Who, What, When, Where, Why, and How of Coral Reef Communities

• Who (organisms) can be found in a coral reef community?

• What is a coral reef community?• When can you find a coral reef community (is it

seasonal)?• Where are coral reef communities found?• Why are coral reef communities so important?• How do coral reef communities interact with other

ecosystems in the ocean?

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Chapter 15Chapter 15Coral Reef CommunitiesCoral Reef Communities

• Let’s talk about primary productivity in the oceans - Photosynthesis by phytoplankton

• Polar oceans are the most productive• Cold water holds more gases• Summer brings 24 hours of daylight for

photosynthesis• Temperate waters are somewhat productive• Tropical waters are the least productive

• That’s why there are nice clear, blue water • Coral reefs are like an oasis in the tropical water

dessert

Coral Reef Communities

• Coral reefs are highly productive, but occur in nutrient-poor waters– This is made possible by the symbiotic

relationship between coral animals and zooxanthellae

– These symbionts + algae form the basis of the community; other reef animals depend on these organisms

Organisms That Build Coral Reefs

• Stony (true) corals are the primary organisms that deposit massive amounts of CaCO3 that compose most of the structure of coral reefs

• Hermatypic: coral species that produce reefs, found in shallow, tropical waters and harbor zooxanthellae

• Ahermatypic: corals that do not build reefs, which can grow in deeper water from the tropics to polar seas– most do not harbor zooxanthellae

• Coralline algae and organisms such as fire coral, deposit lesser amounts of calcium carbonate on reefs– fire corals important in Caribbean reefs– coralline algae important in structure of Pacific

reefs

Organisms That Build Coral Reefs

Organisms That Build Coral Reefs

• Coral colonies– large colonies of small coral polyps, each of

which secretes a corallite (cup of calcium carbonate)

– the coral larva called a planula larva settles and attaches

– a polyp develops, and reproduces by budding to form a growing colony

– polyps’ gastrovascular cavities remain interconnected

– a thin, usually colorful epidermis overlies the colony surface

Organisms That Build Coral Reefs

• Coral nutrition– corals have evolved several strategies for

obtaining food– symbiotic zooxanthellae

• supply 90% of nutritional needs of stony coral• zooxanthella provide glucose, glycerol and amino

acids• coral polyp provides a suitable habitat and

nutrients, absorbed directly through the animal’s tissues

• zooxanthellae remove CO2 and produce O2

• need of zooxanthellae for sunlight limits depths to which stony corals can grow

Organisms That Build Coral Reefs

• Coral nutrition (continued)– corals as predators

• tiny zooplankton or other small animals paralyzed by the cnidocytes (stinging cells in tentacles) are passed into the digestive cavity

Organisms That Build Coral Reefs

• Coral nutrition (continued)– other sources of nutrition

• mesenteric filaments (coiled tubes attached to the gut wall) can be extruded from the mouth to digest and absorb food outside the body

• corals can feed off bacteria living in their tissues, which feed on dissolved organic matter directly from the water

Organisms That Build Coral Reefs

• Reproduction in corals– Reproduction by fragmentation

• in addition to budding, corals can also reproduce asexually by fragmentation

• some branching corals are fragile and tend to break during storms

• if they survive the storm, fragments can attach and grow into new colonies

Organisms That Build Coral Reefs

• Reproduction in corals (continued)– Sexual reproduction in coral

• Many species of coral are hermaphroditic, some have separate sexes

• mostly broadcast spawners—release both sperm and eggs into the surrounding seawater

• some are brooders—broadcast sperm, but retain eggs in the gastrovascular cavity

• spawning is usually synchronous among Pacific reef species, but nonsynchronous among Caribbean species

Reef Formation

• Involves both constructive and destructive phases

• Bioerosion: the destructive phase of reef formation– boring clams or sponges attack exposed surfaces

on the undersides of large corals– the coral stand weakens, then topples in a storm or

ocean surge– accumulating debris smothers boring organisms,

cracks are filled with CaCO2 sediments, and coralline algae cement it together

• Types of reefs:• Fringing• Barrier• Atoll• Patch

Types of Coral Reefs

• Fringing reefs – develop along shores of tropical/subtropical

islands or continental landmasses– Of all reef types, most affected by human

activities because of their proximity to land, develop right next to the land

Types of Coral Reefs

• Barrier reefs– similar to fringing reefs but separated from the

landmass and fringing reef by lagoons or deepwater channels

– Great Barrier Reef is the world’s largest barrier reef

Types of Coral Reefs• Atolls

– usually elliptical, arise out of deep water and have a centrally-located lagoon

– Often eroded dead volcano

• Darwin’s theory of atoll formation:– corals colonize shallow areas around newly-

formed volcanic islands to form a fringing reef– the island sinks and erodes, and a barrier reef

is formed about the island– the island sinks completely, leaving an atoll

Sea Sea

Lagoon

Land Lagoon Sea

Land

SeaReef

ReefPatch reef

Patch reef

ReefReef

Fringing reef

Barrier reef

Atoll Stepped Art

Fig. 15-11, p. 422

Types of Coral Reefs• In addition, patch reefs can occur within

lagoons associated with atolls and barrier reefs

Reef Structure• Different reef types share common

characteristics• Reef front or forereef: portion of the reef

that rises from the lower depths of the ocean to a level just at or just below the surface of the water, on the seaward side– drop-off: a steep reef-front that forms a vertical

wall– spur-and-groove formation or buttress zone:

finger-like projections of the reef front that protrude seaward; disperses wave energy and helps prevent damage

Reef Structure• Reef crest: the highest point on the reef and

the part that receives the full impact of wave energy– where wave impact is very strong, it may consist

of an algal ridge of encrusting coralline algae, lacking most other organisms, and penetrated by surge channels, grooves of the buttress zone

• Reef flat or back reef: portion behind the reef crest– reef flat of fringing reefs ends at the shoreline– reef flat of atolls and barrier reefs descends into

the lagoon

Reef Structure• Coral populations on reef front are massive dome-

shaped brain corals and columnar pillar corals on intermediate slopes, below this region coral species form plate-like formations

• Higher up on reef where wave energy is greatest, branching species of coral are found, e.g., elkhorn coral in Caribbean

• In protected areas behind reef front, in shallow calm waters, small species of coral occur, e.g., rose, flower and star corals

Coral Reef Distribution• Major factors influencing distribution (corals are

sensitive):• temperature – corals do best at 23-25o C• light availability – photosynthetic zooxanthellae need

light, corals not found below 60 meters• sediment accumulation – can reduce light and clog

feeding structures• salinity, corals absent from areas of massive freshwater

outflow, e.g., the mouth of the Amazon• wave action – moderate wave action is beneficial,

brings in oxygenated seawater, removes sediment that could smother coral polyps

• heavy wave action during hurricanes can damage reef structure

• duration of air exposure – can be deadly

Comparison of Atlantic and Indo-Pacific Reefs

• Pacific reefs are older and have a greater depth of reef carbonates

• Buttress zone is deeper on Atlantic reefs and coral growth may extend to 100 m down– Pacific coral growth rarely exceeds 60 m

• Proportion of reef covered by corals may approach 100% on some Pacific reefs, but usually less than 60% on Atlantic reefs

• Algal ridges more common in the Pacific because of wind and waves

Comparison of Atlantic and Indo-Pacific Reefs

• Hydrozoan Millipora complanata (fire coral) is dominant on Atlantic reefs– similar species never dominate

in the Pacific

• Atlantic corals nocturnal (night); Pacific corals diurnal (day)

Comparison of Atlantic and Indo-Pacific Reefs

• Greater sponge biomass in the Atlantic

• Pacific has giant clams and sea stars that prey on corals

Coral Reef Ecology

• Source of nutrients for coral reefs– land runoff for reefs close to land

– That’s why too much runoff can bring unwanted nutrients

– source of nutrients for atolls unclear (usually out in the middle of the ocean)

• possible explanations:– nutrients accumulated over time are efficiently recycled– reef bacteria and filter feeders capitalize on nutrients from

dissolved/particulate organic matter– nutrients are stored in the biomass of the community’s

inhabitants

Coral Reef Ecology• Photosynthesis on Reefs

– photosynthetic organisms: zooxanthellae, benthic algae, turf algae, sand algae, phytoplankton, seagrasses

– more dense than tropical ocean, with greater biomass than reef animals

– associations of producers with other organisms assist in efficient recycling, e.g., zooxanthellae with corals, cyanobacteria with sponges

Coral Reef Ecology

• Reef productivity– ratio of primary production to community

respiration = P-R ratio• P = gross photosynthesis• C = community respiration

– P-R ratio used to measure state of development of a biological community

Coral Reef Ecology

• Reef productivity (continued)– P-R ratio > 1 = primary production exceeds

respiratory needs • biomass increases, excess biomass available for

growth or harvesting

– P-R ratio = 1 = steady state (climax)• little biomass remains available for growth

– P-R ratios for coral reefs are typically close to 1• high productivity balanced by high respiration

Coral Reef Ecology

• Reef productivity – increases in productivity are often the result of

eutrophication• eutrophication: nutrient enrichment

– eutrophication typically manifested as a dramatic proliferation of algae

• if grazing doesn’t increase, algae can grow over and smother corals

• This will happen with too much sewage and fertilizer runoff

The Coral Reef Community

• Competition among corals and other reef organisms– fast-growing, branching corals grow over slower-

growing, encrusting or massive corals and deny them light

– slower-growing corals extend stinging mesenterial filaments from their digestive cavity to kill faster-growing corals

– fast-growing corals can also sting and kill using long sweeper tentacles with powerful nematocysts

The Coral Reef Community

• Competition among corals and other reef organisms (continued)– Slower growing corals are more aggressive than

fast growing corals– Massive corals are generally more shade

tolerant and are able to survive at greater depths

– as a result…• fast-growing, branching corals on many reefs

dominate upper, shallower portions• larger, slower-growing corals dominate deeper

portions

The Coral Reef Community

• Competition among reef fishes– High diversity

• coral reefs - marine habitats with greatest diversity/abundance of fishes

• seems to defy competitive exclusion principle, which suggests that no 2 species can occupy the same niche

– 60-70% of reef fishes are general carnivores– about 15% are coral algae grazers or omnivorous

The Coral Reef Community

• Competition among reef fishes (continued)– hypotheses proposed to explain this:

• competition model: factors such as time of day or night, size of prey, position in the water column, etc. provide each species with a unique niche (hence, no competition)

• predation disturbance model: assumes competition, but suggests that the effect of predation or other causes of death keep populations low enough to prevent competitive exclusion

The Coral Reef Community

• Competition among reef fishes (continued)– hypotheses proposed to explain this:

• lottery model: assumes competition occurs, but suggests that chance determines which species of larvae settling from the plankton colonize a particular area of the reef

• resource limitation model: suggest that available larvae are limited and that limitation prevents fish population from ever reaching the carrying capacity of the habitat

The Coral Reef Community

• Effect of grazing– reef is a mosaic of microhabitats with different

levels of grazing and different algal communities– grazing of larger, fleshier seaweeds permits

competitively inferior filamentous forms or coralline algae to persist

– herbivory decreases with depth– damselfish form territories where they exclude

grazers and permit abundant algal growth• provides habitat for small invertebrates• overgrows corals; fast-growing, branching corals are

most successful near damselfish

The Coral Reef Community

• Effect of predation– predation of sponges, soft corals and

gorgonians provides space for competitively inferior reef corals

– small invertebrates are almost all well hidden or camouflaged, indicating the prevalence of predation in the reef

The Coral Reef Community

• Symbiotic relationships on coral reefs– cleaning symbioses

• cleaner wrasses, gobies, etc. feed on parasites of larger fishes

• cleaning organisms set up a cleaning station

– Other symbiotic relationships• clownfishes and anemones• conchfish and the queen conch• gobies and snapping shrimp• crustaceans and anemones

Evolutionary Adaptations of Reef Dwellers

• Adaptive behaviors to avoid predation– invertebrates hide during the day and forage

at night– producing a poisonous coating of mucus– burying the body in sand to hide– inflating to appear larger– hiding at night when nocturnal predators are

active

• Structural adaptations for feeding– cnidocytes (stinging cells) of cnidarians aid in prey

capture– radioles (hair-like) appendages of Christmas tree

worms are used to capture phytoplankton– non-bivalve mollusks use radula to graze algae– mantis shrimp have extremely sharp forward

appendages– snapping shrimp use sound to defend territory and

stun prey– crinoids (feathers stars) use basket of mucus to feed

Evolutionary Adaptations of Reef Dwellers

• Protective body covering– tough, defensive exteriors help animals avoid

predation, but can limit mobility and growth

• Role of color in reef organisms– color for concealment and protection– Many invertebrates have colors and stripes

that allow them to blend in with the environment

Evolutionary Adaptations of Reef Dwellers

• Role of color in reef organisms (continued)– brilliant color of many fish actually helps them to

blend in with colorful background of the reef– other types of camouflage

• body shape

– warning coloration• e.g., lionfish

– other roles of color• defending territories• mating rituals

Evolutionary Adaptations of Reef Dwellers

Threats to Coral Reef Communities

• Effect of physical changes on the health of coral reefs– hurricanes and typhoons topple and remove

coral formations– El Niño Southern Oscillation (ENSO)

• changes winds, ocean currents, temperatures, rainfall and atmospheric pressure over large areas of tropical and subtropical areas

• can cause massive storms

Threats to Coral Reef Communities

• Why are coral reefs important?– protect coast from high surf conditions– remove large amounts of carbon dioxide from

water and air– provide habitat for a huge diversity of

invertebrates and fish– economical value, many people earn living by

collecting and processing reef products– important place of recreation– have potential for harvesting pharmaceutical

products

Threats to Coral Reef Communities

• Effects of human activities– Destructive fishing practices

• overfishing, i.e., eliminating grazers, allows algae to overgrow reefs

• poisonous chemicals used to capture fish also poison corals

• explosives used to stun and capture fish can cause massive destruction to coral

• bottom trawling for fish also destroys coral structures

Threats to Coral Reef Communities

• Effects of human activities (continued)– Coastal development

• produces runoff containing nutrients, pesticides, toxic wastes

• increases sedimentation and changes patterns of water flow

Threats to Coral Reef Communities

• Effects of human activities (continued)– Other human activities

• coral mined for use as bricks, road-fill, cement component

• removed to make jewelry• inexperienced snorkelers and boaters damage

reefs

Threats to Coral Reef Communities

• Effects of human activities (continued)– effects of human-induced climate change in

atmosphere• increased levels of carbon dioxide from burning of

fossils fuels primary cause of ocean warming• causes corals to become stressed and more

susceptible to coral bleaching and disease

Threats to Coral Reef Communities

• Effects of human activities (continued)– coral bleaching

• a phenomenon by which corals expel their symbiotic zooxanthellae

• most often associated with warming of the ocean water by ENSO or global warming

• if the stress is not too severe, corals may regain zooxanthellae and recover

• if the stress is prolonged, corals may fail to regain zooxanthellae and die

Threats to Coral Reef Communities

• Effects of human activities (continued)– coral diseases

• black band disease: a distinct dark band of bacteria migrates across the living coral tissue, leaving behind a bare white skeleton

• white pox: characterized by white lesions and caused by Serratia marcescens

– other coral diseases:• white band disease• white plague• CYBD (Caribbean yellow band disease) or yellow

blotch disease