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Coral Reef Communities

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Title: Coral Reef Communities


1
Chapter 15
  • Coral Reef Communities

2
Key Concepts
  • Coral reefs are primarily found in tropical clear
    water, usually at depths of 60 meters or less.
  • The three major types of coral reefs are fringing
    reefs, barrier reefs, and atolls.
  • Both physical and biological factors determine
    the distribution of organisms on a reef.

3
Key Concepts
  • Stony corals are responsible for the large
    colonial masses that make up the bulk of a coral
    reef.
  • Reef-forming corals rely on symbiotic
    dinoflagellates called zooxanthellae to supply
    nutrients and to produce an environment suitable
    for formation of the coral skeleton.
  • Coral reefs are constantly forming and breaking
    down.

4
Key Concepts
  • The most important primary producers on coral
    reefs are symbiotic zooxanthellae and turf algae.
  • Coral reefs are oases of high productivity in
    nutrient-poor tropical seas. Nutrients are stored
    in reef biomass and efficiently recycled.

5
Key Concepts
  • Inhabitants of coral reefs display many
    adaptations that help them to avoid predation or
    to be more efficient predators.
  • Coral reefs are huge, interactive complexes full
    of intricate interdependencies.

6
World of Coral Reefs
  • 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

7
Coral Animals
  • Stony (true) corals deposit massive amounts of
    CaCO3 that compose most of the structure of coral
    reefs
  • Hermatypiccoral species that produce reefs,
    found in shallow, tropical waters

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Coral Animals
  • Ahermatypiccorals that do not build reefs, which
    can grow in deeper water from the tropics to
    polar seas
  • most do not harbor zooxanthellae

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Coral Animals
  • Coral colonies
  • large colonies of small coral polyps, each of
    which secretes a corallite
  • 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

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Coral Animals
  • Sexual reproduction in coral
  • mostly broadcast spawnersrelease both sperm and
    eggs into the surrounding seawater
  • some are broodersbroadcast sperm, but retain
    eggs in the gastrovascular cavity
  • spawning is usually synchronous among Pacific
    reef species, but nonsynchronous among Caribbean
    species

16
Coral Animals
  • Reproduction 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
  • fragmentation is a common form of asexual
    reproduction for branching corals

17
Coral Animals
  • Coral nutrition
  • 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 animals
    tissues
  • zooxanthellae remove CO2 and produce O2
  • need of zooxanthellae for sunlight limits depths
    to which stony corals can grow

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Coral Animals
  • Coral nutrition (continued)
  • corals as predators
  • small animals paralyzed by the nematocysts are
    passed into the digestive cavity

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Coral Animals
  • Coral nutrition (continued)
  • other sources of nutrition
  • corals can feed off bacteria living in their
    tissues, which feed on dissolved organic matter
    directly from the water
  • mesenteric filaments (coiled tubes attached to
    the gut wall) can be extruded from the mouth to
    digest and absorb food outside the body

22
Reef Formation
  • Involves both constructive and destructive phases
  • Bioerosionthe 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
  • debris smothers boring organisms, cracks are
    filled with CaCO2 sediments, and coralline algae
    cement it together

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26
Coral Reef Types
  • Fringing reefs border islands or continental
    landmasses

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Coral Reef Types
  • Barrier reefs are similar to fringing reefs but
    separated from the landmass and fringing reef by
    lagoons or deepwater channels

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Coral Reef Types
  • Atolls, usually elliptical, arise out of deep
    water and have a centrally-located lagoon

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Coral Reef Types
  • In addition, patch reefs can occur within lagoons
    associated with atolls and barrier reefs
  • Darwins 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

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Reef Structure
  • Reef front or forereefportion 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-offa steep reef-front that forms a vertical
    wall
  • spur-and-groove formation or buttress
    zonefinger-like projections of the reef front
    that protrude seaward disperses wave energy and
    helps prevent damage

39
Reef Structure
  • Reef crestthe 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 other organisms, and penetrated by surge
    channelsgrooves of the buttress zone
  • Reef flat or back reefportion behind the reef
    crest

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Coral Reef Distribution
  • Major factors influencing distribution
  • temperature corals do best at 23-25o C
  • light availability photosynthetic zooxanthellae
    need light
  • sediment accumulation can reduce light and clog
    feeding structures
  • salinity
  • wave action moderate wave action brings in
    oxygenated seawater, removes sediment that could
    smother coral polyps
  • duration of air exposure can be deadly

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

47
Comparison of Atlantic and Indo-Pacific Reefs
  • Hydrozoan Millipora complanata (fire coral) is
    dominant on Atlantic reefs
  • Similar species never dominate in the Pacific
  • Gorgonians more abundant in the Atlantic
  • Soft corals (subclass Alcyonaria) more abundant
    in the Pacific
  • Atlantic corals nocturnal Pacific corals diurnal

48
Comparison of Atlantic and Indo-Pacific Reefs
  • Atlantic corals often reproduce by fragmentation
    Pacific corals by sexual reproduction
  • Coral diversity is far greater in the
    Indo-Pacific than the Atlantic
  • Greater sponge biomass in the Atlantic
  • Pacific has giant clams and sea stars that prey
    on corals

49
Caribbean reef
50
Pacific reef
51
Reef Productivity
  • Source of nutrients
  • land runoff for reefs close to land
  • source for atolls unclear
  • possible explanations
  • nutrients accumulated over time are efficiently
    recycled
  • reef bacteria and filter feeders capitalize on
    nutrients from dissolved/particulate organic
    matter
  • nutrients brought from other communities

52
Reef Productivity
  • Reef photosynthesis
  • 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
  • turf algae process the most organic carbon

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Reef Productivity
  • Reef succession
  • 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

55
Reef Productivity
  • Reef succession (continued)
  • P-R ratio gt 1 primary production exceeds
    respiratory needs
  • biomass increases, excess biomass available for
    growth or harvesting
  • P-R ratio 1 steady state
  • little biomass remains available for growth
  • P-R ratios for coral reefs are typically close to
    1
  • high productivity balanced by high respiration

56
Reef Productivity
  • Reef succession (continued)
  • increases in productivity are often the result of
    eutrophication
  • eutrophicationnutrient enrichment
  • eutrophication typically manifested as a dramatic
    proliferation of algae
  • if grazing doesnt increase, algae can grow over
    and smother corals

57
Coral Reef Ecology
  • Coral provides
  • foundation for reef food webs
  • shelter for resident organisms
  • Reefs form a complex 3-dimensional habitat for
    many beautiful and strange creatures

58
Coral Reef Community
  • Sponges and cnidarians
  • sessile organisms, though anemones can move if
    necessary
  • filter feed anemones also paralyze and consume
    small fishes and crustaceans
  • Annelids
  • sessile filter feeders include featherduster and
    Christmas tree worms
  • fireworms are mobile predators
  • palolo worms burrow through and weaken coral and
    usually deposit feed

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Coral Reef Community
  • Crustaceans
  • shrimps, crabs and lobsters
  • vary from parasites to active hunters
  • Molluscs
  • gastropods eat algae from coral surfaces
  • giant clams are filter feeders, but also host
    symbiotic zooxanthellae
  • octopus and squid are active predators

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Coral Reef Community
  • Echinoderms
  • feather stars, sea urchins, brittle stars, sea
    stars, and sea cucumbers
  • filter feed, scavenge, or eat sediment
  • Reef fishes
  • most prominent and diverse inhabitant
  • diverse food sources, including detritus, algae,
    sponges, coral, invertebrates, other fish

63
Species Interactions on Coral Reefs
  • Competition among corals
  • fast-growing, branching corals grow over
    slower-growing, encrusting or massive corals and
    deny them light
  • slower-growing corals extend stinging 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

64
Species Interactions on Coral Reefs
  • Competition among corals (continued)
  • slower-growing corals are more tolerant of shade,
    and can grow at greater depths
  • as a result
  • fast-growing, branching corals on many reefs
    dominate upper, shallower portions
  • larger, slower-growing corals dominate deeper
    portions

65
Species Interactions on Coral Reefs
  • Competition between corals and other reef
    organisms
  • sponges, soft corals and algae can overgrow stony
    corals and smother them
  • algae outcompete corals at shallow depths unless
    grazers control the algae growth
  • Other competitive interactions
  • hydrozoans overgrow gorgonians
  • fast-growing colonial invertebrates on coral
    surfaces overgrow many species

66
Species Interactions on Coral Reefs
  • Effect of grazing
  • 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

67
Species Interactions on Coral Reefs
  • Effect of predation
  • predation of sponges, soft corals and gorgonians
    provides space for competitively inferior reef
    corals
  • species that feed on fast-growing coral assist
    slower-growing species to remain
  • corallivores seldom destroy reefs
  • small invertebrates are almost all well hidden or
    camouflaged, indicating the prevalence of
    predation in the reef

68
Coral Reef Ecology
  • 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

69
Coral Reef Ecology
  • Hypotheses proposed to explain this
  • competition modelfactors 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 modelassumes competition,
    but suggests that the effect of predation or
    other causes of death keep populations low enough
    to prevent competitive exclusion

70
Coral Reef Ecology
  • Hypotheses proposed to explain this
  • lottery modelassumes competition occurs, but
    suggests that chance determines which species of
    larvae setting from the plankton colonize a
    particular area of the reef
  • resource limitation modelsuggest that available
    larvae are limited and that limitation prevents
    fish population from ever reaching the carrying
    capacity of the habitat

71
Threats to Coral Reefs
  • 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

72
Threats to Coral Reefs
  • 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

73
Threats to Coral Reefs
  • Coral diseases
  • black band diseasea distinct dark band of
    bacteria migrates across the living coral tissue,
    leaving behind a bare white skeleton

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Threats to Coral Reefs
  • Coral diseases
  • white poxcharacterized by white lesions and
    caused by Serratia marcescens
  • other coral diseases
  • white band disease
  • white plague
  • yellow blotch disease

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Threats to Coral Reefs
  • Human impact on coral reefs
  • overfishing may occur
  • human-sewage bacteria cause white pox
  • nutrient-rich runoff (eutrophication) increases
    algal growth, which covers and smothers corals
  • e.g. Kaneohe Bay in Hawaii

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Evolutionary Adaptations of Reef Dwellers
  • Protective body covering
  • tough, defensive exteriors help animals avoid
    predation, but can limit mobility and growth
  • Protective behaviors
  • 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

82
Evolutionary Adaptations of Reef Dwellers
  • Role of color in reef organisms
  • color for concealment and protection
  • countershading
  • disruptive coloration
  • camouflage (bright colors in reef environment)

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Evolutionary Adaptations of Reef Dwellers
  • Role of color in reef organisms
  • other types of camouflage
  • body shape
  • warning coloration
  • other roles of color
  • defending territories
  • mating rituals

85
Evolutionary Adaptations of Reef Dwellers
  • 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
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