© 2006 Thomson-Brooks Cole Chapter 15 Coral Reef Communities

Chapter 15

Coral Reef Communities

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.

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.

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.

Coral Animals

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

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

Coral Animals

• Sexual reproduction in coral– mostly broadcast spawners—release both

sperm and eggs into the surrounding seawater

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

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

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 animal’s tissues

• zooxanthellae remove CO2 and produce O2

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

Coral Reef Types

• Fringing reefs border islands or continental landmasses

Coral Reef Types

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

Coral Reef Types

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

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

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

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

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

• 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

• 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

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

• 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

• 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

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

• Coral bleaching– a phenomenon by which corals expel their

symbiotic zooxanthellae– most often associated with warming of the

ocean water by 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

• 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. Kane’ohe Bay in Hawaii

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

© 2006 Thomson-Brooks Cole Chapter 15 Coral Reef Communities

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