• stretches over 2,600 kilometers (1,600 mi)

• area of approximately 344,400 square kilometers (133,000 sq mi).[

Great Barrier Reef From Space

Barrier Reefs

Thickest reef almost 1 mile thick.

Coral Reefs

• Thick layer of calcium carbonate covered by thin layer of living organisms– Built up over extremely long periods of time

Cnidarians

Corals contain living animals from the Phylum Cnidaria

Some Members Of Phylum Cnidaria

Cnidarian Morphology

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Coral Polyps

A coral is a colony containing thousands of tiny polyps

Coral Polyps

Coral Feeding

The coral polyps feed on either small living organisms or on dead material (detritus) that floats in the water.

Scleractinian Corals (reef building corals)

• secrete CaCO3

• external skeletons secreted by epidermis

Coral Skeleton

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Hard Corals

Why Should You Care About Corals?

• Cool• Incredibly diverse• Very important economically

• http://www.underseaproductions.com/demo_reels/marine_life_behaviour_video_footage.html

Coral Ecology

• Corals are involved in a lot of interesting ecological interactions.

• Let me tell you about a few.

Energetics and Ecosystems

• Energy is required to do work

– Biological work• Maintaining concentration gradients across membranes

– Active transport

• Biosynthesis– Breaking down and building up bio molecules

• Movement– Cilia– Muscles

First Law of Thermodynamics

• Energy can not be created or destroyed– It can only be converted from one form to another

– Forms of energy• Electromagnetic• Kinetic• Nuclear• Potential

Photosynthesis

• The most important energetic process taking place for life on earth– Converts

electromagnetic energy from the sun (released by fusion reactions in the sun) to potential energy stored in the chemical bonds of glucose

Cellular Respiration

• Energy stored in the chemical bonds of glucose is converted into energy stored in the chemical bonds of ATP

• ATP releases that energy– Used to do biological

work

Energy Flow Through Ecosystem

• Sun• Plants

– Primary producers

• Herbivores– Primary consumers

• Carnivores– Secondary consumers

• Decomposers• Energy lost as heat to

environment

Flow of Energy From One Trophic Level to the Next is Inefficient

• Only about 10% of energy captured by plants is passed on to primary consumers

• About 10% of energy captured by primary consumers is passed on to secondary consumers

Energy Pyramid

Biomass Pyramid

Forests

In terrestrial ecosystems you see lots of plants.Forests are full of trees….

Prairies

Prairies have lots of grasses.

Deserts

You can even see lots of plants in the desert.

Coral Reef Video

• Schooling Fish• http://www.underseaproductions.com/demo_

reels/schooling_fish_video_footage.html

Coral Reef Video

• http://www.underseaproductions.com/demo_reels/seascapes_video_footage.html

• How many plants do you see on this video?

Coral Reefs

• Do not see many aquatic plants (algae) on coral reefs

• Yet coral reefs are teeming with life and are one of the most diverse communities on the planet

• How can this be?????

The Mystery of the “Inverted Energy Pyramid”

Missing Primary Producers

• Two possibilities1) Maybe plants are photosynthesizing but the

plant material is eaten by herbivores as fast as it is produced.

- Therefore we don’t see a build up of plants

Lots of Herbivores Living or Coral Reefs

Parrotfish eat the algae living in corals by scraping off the outer layer using their very sharp teeth. They grind up the rocks, digest the algae, and poop out a lot of sand

Biogenic Sand

So remember, when you are taking a romantic stroll down the beach with your sweetie much of the sand you are walking on is parrotfish poop.

Long-spined Urchin (Diadema antillarum)

Long-spined urchins are important herbivores on coral reefs.

Results of Overfishing and Diadema Die-off

About 20 years ago a disease entered the Caribbean Sea through the Panama Canal. This disease killed almost 90 percent of the urchins. In some places, fishing by humans has reduced the number of herbivorous fishes. The loss of the fishes and the urchins has allowed algae to grow. Because algae grows so much faster than corals algae is able to outcompete the corals so many corals have been covered by algae.

Missing Primary Producers

• In undisturbed reefs, primary productivity of algae is rapidly removed by the herbivores

• But calculations of rate of photosynthesis by algae was not enough to explain the energy and biomass at higher tropic levels

Missing Primary Producers

• Not looking in the right place.

• Scientists (and the video we just saw) looked all over the coral reef for primary producers

– Needed to look “inside” of corals

Symbiotic Zooxanthellae

Little photosynthetic microorganisms called Zooxanthellae are found living inside of the coral polyps.

CORAL REEFS - ZOOXANTHELLAE

--- Are group of algae called dinoflagellates (also form red tides).Symbiodinium spp.)

--- Are different colors; brown, green, yellow.

--- Dinoflagellates mutualistic with other groups; sea slugs, giant clams, tunicates.

--- Can live outside host

Mutualistic Relationship Between Corals Polyps and Zooxanthellae

• Mutualistic interaction– Interaction between two species in which both

species benefit.

Green polyp tissue, red zooxanthellae

Coral – Zooxanthellae Mutualism

• Zooxanthellae provide corals:– Energy (photosynthesis

products) and as a by-product ability to grow and reproduce fast enough to produce reefs.

– Zooxanthellae can provide up to 90% of a coral’s energy requirements

• Corals provide zooxanthellae with:– Protection from predators

via Cnidarian nematocysts.– Removal of dissolved

organic material from water column (to keep water clear)

– Waste products useful for algal photosynthesis (nitrogen and phosphorous)

• )

Coral-Zooxanthellae Mutualism

• Explains one aspect of the distribution of coral reefs

• Coral Reefs are found– Shallow water– Near continents– Tropical– Eastern sides of continents

Distribution of Corals

Coral reefs limited to the “photic zone”

• Zooxanthellae require light for photosynthesis– Corals limited to relatively shallow water

Coral Reef Zonation

• There are consistent patterns of zonation on coral reefs with increasing depth– Water absorbs light so

there is less light as depth increases• Thus, ability of

zooxanthellae to provide corals with energy decreases with depth

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Coral Species Change Growth Form at DepthPlating in Star Coral (Monastrea)

More Sponges and Fewer Corals at Greater Depths

Mutualisms

• Important for the two participant species to be able to find each other

• How do they do this?

Maternally passed from parent to offspring

-vertical transmission

Transmission of Zooxanthellae

Coral Life Cycle

• Corals can reproduce sexually or asexually– Zooxanthellae easily passed

from parent to offspring in asexual reproduction

• Corals also reproduce sexually– Egg and sperm– Mothers can place

zooxanthellae in eggs

Sexual Reproduction in Corals

• Some species of corals release both eggs and sperm in the water– Fertilization occurs in the water column

• Spawners

• Other species hold the eggs but release the sperm in the water– Fertilization occurs in the Mom, later release larvae

• brooders

– Maternal transmission of zooxanthellae occurs more often in brooders than spawners

Transmission of Zooxanthellae

- Environmental transmission each new generation- Free-living Zooxanthellae enter new corals each

generation

• This is very important for some of the issues we will talk about later

Benefits of Coral ReefsFisheries

Benefits of Coral Reefs Protect Shore

Benefits of Coral ReefsTourism

Benefits of Coral ReefsBiodiversity

Many Coral Reefs Are Threatened

Decline of Caribbean Coral Reefs

Threats to Coral ReefsStorm Damage

Threats to Coral ReefsCrown of Thorns Starfish

• Occasionally, there are large population outbreaks of this big (and ugly) starfish. The starfish feeds on coral polyps and can kill large areas of coral reefs.

What Causes Such Large Increases in Population Size of the Starfish?

• One theory is that collecting Tritons to sell their shells has reduced the population size of the most important predator of the starfish.

• Without any predators the population size of the starfish increases greatly.

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• When the land is disturbed by building or farming soil erosion can cause silt to be carried into the ocean.

• When the silt covers the coral the polyps can not feed and the Zooxanthellae can not photosynthesize so the coral dies.

Threats to Coral ReefsSiltation

Threats to Coral ReefsAlgal Blooms

• The addition of nutrients to the ocean from fertilizer run off or human sewage can fertilize faster growth of algae.

• When the algae covers the coral it blocks the light to the Zooxanthellae.

Threats to Coral ReefsBlast fishing

• In some places fishermen capture the fish by stunning them with explosions.

• Obviously, these explosions to a lot of damage to the reef.

• I heard blast fishing for the first time while diving in Malaysia last year!

Threats to Coral ReefsCoral Bleaching

Coral Bleaching

Coral Bleaching

Coral Bleaching

• Environmental stress puts a strain on the symbiotic relationship– fresh water dilution– sedimentation– subaerial exposure– solar irradiance– temperature

Coral Bleaching

• Fresh water dilution and sedimentation are local conditions so coral bleaching due to these factors is limited to certain small areas.

• Solar irradiance and especially temperature are stressors that cause coral bleaching on a global scale– Potentially a much bigger problem

Coral Bleaching

Coral bleaching is occurring all over the world!

Coral Bleaching

• Polyps can live for a while without the zooxanthellae, but growth rate is greatly reduced– If stress is eliminated the zooxanthellae may

return to the polyps and the coral recovers– If stress continue for too long, then the polyps will

die

New Guinea

Temperature and Coral Bleaching

• Coral reefs are vulnerable to increased temperature, which causes corals to lose their symbiotic algae in a process called coral bleaching.

• Small increase in water temperature is enough to trigger bleaching

• Over the last 30 years, average ocean temperatures have increased 0.3 to 0.4 degrees Celsius.– Mass coral bleaching episodes have increased

dramatically over the last 2-3 decades.

Temperature and Coral Bleaching

• El Nino events can change the pattern of ocean currents and bring warmer water to reefs– 16 % of the world’s coral reefs experienced

bleaching in 1997-1998 • mortality approaching 90% in some places

– about half of damaged reefs have not recovered.

Mechanisms of Coral Bleaching

• Not well understood• Often talk about polyps “expelling

zooxanthellae”– This may or may not be an accurate word choice

• This discussion might benefit from a better knowledge of about theories of mutualisms

Mechanisms of Coral Bleaching

• Zooxanthellae may be lost from polyps “unintentionally”

• Cell Adhesion Dysfunction– High temperature shock could result in cell

adhesion dysfunction between the cnidarian endodermal cells and the zooxanthellae cells.• Cell adhesion dysfunction would cause the detachment

and loss of zooxanthellae from the coral.

Mutualisms

• Mutualisms are interactions between two species in which both species benefit– Often think of species behaving altruistically

• Probably more complicated then that.

Mutualisms

• Species are involved in mutualistic relationships because the benefits of interacting with the other species are larger than the costs of that interaction– If something happens to alter the benefits and

costs then species might “reconsider” whether or not they want to be involved in the relationship• Whether or not they can do anything about it can vary

from system to system

Zooxanthellae may “choose” to leave the polyps

Stressed corals may give provide zooxanthellae fewer nutrients for photosynthesis

- less benefit to the mutualism If the fitness of algae living independently is

greater than the fitness of algae living in polyps then the algae may “decide” to leave the polyp and exist independently.

Polyps may “Expell” Zooxanthellae

• Coral polyps might “decide” to end the relationship with the zooxanthellae if– The costs of hosting zooxanthellae increase– The benefits received from the zooxanthellae

decrease

Polyps may “Expell” Zooxanthellae

• Stress might alter the physiology of the zooxanthellae and cause them to release compounds that are harmful to polyps (perhaps free oxygen radicals)

• Polyps will release the zooxanthellae rather than suffer the effects of the toxins.

Polyps may “Expell” Zooxanthellae

• Adaptation Mechanism– If certain strains of zooxanthellae cannot function

when stressed, the polyps expell these zooxanthellae to leave their tissues open to be recolonized by a different strain of zooxanthellae that are better adapted to the current environment

Coral Diseases

• Coral diseases are another threat to coral reefs

• Coral diseases were first identified in the 1970s and their prevalence has increased since then

Black-band Disease

• Black-band disease is characterized by a blackish concentric or crescent-shaped band, 1 to 30 mm wide and up to 2 m long, that “consumes” live coral tissue as it passes over the colony surface, leaving behind bare skeleton.

Black-band Disease

• The disease is caused primarily by a cyanobacteria – sulfide-oxidizing bacteria,

sulfur- reducing bacteria, other bacteria and nematodes, ciliate protozoans, flatworms and fungal filaments also are present.

• The photosynthetic pigments of the dominant cyanobacteria gives the band its maroon to black color

Black-band Disease

• The dead skeleton will be attacked by boring algae, boring sponges, boring clams, and parrot fish which will gnaw away the skeleton– remove about 1 cm per year.

• This means that in 100 years, a 1-meter high coral head will be completely consumed and converted to sediment.

White-band Disease• White-band disease was first

identified in 1977 on reefs surrounding St. Croix. It is now known to occur throughout the Caribbean where it is believed to only affect staghorn and elkhorn corals.

• This disease is characterized by tissue that peels or sloughs off the coral skeleton in a uniform band, generally beginning at the base of the colony and working its way up to branch tips

• The band ranges from a few millimeters up to 10 cm wide, and tissue is lost at a rate of about 5 mm per day

White-band Disease

• The cause of White-band Disease is unknown. – unusual aggregates of rod-shaped bacteria were

found in the tissue of corals affected by White-band Disease• scientists have not determined the role of this

microorganism

White-band Disease• Since the 1980s, Acropora

cervicornis has been virtually eliminated from reef environments throughout the Caribbean.

• In the U.S. Virgin Islands, populations of Acropora palmata declined from 85 percent cover to 5 percent within 10 years

• White-band disease currently is the only coral disease known to cause major changes in the composition and structure of reefs

Yellow Blotch Disease

• Affects only star corals in the genus Montastraea and the brain coral Colpophyllia natans

• First identified in 1994 in the lower Florida Keys. It is now known to occur throughout the Caribbean

Yellow Blotch Disease• Yellow blotch disease begins as

pale, circular blotches of translucent tissue or as a narrow band of pale tissue at the colony margin, with affected areas being surrounded by normal, fully pigmented tissue.

• As the disease progresses, the tissue first affected in the center of the patch dies, and exposed skeleton is colonized by algae . The area of affected tissue progressively radiates outward, slowly killing the coral.

Yellow Blotch Disease

• The rate of tissue loss by corals afflicted with YBD averages 5 t 11 cm per year, which is less than that of other coral diseases.

• However, corals can be affected for many years, and the disease can affect multiple locations on a colony.

• Though the cause of Yellow Blotch Disease remains unknown

Red-band Disease• Red-band disease consists of

a narrow band of filamentous cyanobacteria that advances slowly across the surface of a coral, killing living tissue as it progresses.

• Affects massive and plating stony corals, and also sea fans throughout the wider Caribbean. – exposed skeletal surfaces are

rapidly colonized by algae and other competing organisms.

Sea Fan Aspergillosis

• Caused by the pathenogenic fungus Aspergillus sydowii.

Why has the prevalence of coral diseases increased so much in the last 40 years?

• One theory is that anthropogenic stresses on the environment have made corals more susceptible to infection by coral diseases

Dust Hypothesis

• Changes in global climate and land use in Africa resulted in severe droughts in the Sahara and Sahel of Africa starting in the 1970s.

Dust Hypothesis• Hundreds of millions of tons of

African dust are transported annually from the Sahara and Sahel to the Caribbean and southeastern U.S.

• A similar dust system in Asia carries dust from the Gobi and Takli Makan deserts across Korea, Japan, and the northern Pacific to the Hawaiian Islands, the western U.S., and as far eastward as Europe.

I’ve cleaned this dust off of boats in the Caribbean.

Dust Hypothesis

• African and Asian dust air masses transport nutrients (iron, nitrates, other nutrients), pollutants, and viable microorganisms that may adversely affect human health and downwind ecosystems such as coral reefs.

Dust Hypothesis- Mechanisms

• interfere with a coral's immune system, making it more susceptible to disease pathogens.

• induce pathogenicity in a microorganism in the reef environment.

• trigger a rapid increase in the number of pathogenic microorganisms.

• fuel macroalgae or phytoplankton growth– has been shown for Red tides in the Gulf of Mexico

• directly deposit pathogenic microorganisms.

Lots of topics for future research about the role of microbes in coral reef ecosystems