Chapter 14 Coral Reefs
What is a coral reef? - Coral reefs are massive complex structures made of limestone (CaCO3) deposited by living organisms - Most established coral reefs are between 5,000 and 10,000 years old - Coral reefs support over twenty-five percent of all known marine species - As one of the most complex ecosystems on the planet, coral reefs are home to over 4,000 different species of fish, 700 species of corals and thousands of other plants and animals
-Many types of algae, seaweed, sponge, sediment and even mollusks (giant clams and oysters) contribute to the structure of the coral reef. - Not all coral species build reefs - The actual architects of coral reefs are hard or stony corals, which are referred to as hermatypic or reef-building corals - As stony corals grow, they produce limestone (CaCO3) skeletons
-When the reef-building corals and other organisms die, their skeletons are left behind and are used as foundations for new polyps to attach and grow to - Built up layer by layer by living corals growing on top of the skeletons of past generations, -An actual living coral reef is composed of a thin layer of living coral animals called polyps - Corals grow at different rates, depending on water temperature, salinity, turbulence, and the availability of food
-coral reefs grow upward at rates of 1 to 20 cm per year massive corals grow the slowest while branching and staghorn corals can grow much faster - Coral reefs are tropical, extending to about 30 north and south of the equator and forming only where surface waters are never cooler than 20 C (68 F).
What do corals need to survive? 1. Sunlight: Corals need to grow in shallow water where sunlight can reach them. Since corals depend on the zooxanthellae (algae) that grow inside of them and this algae needs sunlight to survive. Therefore, corals rarely develop deeper than 50 meters (165 feet). 2. Clear Water: Corals need clear water to survive and don't thrive well when the water is opaque. Sediment and plankton can cloud the water which decreases the amount of sunlight that reaches the zooxanthellae.
3. Temperature: Reef building corals require warm water conditions to survive. Different corals living in different regions can withstand different temperature fluctuations. However, corals generally live in water temperatures ranging from 20 to 32 degrees Celsius (68 to 90 degrees Fahrenheit). 4. Clean Water: Corals are sensitive to pollution and sediments. Sediments can settle on coral, blocking out sunlight and smothering coral polyps. Pollution from sewage and fertilizers increases nutrient levels in the water, harming corals. When there are too many nutrients in the water, the ecological balance of the coral community is altered.
5. Saltwater: Corals need saltwater to survive and require a certain balance in the ratio of salt to water. This is why corals don't live in areas where rivers drain fresh water into the ocean.
What are the different types of reefs? Scientists generally divide coral reefs into four classes: fringing reefs, barrier reefs, atolls and patch reefs. 1. Fringing reefs grow near the coastline around islands and continents. They are separated from the shore by narrow, shallow lagoons. Fringing reefs are the most common type of reef that we see. 2. Barrier reefs also parallel the coastline but are separated by deeper, wider lagoons. At their shallowest point they can reach the water's surface forming a "barrier" to navigation. The Great Barrier Reef in Australia is the most famous example and is the largest barrier reef in the world.
3. Atolls are rings of coral that create protected lagoons and are usually located in the middle of the sea. Atolls usually form when islands surrounded by fringing reefs sink into the sea or the sea level rises around them (these islands are often the tops of underwater volcanoes). The fringing reefs continue to grow and eventually form circles with lagoons inside. 4. Patch reefs are small, isolated reefs that grow up from the open bottom of the island platform or continental shelf. They usually occur between fringing reefs and barrier reefs. They vary greatly in size, and they rarely reach the surface of the water.
The Coral Polyp "coral" refers to a small simple animal and the skeleton it produces The polyp is the actual animal Each coral is a colony of coral polyps and the skeleton it produces This skeleton protects the soft, delicate body of the polyp
When these polyps are grouped together, they are referred to as coral colonies Coral polyps are usually nocturnal, meaning that they stay inside their skeletons during the day. At night, polyps extend their tentacles out to feed. A polyp has a sac-like body and an opening or mouth encircled by stinging tentacles The polyp uses calcium carbonate from seawater to build itself a hard, cup-shaped skeleton.. A ring of tentacles surrounds a mouth that leads to a single body cavity for digestion
The tentacles have special cells called nematocyst, which have little harpoons that can deliver a small dose of toxin for capturing prey The tentacles extend from the skeletal cup called a corallite when the polyp is feeding and retract when danger is present
The polyp has three body layers 1. an outer layer of epidermis 2. an inner layer of gastrodermis lining the body cavity 3. a layer of jelly-like material called mesoglea between the epidermis and gastrodermis
Living in the gastrodermis of reef building corals are minute algae called zooxanthellae The zooxanthellae help the polyps in the production of their skeleton and also provide some nutrients for the polyps Without the zooxanthellae the corals would not be able to produce the massive quantities of calcium carbonate that form the reef Some coral polyps live solitary lives but most live in colonies
The variety of growth forms in reef building corals leads to many different shapes of the colonies. Some coral colonies can grow to be enormous as a result of asexual budding Polyps can also reproduce sexually, producing a free swimming planula larva that will settle and form a new coral colony
Methods Of Coral Reproduction Corals exhibit both sexual and asexual reproduction and various methods of fertilization. Asexual Reproduction (Budding) a. Method which allows for colony growth by the continuous division of pre-existing polyps into BUDDS. b. Two Types: 1. Intratentacular (referred to as fission). The oral disk invaginates to produce a new mouth within the parental ring of tentacles. 2. Extratentacular (referred to as BUDDING). The new mouth is produced outside the parental ring of tentacles.
c. New budding polyps form on the horizontal axis. d. New polyps may appear anywhere in the tissue between the polyps. Mouth forms first followed by a ring of tentacles. e. Some species exhibit both traits.
Sexual Reproduction a. Method which allows for the formation of new colonies Hermaphroditic colony Polyps have both male and female gonads, producing ripe eggs and sperm in the same polyp. However, many times the eggs and sperm mature at different times, thus inhibiting self-fertilization. Fertilization may be internal; (brooders), or external (broadcasters).
b. What triggers spawning? In Australia the major trigger is the moon phase (New Moon). Spawning seems to begin one or two nights after the full moon in late spring or early summer.
c. Egg fertilization is internal (brooders) for most species. The sperm swims into the mouth of the female polyp, eggs are fertilized and become a planulae - ciliated and ovalshaped larvae. Upon reaching maturity it swims clear of the female polyp. It may swim for 2 to 3 weeks before settling on the bottom (which must be hard, clean, silt- and algae-free), becoming polyps and starting a new colony. Note: Under the low nutrient conditions of tropical waters, corals will naturally out-compete algae. However, when nitrogen and phosphorous, the main limiting nutrients for algae growth, are artificially introduced into the ecosystem, the delicate ecological balance is altered. The algae will rapidly reproduce and take over substrates that otherwise would be colonized by new coral recruitment.
e. Some species exhibit external fertilization (broadcaster): eggs and sperm shed into the water column.
Reef Zones The side the reef facing the sea is the reef front or fore reef slope reef front is more or less parallel to the shore and perpendicular to the dominant direction of wave travel It slopes downward into deeper water sometimes gently at first, then at a greater angle at the top of the reef front is the reef crest. The upper front and the crest bear the greatest force of the waves and must absorb great energy during storms.
Pieces of coral and other organisms are broken off at such times and thrown shoreward The reef flat thus receives a supply of calcareous material that is eventually broken down into coral sand. The sand is stabilized by the growth of plants such as turtle grass and coralline algae and is ultimately cemented into the mass of the reef by precipitation of carbonates.
A reef is not an unbroken wall facing the sea but is highly irregular, with grooves, caves, crevices, channels through from the flat to the front, and deep, cup-shaped holes ("blue holes"). Hard corals tend to grow in these areas that are more protected from the full force of the waves, as well as on the flat and the deeper areas of the fore reef slope. Enormous numbers of species and individuals of invertebrate groups and fishes populate the reef ecosystem.
Coral Growth Forms The basic growth forms of corals can be categorized as: 1. encrusting 2. massive 3. branching 4. foliaceous 5. columnar 6. platelike
Coral Predators Predation of eggs and larvae by the reef's numerous plankton feeders is extremely heavy Only a tiny percentage of larvae are able to find a suitable bare rock surface to settle on fully mature coral colonies have a variety of predators a. Crown-of-thorns starfish b. parrotfish grind coral skeletons of living polyps into fine sediment with their pharyngeal teeth. c. Many species of butterfly fish feed directly on the fleshy polyps.
Coral Diseases 1. white band disease (WBD) 2. black band disease (BBD) 3. Coral Bleaching 4. others
WBD, which is believed to be caused by a bacteria pathogen yet unknown WBD actually kill coral tissue while advancing in a band around the coral and leaving the white coral skeleton behind WBD destroys the reef structure because the dead coral is colonized by algae, invertebrates, gastropods, and boring sponges that work to weaken the coral skeletons making them more susceptible to breakage during storms.
BBD is characterized by a blackish concentric or crescentshaped band The disease is caused primarily by a cyanobacteria in combination with sulfide-oxidizing and sulfur- reducing bacteria, also, other bacteria and opportunistic organisms such as nematodes, ciliate protozoans, flatworms and fungal filaments also are present in the mix BBD also kill coral tissue while advancing in a band around the coral BBD may have a role in maintaining coral diversity because it most commonly affects massive reef-building corals, but other types of stony corals and sea fans can be affected as well
When BBD kills part of these colonies, the skeleton is available to be colonized by other colonized species such as algae, invertebrates, gastropods, and boring sponges that work to weaken the coral skeletons making them more susceptible to breakage during storms.
When corals are inordinately stressed, they often expel their zooxanthellae, or the concentration of photosynthetic pigments declines This response is known as Coral Bleaching During a bleaching event, a coral s coloration disappears or becomes pale, and the white of the coral skeleton shows through the translucent coral tissue global bleaching has occurred on reefs worldwide
Bleaching usually is not uniform over single coral species but some species are more susceptible to bleaching than others under the same conditions In some instances, only the upper surface or lower surface of the colony is affected Localized bleaching has been attributed to exposure to high light levels, increased ultraviolet radiation, temperature or salinity extremes, high turbidity and sedimentation resulting in reduced light levels, and other abiotic factors In addition, bleaching in some species has occurred in response to a bacterial infection
However, the seven major episodes of bleaching that have occurred since 1979 have been primarily attributed to increased sea water temperatures associated with global climate change and el Niño/la Niña events effects of bleaching include reduced skeletal growth and reproductive activity, a lowered capacity to shed sediments and resist invasion of competing species and diseases Prolonged bleaching can cause partial to total colony death. If the bleaching is not too severe, and the stressful conditions decrease after a short time, affected colonies can regain their symbiotic algae within several weeks to months
Others: only recently described as dark-spots disease (DSD), discolored spots or markings in the tissue of several massive reef-building corals from the western Atlantic Red-band Disease (RBD) consists of a narrow band of filamentous cyanobacteria that advances slowly across the surface of a coral, killing living tissue as it progresses White Plague - is similar in appearance to WBD, but it affects different species Yellow Blotch Disease Affects only corals in the genus Montastraea and the brain corals YBD begins as pale, circular blotches of translucent tissue or as a narrow band of pale tissue at the colony margin. The area of affected tissue progressively radiates outward, slowly killing the coral.
The End