Coral: Polyps, Algae, Eggs, Mass Spawns

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20120517-Corales_en_el_Parque_Marino_Motu_Motiro_Hiva_5.jpg Describing the sensation of diving among a coral reef, David Attenborough wrote: "There is nothing on land that can prepare you for the profusion of shapes and colors of the corals themselves. There are domes, branches and fans, antlers delicately tipped with blue, organ pipes that are blood red. Some seem flower-like yet when you touch them they have the incongruous scratch of stone."

"Often different coral species grow beside one another, mingled with seas pens arching above and beds of anemones that wave long tentacles in the current. Sometimes you swim over great meadows that consist entirely of one kind of coral; sometimes in deeper water, you discover a coral tower hung with fans and sponges that extends your sight into depths of darkest blue."

Coral reefs are sometimes called the rain forests of the sea because of their beauty, biodiversity and the stresses they are under. They contain more species per unit than any other marine environment. They go a step further than rainforests in arguably being the most colorful place on earth. In the rainforest you generally don’t find the brilliant yellows, shocking blues and brilliant reds that you find in the reef.

What makes reefs so colorful and why? In recent years scientists have begun to develop a better understanding into these questions by studying how light and color appear at different depths to the eyes of the creatures that inhabit the reef. See Reef Fish

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); “Introduction to Physical Oceanography” by Robert Stewart , Texas A&M University, 2008 ; Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal ; Woods Hole Oceanographic Institute ; Cousteau Society ; Monterey Bay Aquarium ; MarineBio ; Websites and Resources on Coral Reefs: Coral Reef Information System (NOAA) ; International Coral Reef Initiative ; Wikipedia article Wikipedia ; Coral Reef Alliance ; Global Coral reef Alliance ;Global Coral Reef Monitoring Network


Sea anemones, jellyfish and corals belong to the family of colonizing organisms called coelenterates (Greek for cavity) and the 9,000-species phylum Cnidarians (meaning "stringing thread"), a group of tentacled creatures which also includes anemones, jellyfish and corals and hydriods. Most reproduce asexually without mating by producing buds from their own bodies.

All coelenterates are simply a hollow sac or shallow cup of cells with a mouth at one end surrounded by tentacles. Armed with stinging cells, the tentacles help them to paralyze small swimming animals which are then pushed into its mouth. Coelenterates have a primitive gut for digestion and their mouth also serves as an anus.

Coelenterates take in food through their mouthes and ingest it in the stomach, with the indigestible parts being expelled back out the mouth. They are almost exclusively carnivores but have no teeth. Instead they have tentacles lined with whiplike structures called nematocysts that release poison barbs that are strong enough to paralyze prey and allow it to be pulled into the coelenterate’s mouth and gut. Jellyfish have their tentacles pointed downward while anemones and corals have theirs pointed upwards.


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coral polyps
Cnidarians are essentially the same as coelenterates — cup-like animals — but looked at in a slightly different way. The tube can either be a medusa, flattened into a bell shape, or a polyp, with the closed end attached to a hard surface. Corals and sea anemones are polyps. Medusas are mostly jellyfish. Some hydriods and jellyfish exist in both medusa and polyp forms in their lifetimes.

Cnidarians include corals, sea anemones, jellyfish, and relatives. Phil Myers wrote in Animal Diversity Web: The Phylum Cnidaria includes such diverse forms as jellyfish, hydra, sea anemones, and corals. Cnidarians are radially or biradially symmetric, a general type of symmetry regard as primitive. They have achieved the tissue level of organization, in which some similar cells are associated into groups or aggregations called tissues, but true organs do not occur. Cnidarian bodies have two or sometimes three layers. A gastrovascular cavity (coelenteron) has a single exterior opening that serves as both mouth and anus. Often tentacles surround the opening. Some cells are organized into two simple nerve nets, one epidermal and the other gastrodermal, that help coordinate muscular and sensory functions. [Source: Phil Myers, Animal Diversity Web (ADW) /=]

Cnidarians have two basic body forms, medusa and polyp. Medusae, such as adult jellyfish, are free-swimming or floating. They usually have umbrella-shaped bodies and tetramerous (four-part) symmetry. The mouth is usually on the concave side, and the tentacles originate on the rim of the umbrella. Polyps, in contrast, are usually sessile. They have tubular bodies; one end is attached to the substrate, and a mouth (usually surrounded by tentacles) is found at the other end. Polyps may occur alone or in groups of individuals; in the latter case, different individuals sometimes specialize for different functions, such as reproduction, feeding or defense. /=\

Reproduction in polyps is by asexual budding (polyps) or sexual formation of gametes (medusae, some polyps). Cnidarian individuals may be monoecious or dioecious. The result of sexual reproduction is a planula larva, which is ciliated and free-swimming. /=\

If collar cells and spicules are defining characteristics of the Phylum Porifera, then nematocysts define cnidarians. These tiny organelles, likened by Hickman to cocked guns, are both highly efficient devices for capturing prey and extremely effective deterrents to predators. Each contains a coiled, tubular thread, which may bear barbs and which is often poisoned. A nematocyst discharges when a prey species or predator comes into contact with it, driving its threads with barb and poison into the flesh of the victim by means of a rapid increase in hydrostatic pressure. Hundreds or thousands of nematocysts may line the tentacles or surface of the cnidarian. They are capable even of penetrating human skin, sometimes producing a painful wound or in extreme cases, death. /=\

Are Corals Animals, Plants or Rocks?

Corals are animals because they do not make their own food, as plants do. Corals have tiny, tentacle-like arms that they use to capture their food from the water and sweep into their inscrutable mouths. Corals are sessile animals — permanently attach themselves to the ocean floor — that rely on their relationship with plant-like algae, They "take root" on the ocean floor so it no wonder that many people think corals are plants! [Source: NOAA]

Most structures that we call "coral" are, in fact, made up of hundreds to thousands of tiny coral creatures called polyps. Each soft-bodied polyp — most no thicker than a nickel — secretes a hard outer skeleton of limestone (calcium carbonate) that attaches either to rock or the dead skeletons of other polyps. In the case of stony or hard corals, these polyp conglomerates grow, die, and endlessly repeat the cycle over time, slowly laying the limestone foundation for coral reefs and giving shape to the familiar corals that reside there. Because of this cycle of growth, death, and regeneration among individual polyps, many coral colonies can live for a very long time.

Most corals contain algae called zooxanthellae (pronounced zo-UH-zan-thuh-lay), which are plant-like organisms residing within the coral's tissues. More than merely a clever collaboration that has endured between some of the tiniest ocean animals and plants for some 25 million years, this mutual exchange is the reason why coral reefs are the largest structures of biological origin on Earth, and rival old-growth forests in the longevity of their ecological communities.

Coral and Coral Polyps

Coral polyp
Coral is the hard skeleton secreted by a tiny organism called a polyp. The scientific name for coral, “ Anthozoa”, means "flower animals." These animals are partly carnivorous with stinging cells that catch microscopic prey with tentacles and mucous coating. They get 90 percent of their nourishment from algae. Corals have changed little since they first evolved around 230 million years ago. There are deep-sea corals that are more than 8,000 years old.

Coral looks like plants but in fact are animals. This fact is evident at night when the polyps emerge and cover the rocky exoskeletons like fur. Attenborough wrote: "Each polyp is connected to its neighbor by strands that extend laterally. As the colony develops, new polyps form, often on these connecting sections and their skeletons grow over and stifle earlier polyps."

Polyps are about the size of pencil erasers. They extract calcium carbonate (limestone) from seawater and deposits this beneath themselves as a exoskeletons (protective calcium shells). The polyps live inside the limestone exoskeletons during the day and emerge at night to feed.

Coral Polyps and Algae

Corals comprise an ancient and unique partnership, called symbiosis, that benefits both animal and plant life in the ocean. Most corals contain tiny one-celled plants, an algae called “ zooxanthellae” (pronounced zo-UH-zan-thuh-lay). Residing within the coral's tissues, the microscopic algae are well protected and make use of the coral's metabolic waste products for photosynthesis, the process by which plants make their own food. The corals benefit, in turn, as the algae produce oxygen, remove wastes, and supply the organic products of photosynthesis that corals need to grow, thrive, and build up the reef.

The outside layer of each polyp is inhabited by tiny “zooxanthellae”, which give the coral its rich orange, purple, red and yellow colors. In this way coral is part animal, part plant and part mineral. The name of most kinds of corals — elkhorn, staghorn, brain, blue, black, sea fans, star — describes the appearance and color of the coral colonies. Algae lies in tissues of these colonies at a density of millions per square inch. [Source: Walter A. Stark II, National Geographic, November 1966 [╆]

Algae live within coral polyp cells and absorb light from the sun. Through photosynthesis they are able to meet nearly all the coral’s energy needs. Corals produce compounds that act as sunscreen on new growth until it acquires algae that absorb UV light. A layer of mucus excreted from the coral polyps removes sediment, preventing it from blocking sunlight.[Source: Fernando G. Baptista, Lawson Parker and Eve Conant, National Geographic, April 15, 2021

The polyps in coral secrete a digestive solution that weakens the skins of the algae and caused 80 percent of the food produced by the algae to leak out and into the polyps cells, nourishing the polyps. Algae provided coral with 90 percent of its energy needs. The remaining 10 percent comes from plankton gathered by the polyps tentacles when they emerge at night. The relationship between coral and algae has allowed coral to thrive in an environment that has relatively few nutrients. Without algae the coral eventually starves, dies and turns white.

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pretty algae that looks like coral

Coral polyps and the algae living within the polyps's tissue have a symbiotic relationship. The algae provides fat and sugars for the polyp and gives off oxygen though photosynthesis that the polyps need to survive when they are is inside their exoskeletons during the day. The algae also absorbs the waste of the polyps, converting phosphates and nitrates into proteins and, with the help of the sun, uses carbon dioxide to make carbohydrates through photosynthesis. The algae also supplies the polyp with chemicals that act as sun cream against harmful ultraviolet radiation from the sun.

The algae also accelerate the conversion of calcium, providing the coral with minerals that help it build it exoskeletons and produce larger and stronger reefs. The polyps in turn produce carbon dioxide through respiration which the algae needs to sustain itself. "Coral eating" fish such as parrotfish, triggerfish and surgeon fish feed on the algae rather than the coral polyps themselves.

Scientists used to think there was just one species of zooxanthellae but research in the 2000s revealed that there are more than a dozen different kinds and some of them have significant attributes such as surviving better in warm water than others.

Coral Characteristics

During the day polyps expel water from their bodies and sink as deeply into their exoskeletons as possible, where they are reasonably safe from the jaws of grazing fish. After sunset, when the coral-eating fish hide in their crevasses to escape nighttime predators, the colorful polyps inflate their bodies with water and emerge from their exoskeletons, their stinging tentacles snatching plankton in the water.

Corals fight off competitors with chemical weapons. Some species extends threads from the gut, called mesenterial filaments, that deposit mucus with tissue-dissolving enzymes on competitors, which sometimes fight back with extra-long stinging tentacles. Corals deal with stresses like silt and pollution by secreting mucous proteins on their outer tissues.

Coral polyps in symbiosis
with unicellular dinoflagellates
Some corals turn strange colors — like black light posters from the 1960s — when ultraviolet light is shined on them. The orange, blue and green colors are caused by fluorescence, a phenomena in which a surface absorbs ultraviolet light and emits lights of different colors. Corals exude chemicals that protect them from sunburn during low tide. Scientists are examining these chemicals to develop a new waterproof protection cream.

Ove Hoegh-Guldberg, a biologist at Queensland University, told National Geographic, "At shallow depths sunlight here can actually be toxic. Corals have special pigments to absorb ultraviolet rays, and their symbiotic algae hide in the shade beneath bundles of these pigments for protection. Conversely, at depths where little sunlight penetrates, the algae nestle right inside the coral's pigment bundles. Then, as the pigments re-radiate the light energy they have gathered, the algae can use it for photosynthesis.” This re-radiation causes corals to fluoresce.

Coral Growth and Movement

Corals are very delicate and very slow growing. This is why it is so important not to touch or disturb them when snorkeling or diving. Most corals only grow about one centimeter a year. Staghorn corals are among the fastest growing. The grow as fast as 10 centimeters a year. Those that grow fast also are among the most sensitive. Nancy Knowlton, a biodiversity expert at Scripps Institute, told the New York Times, “They can slip from growing really fast to dying really fast.”

Coral forms are dictated by species as well as exposure to light and waves. Branching corals, specialized to absorb light, are the fastest growing but most susceptible to bleaching. [Source: Fernando G. Baptista, Lawson Parker and Eve Conant, National Geographic, April 15, 2021]

Coral generally don't move because their exoskeletons are fixed to a reef or coral head. There is one species of "walking" coral (“ Heteropsammia michelenii” ) that is about an inch-long and moves around with the help of a specific worm which has a symbiotic relations with the coral polyp and live in its exoskeleton.

Since coral doesn’t swim or fly, scientists have long wonder how different species are transported around the globe and how remote islands can have so many species of coral. Based on the fact that some places down current from relatively recent volcanic eruptions and pieces of coral have been observed floating on pumice, it is believed that fertilized coral eggs and coral itself are transported through the water and carried like a hitchhiker by various means, reaching destinations a considerable distance from where they originated.

It is believed, can be transported by flotsam such as pieces of driftwood and manmade objects like plastic and litter. Christmas Island, which is in the South Pacific, down current, from Krakatau, which erupted violently in 1883, possesses an especially rich variety of coral.

Coral Reproduction

20110307-NOAA coral bubble reef2080.jpg
coral bubble
Some corals reproduce asexually (without mating) by budding in which mature corals sends out shoots called buds from their own body that develop into new polyps. In some cases the new polyp separates completely; in many cases the budding is incomplete and individual remain joined together in a colony.

Most of the reef-building species are hermaphroditic. Some species have polyps that contain reproductive bundles with around 100 eggs and 2 million sperm. The bundles are released once a year and drift in the open sea to a new homes. Others raise their larvae to an advanced stages and then release them. Some corals cycle reproduction by the amount of blue light in the second full moon of springtime.

Coral eggs are filled with nutrients and fat. In addition to providing food the fat makes the eggs buoyant and able to float to the ocean surface. After being released by the coral, the egg-and-sperm bundles can collect en mass and form a spongy carpet on the ocean surface. After a half hour or so the bundles break open allowing the eggs and sperm of different polyps to mix.

Coral Mass Spawns

Several species of reef-building star corals reproduce once a year by releasing billions of eggs and sperm into the water at one time, a phenomena that wasn't described in scientific journals until 1984. Before then most scientists thought that coral reproduced like plants. [Source: John Ross. Smithsonian magazine]

Hard (or stony) corals reproduce by releasing their eggs and sperm all at the same time. This spawning cycle is one of nature’s most spectacular events. Once a year, on cues from the lunar cycle and the water temperature, entire colonies of coral reefs simultaneously release their tiny eggs and sperm, called gametes, into the ocean. The phenomenon brings to mind an underwater blizzard with billions of colorful flakes cascading in white, yellow, red, and orange. [Source: NOAA]

In ways that scientists still do not fully understand, mature corals release their gametes all at the same time. This synchrony is crucial, because the gametes of most coral species are viable for only a few hours. The “blizzard” makes it more likely that fertilization will occur. The gametes, full of fatty substances called lipids, rise slowly to the ocean surface, where the process of fertilization begins.

Describing a mass-spawning event in the Caribbean, John Ross wrote in Smithsonian magazine, "I can see the slightly pink, tiny egg-sperm bundles in the stars of the coral, ready to fire like cannonballs...I half expect to hear a noise then the corals launch their precious packages...I watch a Montastraea coral head as the eggs pop out in a quick wave that moves across its face. Suddenly streams of these tiny balls are wafting up and around me, and I'm reminded of a snow-globe toy...The egg bundles rise to the surface like tiny bubbles."

In the Caribbean, “ Acropora” (a common elkhorn species) and “ Montastraea” mass spawn according to the lunar calendar. Acropora begins to spawn two to four nights after the full moon in August. Montastraea spawn six nights after the same moon.

In addition to reproducing on a certain day, mass-producing coral species also release their eggs and sperm at a specific time. How they coral know when to do all this is still a mystery. Coral species in Japan release eggs en mass during high tides with a full moon in late July or early August. The one-millimeter-in-diameter eggs emerge from the coral for about 30 minutes.

Coral Mass Spawning Events

The mass spawning provides a feast for a number of sea creatures. Every year whale sharks gather in large numbers at Ningaloo reef in Western Australia to a feast on spawning of coral. This is the largest known gathering of whale sharks in the world. It is not unusual for divers to see 20 or 30 of them in a single day, and manta rays, minke whales and mobula rays also show up at the 160 mile long reef for the mass spawning which only occurs once every year. Groups at Ningaloo tend to be immature males.

Coral Eggs and Larvae

When a coral egg and sperm join together as an embryo, they develop into a coral larva, called a planula. Planulae float in the ocean, some for days and some for weeks, before dropping to the ocean floor. Then, depending on seafloor conditions, the planulae may attach to the substrate and grow into a new coral colony at the slow rate of about .4 inches a year.

About 24 hours after being fertilized coral polyp eggs evolve into larvae with cilia hairs that allow them move about. After five to eight days the larvae have developed enough to "settle" and attach themselves to a hard surface and grow into a mature coral animal. Before then many are gobbled up by a host of creatures. The survival rate for coral larvae is generally only around 1 or 2 percent.

For many years scientists believed that the larvae found their homes by chance like windblown-seeds. But that doesn't always seem to be the case. Recent research has shown that the larvae have a sophisticated development system that triggers metamorphosis after the larvae comes in contact with a hard surface with algae. If it hits a hard surface with no algae, the metamorphosis mechanism is not activated and the larvae keeps drifting and eventually dies or is eaten.

During metamorphosis the larvae is transformed from a soft bodied organism into a young polyp with an exoskeleton. The upper part of the body becomes dome-shaped. The creature also develops a stomach and a mouth. Around the mouth grows a number tentacles. After the metamorphosis is mostly complete the polyp builds a platform from lime between its body and the surface to which it is attached. Then it builds up an exoskeleton of lime. When the skeleton is complete only the tentacles reach above it.

Moon's Blue Light — a Coral Aphrodisiac?

Michael Perry of Reuters wrote: Ancient light-sensitive genes may be the trigger for the annual mass spawning of corals shortly after a full moon on the Great Barrier Reef, according to a study by Australian and Israeli scientists. The cryptochromes genes occur in corals, insects, fish and mammals — including humans — and are primitive light-sensing pigment mechanisms which predate the evolution of eyes. [Source: Michael Perry, Reuters, October 19, 2007]

“The Cry2 gene, stimulated by the faint blue light of the full moon, appears to play a central role in triggering the mass synchronized coral spawning, said the scientists in a paper published in the international journal Science on Friday. “This is the key to one of the central mysteries of coral reefs,” said Professor Ove Hoegh-Guldberg, who lead the University of Queensland laboratory which discovered the genes. “We have always wondered how corals without eyes can detect moonlight and get the precise hour of the right couple of days each year to spawn,” Hoegh-Guldberg said in a statement.

“The annual mass spawning of corals occurs across a third of a million square kilometers of Australia’s Great Barrier Reef, shortly after a full moon. Exposing corals to different colors and intensities of light and sampling live corals on reefs around the time of the full moon, Israeli researcher Oren Levy found the Cry2 gene at its most active in Acropora corals during full moon nights. The genes developed in primitive life forms in the Precambrian, more than 500 million years ago, as a way of sensing light to synchronies their body clocks and breeding cycles, said the researchers.

Image Sources: Wikimedia Commons; YouTube, Animal Diversity Web, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated March 2023

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