Transcript Coral Reef Communities

Chapter 15
Coral Reef Communities
Karleskint
Turner
Small
Key Concepts
• Coral reefs are primarily found in tropical
clear water, usually at depths of 60 meters
or less.
• The 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
• Scleractinian (stony) corals are responsible
for the large colonial masses that make up
the bulk of a coral reef.
• Corals obtain up to 90% of their energy
from zooxanthellae, symbiotic
dinoflagellates that use coral wastes,
produce carbohydrates and aid in calcium
carbonate deposition.
• 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. Nutrients are
stored in reef biomass and efficiently
recycled.
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.
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
• Turf algae is the other main primary
producer
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
– 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
• Reproduction in corals
– Reproduction by fragmentation
– 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
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
Types of Coral Reefs
• Barrier reefs are 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
Land
Sea
Reef
Fringing reef
Patch Lagoon
reef
Land
Reef
Sea
Barrier reef
Lagoon
Sea
Sea
Reef
Atoll
Patch
reef
Reef
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
• Darwin’s theory of atoll formation:
– corals colonize shallow areas around newlyformed 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
Lagoon
Active volcano
Atoll
Lagoon
Fringing reef
Barrier reef
Stepped Art
Fig. 15-12, p. 423
Coral Reef Distribution
• Major factors influencing distribution:
– 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
• Gorgonians more abundant in the Atlantic
• Soft corals (subclass Alcyonaria) more
abundant in the Pacific
• Atlantic corals nocturnal; Pacific corals
diurnal
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 IndoPacific than the Atlantic
– Indo-Pacific has 500 species of stony corals,
only about 62 species occur in Atlantic
Comparison of Atlantic
and Indo-Pacific Reefs
• Coral reef communities also differ:
– Indo-Pacific reefs have > 5,000 species of
molluscs and ~ 2,200 species of ray finned
fish
– Atlantic reefs have ~ 1,200 species of
molluscs, and 550 species of fish
• Greater sponge biomass in the Atlantic
• Pacific has giant clams and sea stars that
prey on corals
Caribbean reef
Pacific reef
Coral Reef Ecology
• Source of nutrients
– land runoff for reefs close to land
– source of nutrients 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 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
– turf algae most abundant, process the most organic
carbon
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 (continued)
– 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
The Coral Reef Community
• Competition among corals and other reef
organisms
– fast-growing, branching corals grow over slowergrowing, 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 between corals and other reef
organisms (continued)
– sponges, soft corals and algae can overgrow
stony corals and smother them
– algae outcompete corals at shallow depths
unless grazers control the algae growth
– massive die-off of Diadema (grazing sea urchin)
in Jamaica in 1982 resulted in dramatic decline
of coral cover and substantial increase in algal
cover
The Coral Reef Community
• Competition among reef fishes
– 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
– 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
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
Evolutionary Adaptations of Reef Dwellers
• 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
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