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 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

Figure 15-4 FORMATION OF A CORAL COLONY.

Figure 15-6 CORAL POLYP FEEDING.

Figure 15-7 CORAL POLYP AND SYMBIONTS.

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

Figure 15-11 (a) TYPES OF CORAL REEFS.

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

Figure 15-11 (b) TYPES OF CORAL REEFS.

Figure 15 -11(d) TYPES OF CORAL REEFS

Types of Coral Reefs Atolls, usually elliptical, arise out of deep water and have a centrally-located lagoon FIGURE 15-11 (c) TYPES OF CORAL REEFS.

Land Sea Reef Fringing reef Land Lagoon Sea Reef Patch reef Barrier reef Sea Lagoon Reef Patch 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

Figure 15-12 (1) ATOLL FORMATION.

Figure 15-12 (2) ATOLL FORMATION.

Figure 15-12 (3) ATOLL FORMATION.

Figure 15-12 (4) ATOLL FORMATION.

Lagoon Active volcano Atoll Lagoon Fringing reef Barrier reef Stepped Art Fig. 15-12, p. 423

Figure 15-13 9 (b) REEF CHARACTERISTICS.

Figure 15-14 (a) TYPES OF CORAL.

Figure 15-14 (b) TYPES OF CORAL.

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

Figure 15-15 DISTRIBUTION OF CORAL REEFS.

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 Indo-Pacific 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

Figure 15-16 (a) CORAL REEFS. Caribbean reef

Figure 15-16 (b) CORAL REEFS. 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

Table 15-1 Primary Productivity of Component Producer Communities on Coral Reefs and Their Distribution

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

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

Figure 15-20 GIANT CLAM.

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

Figure 15-25 REEF FILTER FEEDERS.

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

Figure 15-31 (a) CORAL BLEACHING AND DISEASE.

Figure 15-31 (c) CORAL BLEACHING AND DISEASE.