October 31st, 2009 – From the On-Board Scientist
by Dr. Michael Reynolds
First thoughts on Coral
We are currently sailing through a part of the world where coral reefs are found (The West Indies, Caribbean Sea, and the north coast of South America), so it is a good time to begin a discussion of coral and coral reefs. The purpose of this and following reports on coral and related research is to help the reader gain a better understanding and appreciation of coral. This article draws upon excellent online resources, principally material from National Oceanographic and Atmospheric Administration (NOAA) and Wikipedia. These, and a few additional sources are listed at the end of this report.
- Shallow-water coral are reef building animals that are are found in tropical waters around the world.
- Hundred of thousands of coral polyps work together to build vast reef complexes.
- Survival depends on a relationship with one-celled algae plants, zooxanthellae.
- When zooxanthellae die, the reef turns white, bleaching, and dies soon after.
- Shallow water coral are dying and the reasons are not fully understood.
- Dying comes from natural processes and man made activity.
- However, much permanent dying is attributable to human activity.
- Causes include commercial harvest, rising temperature, trash, pollutants, garbage, and physical ravaging.
The Take-Home Messages
What is a coral?
First, a coral is an animal, not a plant, so corals do not require sunlight or warm water to live. Most people associate corals with clear, warm tropical seas and fish-filled reefs. In fact, stony, shallow-water corals-the kind that build reefs-are only one type of coral. There are also soft corals and deep water corals that live in dark cold waters. We will discuss deep water corals in a subsequent article; here, we will focus on shallow water coral communities.
Coral are sessile (they are permanently attached to a rock or self-made reef) creatures with tentacles that surround a centrally located mouth. They are relatives of the larger anemone who transform the ocean floor into magic flowing carpets of color. They feed, mostly, by capturing small fish, plankton, or detritus with their tentacles. Stinging cells called nematocysts in the tentacles are often used to disable tiny creatures, making them easier to eat. Everything that comes to the tentacles of a coral is eaten and unfortunately this includes plastic, styrofoam, and other pollutants.
Except for a few rare cases, corals live in large communities called reefs. Each individual coral is called a polyp and a single community can have hundreds of thousands of member polyps. Most of the time polyps are hidden during the day and they come out to feed at night, so casual divers must go out in the night to see a reef glory in full color.
What are zooxanthellae?
Life in tropical waters is tough, especially for sessile creatures. For example, tropical waters are clear because they are defic ient in nutrients such as iron, minerals, protein, and vitamins. To overcome this deficiency shallow water coral have established a close relationship with a species of one-celled algae called zooxanthellae. The corals and algae have a mutualistic relationship, which is an association between organisms of two different species in which
|A map of the distribution of shallow water coral reefs. See http://www.nasa.gov/vision/earth/lookingatearth/coral_assessment.html. NASA produced the map by examining over 7000 Landsat images for evidence of reef. This map would not include the deep water coral which are being discovered around the globe. (Note: don’t be fooled by the Mercator map projection. One half of the surface of the globe lies between 30N and 30S latitudes, approximately from Jacksonville Florida to Buenos Aires Argentina. So we can say that coral reefs cover half the surface of the Earth.)|
each member benefits. The coral provide the algae with a protected environment and compounds they need for photosynthesis. In return, the algae produce oxygen for the coral and help it remove waste. Most importantly, zooxanthellae supply the coral with the products of photosynthesis: glucose, glycerol, and amino acids. Coral use these products to make proteins, fats, and carbohydrates, and produce calcium carbonate, which is the composition of their hard exterior.
Sometimes when corals become physically stressed, the polyps expel their companion algal cells causing the
|A large single coral polyp under filtered light. This form of chromatography is used to diagnose disease in corals. The mouth is the vertical slit in the center.|
colony to take on a stark white appearance referred to as “coral bleaching”. If the polyps go for too long without the zooxanthellae algae, coral bleaching will lead to the coral’s death.
Why are coral reefs important?
Coral reefs provide the foundation for a vast ecosystem of other creatures; the entire food chain from algal plants to sharks, coexist in this important environment. Coral reefs support over 4,000 species of fish, 800 species of hard corals and hundreds of other species – more species per unit area than any other marine environment- and are among the most diverse and valuable ecosystems on Earth. It is estimated there may be another 1 to 8 million undiscovered species of organisms that live in and around reefs. This biodiversity is considered key to finding new medicines for the 21st century; many drugs are now being developed from coral reef animals and plants that could serve as possible cures for cancer, arthritis, bacterial infections, viruses, and other diseases.
Does this all sound like the way we discuss tropical rain forests?
Why are coral reefs in danger?
Coral reefs are vulnerable to both natural and man-made (anthropogenic) activities. Naturally occurring processes that can damage coral reefs are waves from hurricanes and storms, as well as low tides that expose reefs to excess sunlight, and cold or warm ocean temperatures from seasonal variability. Increased sea surface temperatures, decreased sea level and increased salinity from altered rainfall occur naturally, caused by weather patterns such as El Niño. Together these conditions can have devastating effects on a coral’s physiology (Forrester, 1997.) During the 1997-1998 El Niño season, extensive and severe coral reef bleaching occurred in the Indo-Pacific and Caribbean oceans, and approximately 70 to 80 percent of all shallow-water corals on many Indo-Pacific reefs died as a consequence.
Predation from other animals is a periodic problem that can also cause severe damage. Fish, marine worms, barnacles, crabs, snails and sea stars all prey on the soft inner tissues of coral polyps. In an extreme case, in 1978 and 1979, there was a massive outbreak of the crown-of-thorns starfish (Acanthaster planci) that threatened to destroy the entire Great Barrier Reef in Australia.
|Bleached coral with damsel fish. From http://www.coralcoe.org.au|
As if natural threats weren’t bad enough, living near humans has been especially lethal to coral. Coral reefs exist along shorelines as does much of the human population. This is an unfortunate juxtaposition for animals with a fragile constitution. Coral bleaching is currently occurring on an epidemic scale and large areas of dead or dying coral can be found just about everywhere. To try to stave off this threat, there is a large US and international response to coral disease (described in some links provided below).
Pollution, destructive fishing practices using dynamite or cyanide, overfishing, collecting live corals for the aquarium market and mining coral for building materials are some of the many ways that people damage reefs all around the world every day. Fifty years ago the world population stood at 2.2 billion and today it is 6.6 billion. Human pressure on all ocean life, particularly those living close to the continental coasts, is enormous.
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References — an incomplete list, but good
NOAA tutorial on coral
Wikipedia is always a good first stop.
Center for Coastal Environmental Health and Biomolecular Research
Center for Human Health Risk (CHHR) – Hollings Marine Laboratory
Coral Disease & Health Consortium
U.S. Coral Reef Task Force