Reef-building or hermatypic corals live only in the photic zone (above 50 m), the depth to which sufficient sunlight penetrates the water, allowing photosynthesis to occur. Coral polyps do not photosynthesize, but have a symbiotic relationship with zooxanthellae; these organisms live within the tissues of polyps and provide organic nutrients that nourish the polyp. Because of this relationship, coral reefs grow much faster in clear water, which admits more sunlight. Without their symbionts, coral growth would be too slow for the corals to form significant reef structures. Corals get up to 90% of their nutrients from their symbionts.much of the reef interior consists of toppled and degraded corals, sediment, and open cavities. Most of the sediment is produced by (1) fish and urchins grazing on algae that cover dead coral surfaces and (2) sponges, molluscs and worms that excavate into the carbonate substrate seeking shelter (a process known as bioerosion). This nearly equal mix of intact and broken corals plus loose sediment is subsequently bound together by encrusting organisms (for example, coralline algae) and carbonate cement. Thus, the edifice that is the coral reef owes its final structure as much to bioerosion, cementation, and encrustation as it does to skeletal production by corals.
As a general rule, as the water depth increases then:Light intensity drops. Wave surges become less intense, but currents can still remain strong.Water temperature falls and becomes more constant.In the shallow, high light intensity and extreme water motion zones they will form fingered or massive (domed) structures. As the depth increases, light intensity rapidly drops off and wave surges are reduced, but the currents can still remain strong. To adapt to this lower availability of light, stony corals then take on thin, flattened plates therefore increasing the surface area that is exposed to the light.
Coralline algae are made up of masses of very fine thread-like filaments, that spread out in thin layers over the reef rock surface. These filaments produce calcium carbonate thus giving the algae anappearance more like a rock than a plant. The encrusting filaments trap sediments of sand, as well ascement the particles of sand together. Thus coralline algae help to stabilize the coral reef structure.Calcareous algae do not encrust like coralline algae, but grow erect. They too produce calciumcarbonate (limestone). When these algae die, the limestone remains produce sand. One type ofcalcareous algae known as Halimedaproduces about fifty percent of the sand found on some of ourbeaches in the Caribbean.Seagrassesserve as a habitat and shelter for reef animals such as the young or juveniles of conch andlobsters. These plants also provide food for many herbivorous reef fish. The leaves of seagrass arealso a habitat for very tiny organisms.Seagrasses have extensive interwoven underground creeping stems with roots attached. These stemsare called rhizomes. They anchor the plant in the sandy sea bed and help to keep the sea water cleanby filtering and trapping large amounts of fine sediments. These rhizomes also bind the sand on thesea bed and prevent the sand from being carried away by water currents. They are important inpreventing beach erosion.
Sea Snake: Adaptations for aquatic life include paddle-like tails and laterally compressed bodies, both of which enhance swimming ability. Also highly venomous.
Symbiotic Adaptationsor - example, the coral gives off ammonia as waste, while the zooxanthellae eat the ammonia for nourishment. This relationship helps to clean the coral. If the zooxanthellae die, the coral turns white (or bleaches). In many cases, if a coral bleaches, it will die. However, if bleached corals are given optimal conditions, the zooxanthellae will return and the coral will survive.Gobies---small fish that live in burrows in the reef sand---and pistol shrimp have an adaptation in which they partner to live together. The shrimp digs a burrow big enough for itself and the goby. The shrimp's digging stirs up food for the goby, and the goby protects the shrimp, since the shrimp has poor vision and cannot easily see approaching danger.Feeding: Butterfly fishes have small, pointed mouths adapted to eat the tiny polyps that make up corals. The coral-eating fish are constantly searching for and nipping at polyp-shaped food. But while their small, pointed mouths and nipping behavior give them an advantage over coral-eating competitors, the Butterflyfish varieties with the smallest mouths do not easily adapt to eating different foods.One of the most abundant reef food sources is plankton (microscopic plant, animal and bacterial organisms that circulate throughout the ocean's currents). Numerous reef species are very efficient plankton harvesters, using long tentacles designed to adhere to plankton cells. The tentacles are filled with toxins and stretch for great lengths to maximize their catch.Body shape:The latter are generally built primarily for sheer speed, and have evolved appropriate torpedo-like shapes that offer low frictional resistance (drag) to movement through water.In the complex coral reef environment however, a premium is placed upon maneuverability rather than sheer speed. Thus, many reef dwelling fishes have evolved a body plan that maximizes their ability to make rapid turns and stop quickly, highly useful traits for an animal attempting to avoid predators in physically complex habitats.By quickly dodging into fissures in the reef, swiftly circling around coral heads, or coming to a sudden halt next to a solid object (like a hard coral colony), prey can more readily avoid predators that lack such abilities.
CORAL REEFS Often called “rainforests of the sea”, they are one of the most diverse ecosystems on Earth. They occupy less than 0.1% (around 284,300 km2) of the worlds ocean surface, yet provide a home for 25% of all marine species, includingfish, mollusks, worms, crustaceans, echinoderms, sponges , and other cnidarians. They are most commonly found at shallow depths in tropical waters. Coral colonies thrive at 21-29OC, saline environments, and clear waters.
LOCATIONIndo-Pacific region (including the Red Sea, Indian Ocean, Southeast Asia and the Pacific) account for 91.9%.
A fringing reef can taketen thousand years toform, and an atoll can take up to 30 million years.
STRUCTURE Corals are modular animals, anemone-like cylindrical polyps with prey-capturing tentacles surrounding the opening or mouth. Most form sessilecolonies supported on the tops of dead colonies and cease growth when they reach the surface of the water Coralline algae are important contributors to reef structure in those parts of the reef subjected to the greatest forces by waves. These algae strengthen the reef structure by depositing limestone in sheets over the reef surface.
General abiotic conditions: Shallow waters, Temperature 21-29 C, Clear low-nutrient waters, salinity 30-40 ppt. However, the coral reef can be divided into different zones with varyingabiotic characteristics and consequently different biological communities. The environmental factors responsible for these differences are wave action, current direction and intensity, light intensity, etc. Light sets the depth at which zooxanthellae can survive. Diversity is lowest at the crest near the surface, where only species such as massive pillar- shaped corals tolerant of intense or frequent disturbance of waves cansurvive. Diversity increases with depth to a maximum of about 20 m. Then it decreases as light attenuates, eliminating shade-intolerant species.
Branching corals occur along the reef crest where abundant light drives photosynthesis needed to support rapid growth. Vigorouswave energy clears sediment fromthe branching species that have no biological means of sediment removal. Further down the reef front, slower-growing hemispherical corals (Mixed/Massive) are better suited for removing sediment coming down the slope. In even deeper water, platelike colonies respond to continually decreasing light levels. This shape places all the polyps on upward- facing surfaces, optimizing the colonys ability to gather light— much like solar panels.
Adding to the productivity of the coral reefs are crustose, coralline algae, turf algae, macroalgae, sea grass, sponges, phytoplankton, and a large bacterial population. Coral reefs are among the most highly productive ecosystems on earth.This productivity and the varied habitats within the reef support a high diversity of life –thousands of kinds of invertebrates (some of which, such as sea urchins, feed on coral animals and algae), many kinds of herbivorous fish that graze on algae, and hundreds of predatory species. Some of these predators such as the puffers and filefish, are corallivores, feeding on coral polyps. Others lie in ambush for prey in coralline caverns. In addition, there is a wide array ofsymbionts such as cleaning fish and crustaceans that pick parasites and detritus from larger fish and invertebrates.
• Loss of zooxanthellae• causes – higher than usual ocean temperature – sharp changes in salinity – heavy UV light exposure
Pathogens• Diseases on the rise – new pathogens (8+) – land pathogens (Aspergillus) – occurring at all depths – More susceptible when stressed
Threats from Nature• Unusually strong waves such as those from a hurricane• Water temperature changes• Dramatic changes in saltiness of water• Predators, such as snails and crown of thorns starfish• Overgrowth of algae
Threats to coral and coral reefs: • 1) Natural • 2) Anthropogenic –of human origin
Anthropogenic threats to coral reefs:• Overfishing• Development• Mining and dredging• Recreation
Threats from Humans• Pollution – Sediments block light from zooxanthellae – Chemicals either poison corals or allow too much algae to grow• Power plants – Filter water and kill fish and plankton – Releasing hot water kills organisms
More Threats from Humans• Deforestation – Causes erosion which clouds the water – Burning of trees could be a factor in climate change• Destructive fishing – Blasting with dynamite – Cyanide poison – Boats running aground, anchors – Overfishing
Synergistic effects• interaction of natural changes & human activity• multiple insults
Global climate change• Reduced reef building• Increase in frequency and intensity of hurricanes• Increases in bleaching• Increased in disease
A New Home• Corals now have a choice of where to live• Corals are very sensitive and are rapidly dying• People are trying to help by creating artificial reefs
Arguments for Artificial Reefs• It can help build or rebuild a reef• It will increase fish populations by making new habitats
Arguments Against Artificial Reefs• Moveable• Chemical leaking and leaching into the ocean• It does not increase fish populations but rather moves them to one location away from their natural habitat which makes them easier to catch