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