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Turtles in turmoil:
Fibropapilloma tumors a threat to Hawaiian green sea turtles

by Dava Della, Associate S&E editor


More than 70 percent of the nation’s coral reefs are found in the Northwestern Hawaiian Islands, which extend 1,200 miles northwest of Honolulu. Many of these corals are prone to a major reef-building disease known as coral bleaching.

In September, scientists with the federal and state governments, the Universities of Hawai‘i and California at Santa Cruz, and the Bishop Museum, explored the reefs around the northwestern islands and found evidence of coral bleaching at Pearl, Hermes, Midway, and Kure atolls.

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What are corals?
Corals are marine organisms that have evolved into modern reef habitats for vertebrates and species over the last 25 million years. Also known as anthozoans, corals are comprised of over 6,000 species and classified within the phylum Cnidaria. Stony corals, or sceleractinians, make up the largest order of anthozoans, and are primarily responsible for building up reef structures.

What is bleaching?
Corals have a symbiotic relationship with algae known as zooxanthellae, or the yellow and brown microscopic plants with photosynthetic pigments. Zooxanthellae are responsible for giving the coral its normal and healthy color and providing it with food.

When corals experience prolonged changes in their environment, they start to lose their color, skeletal ability to grow, and reproductive capacity because they expel their zooxanthellae and may die ( Since the algae give them their color, nutrients, and oxygen, we call this process bleaching. Coral bleaching, which has increased significantly over the last 10 years, is a stress condition in which the algae inside a coral dies.


Causes of bleaching
A variety of factors can trigger bleaching. Coral diseases generally occur in response to thermal and environmental stress such as increased sea water temperature, UV radiation, and natural disturbances. These threats and hazards can further weaken coral systems and compromise their ability to recover.

Thermal stress
In order for growth to occur (the optimal growth for corals is 0.5 to 2 centimeters per year), coral reefs require a water temperature of 23 to 29 degrees Celsius. Some species can tolerate temperatures as high as 40 degrees Celsius, allowing them to grow up to 4.5 centimeters.

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Another form of thermal stress is ultraviolet radiation. Coral reefs are more exposed to the UV radiation during the day. Because they thrive on obscure lighting, deprivation can overheat, starve, and dry out the coral. The expedition’s chief scientist, Rusty Brainard, explained that many of the bleaching discovered in September was “most pronounced in back reef and flat reef habitats exposed to the most intense sunlight, warmest temperature, and strongest ultraviolet radiation.”

Environmental stress
Corals are especially vulnerable to natural disasters such as hurricanes, where large, powerful waves can break apart or flatten parts of a reef colony. Prolonged exposure to cold and rainy weather can decompose coral tissue.

Human activity
In addition to natural threats, human activities can endanger the life of coral reefs. Careless and untrained divers or swimmers can often destroy fragile corals. Also, the pollution or leakage of fuel, paints, and other harmful chemicals from fishing vessels or as runoff from coastal areas are equally threatening.

Coral bleaching can be deadly. Some environmentalists have even warned that corals may be headed for extinction, with 60 percent of the world’s coral reefs dying completely by the year 2050 ( However, according to Greta Aeby, a coral biologist and coral disease expert with the State Department of Land and Natural Resources, some bleaching in healthy coral reefs is actually normal. “The higher-than-normal levels of bleaching found in these northwestern islands are unusual, but are not necessarily indicative of long-term environmental problems. Most mildly bleached colonies will recover in a few weeks,” said Aeby.

Zooxanthellae can be regained if species are exposed to their normal consistency of lighting and water temperature, otherwise reef growth may be disrupted for up to 15 years or more. Since coral biology and coral ecology are new fields and research is poorly funded, the recovery process of coral bleaching is still being studied. For a list of other coral diseases, visit the National Oceanic and Atmospheric Administration Coral Disease Identification and Information Web Site at




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