One Third of Reef-Building Corals Face Extinction Risk
In terms of promoting and supporting the greatest variety of life, reef corals are the “rain forests” of the oceans.
The corals themselves also have commercial value (for jewelry, aquarium decoration, etc.). Corals–symbiotic colonies of plant-like and animal-like creatures–have existed on this planet for tens of millions of years. However, some 45% of all coral species went extinct around the same time as the dinosaurs did. Hardest hit were a group of photosynthesizing, algae-harboring corals known as zooxanthellates. The symbiotic , red-brown algae that co-exist with these corals (and provide their energy source) are known as zooxanthellae.
Marine scientists have noted for several years now the spread of a coral disease known as Yellow Band Disease (YBD)–so named for the yellowish bands that spread across coral polyps–but had attributed its spread to thermal stress caused by global warming. Thermal stress is known to weaken much marine life, including corals, shellfish, and some species of zooplankton. But in November of 2008, researchers at Woodshole Oceanographic Institute (WHOI), on Cape Cod, Massachusetts, isolated the bacteria that caused YBD–actually four species of Vibrio bacteria that combine with the indigenous Vibrio bacteria to attack zooxanthellae (see the paper in the Journal of Applied Microbiology).
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Both factors are contributing to the decimation; the coral and its symbiont are getting hit with a double whammy–thermal stress (which weakens the colony) and bacterial disease (which kills the photosynthesizing algae). In addition to this, there are other detrimental factors at work.
This past Summer 2008, the status of 845 species of zooxanthellate reef-building coral species was assessed (Using the IUCN red list criteria). The assessment found that 32.9% of the studied species faced an elevated risk of extinction. Population (size of reef colonies) declines are associated with bleaching and disease caused by elevated sea surface temperatures. In addition, the build up of CO2 in the atmosphere leads to an increase in the amount of CO2 dissolved in water. When CO2 dissolves in water, some of it forms carbonic acid. This acidification of ocean waters (the lowering of ph) means fewer carbonate ions available for coral species (and shell fish) to build their calcareous skeletons. Climate change, it seems, is a triple whammy for many corals.
Thus, climate change would seem to be the background, material cause for these declines, with bacteria and acidification serving as the efficient causes. But local, human impacts also play a role. Such local impacts include over-harvesting (for aquariums), drag-netting, and, in some cases, the inadvertent disruption/destruction of reefs from commercial divers seeking prized, fish delicacies, such as the Napoleon Wrasse (which is captured alive through stunning), favored by Korean and Japanese gourmands.
In terms of oceanic regions with the highest extinction risks, the Caribbean ranks first. In terms of over-all risk, the Coral Triangle (Western Pacific Ocean) has the highest proportion of species in all extinction risk categories, as described by the IUCN red list criteria. To date, assessment of coral populations has suffered from a lack of a standardized, quantitative method for measuring declines. These recent studies were more conservative in their estimates, and took into consideration both habitat loss (population decline is generally a function of loss of habitat) and species-specific responses to habitat loss. Some species are more resilient than others. Certain types of corals, like the phytolithophores actually thrive in a warmer, high CO2 environment.
Ocean scientists fear that these declines will push these corals below sustainable levels, critically damage a vital component of the shallow-sea ecosystem, and thus further accelerate the depletion of many fish stocks, already in rapid decline.
Image credits: (top) NOAA, as public domain; (middle, thumbnail) photo by James Cervino, Pace University, courtesy of WHOI
[...] the creature more susceptible to predation and even disease (as in the case of many corals; see: One Third of Reef-Building Corals Face Extinction Risk [...]
[...] Much current marine research has focused on the “acidification” of the oceans–especially the shallower seas and sea beds where corals thrive. Too much acid (or a surplus of H+ ions) in the water makes for fewer carbonate ions available to build shells and coral structures (Hydrogen bonds with carbonate, forming bicarbonate, not CaCO3). This is part of the complex, acid-base reaction dynamics that go on continuously in the world’s oceans. It is believed that the recent decline in some coral species is the result of this acidification (see my earlier post : One Third of Reef-Building Corals Face Extinction Risk ). [...]