Karleskint Small Turner Chapter 15 Coral Reef Communities.

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Chapter 15Chapter 15Coral Reef CommunitiesCoral Reef Communities

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


• 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


• 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


• 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


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

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

• 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

Lagoon

Fringing reef

Lagoon

Barrier reef

AtollActive volcano

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


• 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


• Atlantic corals often reproduce by fragmentation; Pacific corals by sexual reproduction

• Coral diversity is far greater in the Indo-Pacific than the Atlantic– Indo-Pacific has 500 species of stony corals,

only about 62 species occur in Atlantic


• 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 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• 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


• Effects of human activities (continued)– Coastal development

• produces runoff containing nutrients, pesticides, toxic wastes

• increases sedimentation and changes patterns of water flow


• 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


• 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


• 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


• 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

Karleskint Small Turner Chapter 15 Coral Reef Communities.

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