Transcript coral reef - honorsmarinebiology

Chapter 15
Coral Reef Communities
© 2006 Thomson-Brooks Cole
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.
© 2006 Thomson-Brooks Cole
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.
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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.
© 2006 Thomson-Brooks Cole
Coral Animals
• Stony (true) corals deposit massive
amounts of CaCO3 that compose most
of the structure of coral reefs
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Coral Reef Types
• Fringing reefs border islands or
continental landmasses
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Coral Reef Types
• Barrier reefs are similar to fringing
reefs but separated from the landmass
and fringing reef by lagoons or
deepwater channels
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Coral Reef Types
• Atolls, usually elliptical, arise out of
deep water and have a centrallylocated lagoon
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
Caribbean
reef
Pacific reef
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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
© 2006 Thomson-Brooks Cole
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 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
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
© 2006 Thomson-Brooks Cole
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. Kane’ohe Bay in Hawaii
© 2006 Thomson-Brooks Cole
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