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Showing posts from March, 2013

Sea Grasses and Mangroves

Although the green, brown, and red macroalgae play key roles in supporting the reef ecosystem, they are not the only large plants growing there. Two types of vascular plants, sea grasses and mangroves, are common around coral reefs. The ancestors of these true plants evolved to live on the land then moved back to the sea; therefore, they have many of the typical terrestrial plant adaptations such as roots and vascular systems. In addition, both mangroves and grasses form pollen and seeds. Once a year, grasses produce thousands of tiny underwater flowers that yield pollen grains that float in the water from one plant to another. After pollination, fertilized eggs mature into seeds that float away from the parent plants, sink, and start new beds of grass.
Besides reproducing sexually, sea grasses can spread asexually by sending out runners that generate upright shoots. In this way, they can quickly form large meadows on the sandy bottoms of lagoons. Sea grasses are not tolerant of inte…

Brown Algae

A few species of brown algae live on the reef. Some are small and grow inconspicuously next to the corals, while others are prominent on the reef flat. Like many red algae, browns have complex life cycles that involve alternation of sexual and asexual generations.
Brown algae vary in form from encrusting growths to thin filaments, fleshy stalks, and giant kelps that reach sizes of 328 feet (100 m) long. Most brown algae cling to the bottom with holdfasts. Exceptions are the encrusting forms and a type referred to as sargassum, which floats on inshore reef waters.

Turbinaria is a reef brown alga that has a tough, spiny thallus capable of deterring would-be grazers. Padina, a fanshaped brown alga, contributes to reef building. On the outer surface of its thallus, it forms bands of calcium carbonate that helps cement the reef together.

Differences in Terrestrial and Aquatic Plants

Even though plants that live in water look dramatically different from terrestrial plants, the two groups have a lot in common. Both types of plants capture the Sun’s energy and use it to make food from raw materials. In each case, the raw materials required include carbon dioxide, water, and minerals. The differences in these two types of plants are adaptations to their specific environments.
Land plants are highly specialized for their lifestyles. They get their nutrients from two sources: soil and air. It is the job of roots to absorb water and minerals from the soil, as well as hold the plant in place. Essential materials are transported to cells in leaves by a system of tubes called vascular tissue. Leaves are in charge of taking in carbon dioxide gas from the atmosphere for photosynthesis. Once photosynthesis is complete, a second set of vascular tissue carries the food made by the leaves to the rest of the plant. Land plants are also equipped with woody stems and branches that …

Red Algae

Most red algae are multicellular plants that grow on rocks or other algae. Some of the most common red algae are encrusting forms that spread over hard surfaces, forming pink or red films on rocks and animal shells. Some other species of red algae develop tall plants with leafy fronds.
The encrusting coralline red alga Porolithon is especially noticeable on reef crests where it makes up much of the plant population. Porolithon is a tough plant that can endure occasional drying as well as intense wave action. Like the green alga Halimeda, Porolithon deposits calcium carbonate in its cell walls, giving it a rocklike texture. In all parts of the reef, this alga forms calcium carbonate that eventually finds its way into the reef. Calcium carbonate in Porolithon also acts as a mortar, cementing loose pieces of sand and rubble onto the reef structure. As a result, it improves the reef’s overall strength and replaces much of the material that is lost to erosion.

Upright varieties of red cora…

Green Algae

There are between 50 and 100 species of green algae that make their homes on coral reefs. A few species are covered with calcium carbonate skeletons very similar to the ones that protect coral animals. These plants—called coralline, or calcareous, algae—play special roles in building reef structure.
Coralline algae occur in a wide variety of shapes and sizes. Some look like masses of fine, threadlike filaments spreading over the reef and rock surfaces. The filaments are able to trap sediments and cement the particles together. In this way, coralline algae strengthen and support the coral reef structure. Even if a storm hits and many coral colonies are broken, coralline algae quickly bind the pieces back together. Unlike the encrusting forms, other species of coralline algae grow upright. They, too, produce calcium carbonate for structural strength and protection. When these algae die, the limestone in their bodies is converted to sand. About 50 percent of the sand found on coral reef…

Light and Algal Coloration

Light is a form of energy that travels in waves. When the Sun’s light arrives at Earth, it has a white quality to it. White light is made up of the colors red, orange, yellow, green, blue, indigo, and violet. The color of light is dependent on the length of the light wave. Light in the visible spectrum contains colors and has wavelengths between 0.4 and 0.8 microns (1 micron equals 1/1000000 m; a micron is also known as a micrometer). Violet light has the shortest wavelength in the visible spectrum and red has the longest.
Light is affected differently by water than it is by air. Air transmits light, but water can transmit, absorb, and reflect light. Water’s ability to transmit light makes it possible for photosynthesis to take place beneath the surface. All of the wavelengths of visible light are not transmitted equally, however; some penetrate to greater depths than others.
Light on the red side of the spectrum is quickly absorbed by water as heat, so red only penetrates to 49.2 fe…

Plants in the Coral Reef

Two important groups of plants in the coral reef are seaweeds, also known as macroscopic algae, and sea grasses. Both types of organisms are autotrophs. Along with the one-celled autotrophs, these marine plants support the food chains in the reef.
Compared to other marine ecosystems, the number and diversity of plants in coral reefs are relatively low. Small plant populations may be due to the fact that competition for space on reefs is very high, and corals often outcompete plants for the best reef locations. In addition, many of the reef animals are grazers, and they may hold down the size of seaweed populations; in experiments where grazers are removed from an area of reef, plant density increases dramatically. Another reason may be that coral reef waters are low in nutrients needed to support abundant plant life. Despite all of these hurdles, several species of green, red, and brown macroalgae, as well as grasses, flourish in the reef environment.

Advantages of Sexual Reproduction

Even though asexual reproduction seems like a simple solution to continuing a species, many monerans and protists also undergo sexual reproduction. While asexual reproduction expands a population, it does not make it possible for the population to change in any way. All of the organisms created in asexual reproduction are clones, so they have the same genetic information and the same characteristics as the parent organism. As long as environmental conditions remain steady, asexual reproduction maintains a healthy population; however, if anything in the environment changes, the population may suddenly be at risk. Because all the individuals are alike, any problem that may befall one cell will probably visit them all, possibly resulting in the loss of the entire population.
In organisms that reproduce sexually, all of the offspring are different. Each one contains a unique set of genetic information, half of it inherited from one parent and half from the other. Since individuals in the…

Protists and Fungi

Protists are another group of one-celled organisms, although these cells are larger and more complex than the cells of monerans. There are many types of protists living on the reef. Some of the key players are diatoms, dinoflagellates, and foraminiferans.
Like most groups of protists, diatoms are a highly diverse group. They use all types of nutritional strategies, and their ranks include autotrophs, heterotrophs, and mixotrophs (organisms that are both autotrophic and heterotrophic). Some species are capable of rapid movement, while others are stationary during their entire lives. Despite their differences, all diatoms have some characteristics in common.

The fragile cells of diatoms are covered with protective shells, or frustules. Each frustule contains a large component of silica, the same material that is found in sand and glass; therefore, diatom frustules look like tiny glass hat boxes that are topped with lids. The silica shells of several marine diatoms are visible in this Fi…

Kingdoms of Living Things

There are millions of different kinds of living things on Earth. To study them, scientists called taxonomists classify organisms by their characteristics. The first taxonomist was Carolus Linnaeus (1707–78), a Swedish naturalist who separated all creatures into two extremely large groups, or kingdoms: Plantae (plants) and Animalia (animals). By the middle of the 19th century, these two kingdoms had been joined by the newly designated Protista, the microscopic organisms, and Fungi. When microscopes advanced to the point that taxonomists could differentiate the characteristics of microorganisms, Protista was divided to include the kingdom Monera. By 1969, a five-kingdom classification system made up of Monera (bacteria), Protista (protozoans), Fungi, Animalia, and Plantae was established. The five-kingdom system is still in use today, although most scientists prefer to separate monerans into two groups, the kingdom Archaebacteria and the kingdom Eubacteria.
Monerans are the smallest cr…

Simple Coral Reef Microbes

Cyanobacteria are the smallest and simplest producers on the coral reef. Producers, or autotrophs, are organisms that are capable of making food molecules. Cyanobacteria are members of the kingdom Monera and have been on Earth longer than any other living thing. Cells just like them are believed to have formed the reefs of ancient seas on early Earth. Cyanobacteria are still abundant on present-day coral reefs, although they play different roles than those of their ancestors. Some types of cyanobacteria provide life-sustaining food and oxygen for the coral reef system, but others are responsible for disease and death.

A few species of cyanobacteria are capable of nitrogen fixation, a job that falls to heterotrophs, organisms that cannot produce food but must consume it, in many other ecosystems. In all cells nitrogen is an essential element that is used to make proteins and DNA, the genetic material that carries each cell’s blueprint. There is plenty of nitrogen gas in the atmosphere…

Food Chains and Photosynthesis

Living things must have energy to survive. In an ecosystem, the path that energy takes as it moves from one organism to another is called a food chain. The Sun is the major source of energy for most food chains. Organisms that can capture the Sun’s energy are called producers, or autotrophs, because they are able to produce food molecules. Living things that cannot capture energy must eat food and are referred to as consumers, or heterotrophs. Heterotrophs that eat plants are herbivores, and those that eat animals are carnivores. Organisms that eat plants and animals are described as omnivores.

When living things die, another group of organisms in the food chain—the decomposers, or detritivores—uses the energy tied up in the lifeless bodies. Detritivores break down dead or decaying matter, returning the nutrients to the environment. Nutrients in ecosystems are constantly recycled through interlocking food chains called food webs. Energy, on the other hand, cannot be recycled. It is e…

Simple Organisms and Algae on the Coral Reef

Few places on Earth rival the abundance and splendor of life on the coral reef. A reef visitor can spot living things in almost every size, shape, and color; however, some of the most important reef inhabitants cannot be seen with the naked eye. These invisible organisms live on the reef floor or float in the water column, the huge expanse of water below the surface.
The organization of living things on coral reefs is unique. In most oceans, upper regions of the water teem with plankton, communities of tiny, drifting organisms. The plantlike members of this community, the phytoplankton, are able to carry out photosynthesis. The rest of the community is zooplankton, and it is made up of very small living things that cannot photosynthesize. In seas where the populations of plankton are substantial, waters are also rich in minerals and nutrients. The waters around coral reefs are low in nutrients and have very small populations of plankton. It is this very lack of nutrients and plankton …

Coral Reef

Each coral reef is a unique and highly productive ecosystem. A reef can support thousands of different species from almost every known group of living things. All of these species depend on one another and the coral itself for their survival. As in all ecosystems on Earth, organisms that live there maintain a delicate biological balance of competition and cooperation.
Reefs are busy centers of activity in an otherwise scantily populated ocean landscape. Their nooks and crannies provide hiding places, nurseries, and spawning grounds for many types of sea organisms. Each group of organisms that moves into a coral reef helps attract and maintain other kinds of living things. The mature reef hosts hundreds of species in a bright display of color and activity.
Coral reefs are small but invaluable pieces of the Earth’s ecosystem. Reefs are more sensitive to pollution and other changes in environment than most other ecosystems are and are the first to reflect damage. Scientists watch them c…

Deep Water Reefs

Although the best known reefs are those in warm, tropical waters, coral reefs exist in other locations. Deepwater coral reefs can be found near landmasses around the globe in waters from 656.17 feet (200 m) to 4,921.26 feet (1,500 m) in depth. Deepwater reefs are similar in many ways to those in shallow waters. The most obvious differences in the two environments are temperature and available light.
A rocky or firm surface provides deepwater coral animals a point of attachment. For this reason, most reefs in deep water are located on underwater mounds, ridges, slopes, and mountains. Strong, fast-moving currents are almost always associated with these communities because they continuously supply water that is laden with oxygen and particles of food. Strong currents also help disperse the reproductive cells of corals and keep their surfaces free of sediments.
Scientists have been aware of deepwater reefs for more than 200 years, but gathering information on them has been a challenge. T…

Evolution of a Coral Reef

Scientists have studied the structures of coral reefs for decades, trying to determine how they were formed. The theory that most present-day scientists accept was among the earliest proposed. Naturalist Charles Darwin first presented his ideas on reef evolution in the 1830s.
Darwin believed that coral reefs changed over long periods of time, evolving from fringing reefs to barriers and finally to atolls. He explained that the process began when the eruption of an active volcano in the ocean created a small island of lava. After the volcano became inactive, it cooled, leaving part of its surface (the island of lava) jutting above sea level. At first this tip of the volcanic mountain lacked life. Ocean waters carried immature coral animals to the mountain island’s rough, rocky shores. These young corals attached to
the volcano in the shallow waters and grew into adults with hard skeletons. As the corals grew and reproduced, they spread around the entire volcanic island, eventually cre…

The Great Barrier Reef

The largest reef in the world is the Great Barrier Reef located off the coast of Australia. Bigger than the entire country of Italy, this reef system measures 1,249.1 miles (2,011 km) in length and 44.7 miles (72 km) across at its widest point. The reef is not a continuous structure but is made of more than 2,800 individual reefs. More than 400 types of coral, 1,500 species of fish, 4,000 types of mollusks, and 400 kinds of sponges make their homes in the Great Barrier Reef. Other animals there include anemones, worms, crustaceans, and echinoderms.

The reef supports sea grass beds that are feeding grounds for the dugong, an endangered mammal, as well as for the endangered green and loggerhead turtles. The reef is also used by humpback whales that travel from the Antarctic to give birth in its warm waters.
The Great Barrier Reef is the bestknown marine protected area in the world. Because it is a living classroom of natural history and science, the reef was declared a marine park in 1…

Types of Coral Reefs

Depending on where they are located and how they are formed, shallow- water tropical reefs can be classified into one of three major groups: fringing reefs, barrier reefs, and atoll reefs. The Figure above illustrates the structure of each reef type. Fringing reefs, which form along a coastline, are the most common type. These develop at the margin of a landmass where conditions are suitable for coral growth. They are normally located only in shallow waters and border the coast very closely with only a narrow stretch of water separating the reef from the shore. Because sediment washes from the land out to the sea, most fringing reefs have very little coral growing on the shore side. However, the ocean side, which is not exposed to as much sediment, is home to large populations of live coral. Fringing reefs are common in the Caribbean and around the Hawaiian Islands.

Like fringing reefs, barrier reefs run parallel to the shoreline, but they are located further out in the ocean. A barr…

In the Zone

No two coral reefs are exactly alike. Each one is a dynamic and ever-changing structure. Despite their differences, most coral reefs display several distinct zones that are created by environmental conditions such as wave and current strength, suspended sediment content, temperature, and depth of the water. Zones vary somewhat, depending on ocean location and type of reef, but most reefs have four typical zones: the reef flat, reef crest, buttress, and seaward slope.
The part of the reef that is closest to the shore is called the reef flat, or back reef. In this area, living things are protected from the full force of the breaking waves; however, water on the reef flat is relatively shallow, ranging in depth from a few centimeters to a couple of meters. Shallow-water inhabitants are exposed to wide variations in temperature and salinity. They must also deal with changing water levels and occasional periods when the low tide leaves them stranded without water. These factors limit the …

Chemical and Physical Characteristics of Water

Water is one of the most widespread materials on this planet. Water fills the oceans, sculpts the land, and is a primary component in all living things. For all of its commonness, water is a very unusual molecule whose unique qualities are due to its physical structure.
Water is a compound made up of three atoms: two hydrogen atoms and one oxygen atom. The way these three atoms bond causes one end of the resulting molecule to have a slightly negative charge, and the other end a slightly positive charge. For this reason water is described as a polar molecule.
The positive end of one water molecule is attracted to the negative end of another water molecule. When two oppositely charged ends of water molecules get close enough to each other, a bond forms between them. This kind of bond is a hydrogen bond. Every water molecule can form hydrogen bonds with other water molecules. Even though hydrogen bonds are weaker than the bonds that hold together the atoms within a water molecule, they …

Physical Characteristics of Coral Reefs

Coral animals can be found in several parts of the ocean, but the reef-building types only live in places that meet a narrow range of environmental conditions. Reef-building corals have very specific habitat requirements. They are finicky about the amount of salt in the water, water temperature and depth, movement of currents, and available nutrients.
Salinity refers to the amount of dissolved minerals, or salts, in ocean water. The average salinity of ocean water is 35 parts per thousand, which can be written as 35‰. The symbol ‰ is similar to percent but refers to parts per thousand instead of parts per hundred. Salinity is low in areas where freshwater flows into the ocean, such as near the mouths of rivers. Salinity is high in places where water evaporates from slowmoving or stagnant pools of salt water.
Reef-building corals favor waters where the salinity is about 34 parts per thousand by weight, a little lower than average sea salinity. Coral reefs do not exist in places where …

Geologic Time

The Earth is about 4.5 billion years old. Fossil evidence suggests that the first living things were simple cells that appeared about 3.5 billion years ago. The time line in Table above shows that the period of time from the beginning of Earth to 700 million years ago, the largest part of the Earth’s past, is known as the Precambrian era. 
The Paleozoic era began about 570 million years ago and lasted until 280 million years ago. Fish, insects, amphibians, and reptiles were some of the major groups of animals that developed in this period. Both terrestrial and aquatic plants also formed in this time span. The Mesozoic era extended from 250 million years ago until 135 million years ago. A period dominated by reptiles, the Mesozoic is known as the age of the dinosaur. Late in the era, mammals and birds developed. The most recent period, the Cenozoic era, began 65 million years ago and extends to the present. During this time, birds and mammals flourished. Humans made their appearance l…

Origins of Coral Reefs

A visitor to a coral reef millions of years ago would have witnessed a seascape that is quite different from the one that exists today. Over time, both the appearance and composition of reefs have changed dramatically. Reefs have been subjected to countless alterations over their history. Ice ages, mass extinctions, shifting of landmasses on continental plates, and fluctuating sea levels are just a few of the global events that reefs have endured.
Geologic records document the existence of reefs 2 billion years ago, in a period of time referred to as the Precambrian era. The architects of the ancient reefs were not coral but simple microbes called cyanobacteria. Then, as now, cyanobacteria were algaelike organisms that formed long, mucus-producing filaments. Their sticky filaments trapped and held debris and grains of sand. Individual algae, with their ensnared soil particles, stuck to one another, forming tall, gray towers, or stromatolites, that rose several meters upward from the …

Carbon Dioxide Grabbers

Coral reefs help keep the Earth’s biosphere, the part of the planet where living things are found, in balance. One of the coral reef’s important functions is in maintaining normal levels of carbon dioxide in the atmosphere. At the point where the atmosphere meets the sea, carbon dioxide and other gases from the air dissolve in ocean water. In places where coral reefs exist, much of this dissolved carbon dioxide is removed from the water by coral organisms. The organisms then use the gas to build calcium carbonate, or limestone, skeletons. As the skeleton-building proceeds, levels of the dissolved gas in ocean water decrease, permitting more carbon dioxide to enter the water from the atmosphere. For this reason, reefs act as carbon “sinks.” Greenhouse Gases Carbon dioxide is one of several so-called greenhouse gases that form an invisible layer around the Earth. As shown in Figure greenhouse gases trap the Sun’s heat near the Earth’s surface, very much like the windows in a greenhouse…


Biodiversity, or biological diversity, refers to the variety of living things in an area. Diversity is higher in complex environments than in simple ones. Complex physical environments have a lot to offer organisms in the way of food and housing. Estuaries, shorelines, and coral reefs are extremely complex marine environments, and each of them provides a wide assortment of nutritional resources for living things.
There are thousands of habitats in estuaries, coastal systems where fresh and salt water meet and mix. The bottom of the estuary provides homes for different kinds of organisms. Some spend their entire lives on the surface of the sediment, many burrow just under the surface, and others dig deep into the sediment. Organisms also select locations that accommodate their abilities to tolerate salt, so those that are adapted to high salinity are on the seaward side while the freshwater-dependent ones are on the river side. In between the two extremes, organisms live in zones that…