Wednesday, 20 March 2013

Some confusing facts about corals

I love coral reefs. I also love news articles about coral reefs, especially when they mention the problems corals face today that challenge their survival: mostly pollution, overfishing, and climate change. But I hate when somewhat minor inaccuracies undermine the strength of the articles. Today I came across just such an article, on one of National Geographic TV's blogs. First, I griped about it on Twitter. A few hours later, DNLee tweeted and suggested writing up corrections to news articles and sending them to the author. So, here goes my #scisplain.

The quotes below come from the National Geographic blog post, and my gripes about them follow.

"Coral Is Smaller than a Tea Cup"

Well, yes, some coral colonies can be smaller than a tea cup. But many coral colonies are huge, the size of cars and tables. The author, Rebecca O'Connor, clarifies in the next paragraph that she means individual coral polyps are smaller than a tea cup. Well, that is often true, but some coral polyps (especially those of solitary corals, which are only composed of a single polyp), are larger. To be more accurate then, this sub-headline above should say something like Individual coral animals are often smaller than a tea cup. 

Ok, my red lines are really faint...but this is a picture of a bunch of skeletons of solitary corals (and some Tridacna clams). Some of these corals, which are comprised of single polyps, are 15 cm in diameter. Maybe your tea cups are really big?

O'Connor goes on: "The animals that make up a coral reef are called polyps and they can actually live on their own, but are primarily associated with the spectacularly diverse limestone communities or reefs, they construct."

Ack! Ok, first, many different types of animals make up a coral reef, not just corals--sponges, algae, soft corals, bivalves...etc. So, no. The animals that make up a coral reef are not called polyps. Instead, this sentence could read something like: Corals are typically colonial animals; each colony is made up of many individuals called polyps, which are usually smaller than a tea cup. I'm not sure what she means by the rest of that sentence. Does she mean that some corals are not colonial, and are just single polyps? Or does she mean that sometimes corals live relatively far away from other corals, for instance in places like São Tomé where individual corals grow on rocks?

I kind of like the "spectacularly diverse" part but why are the communities limestone? Fish, sponges, and many types of algae are not limestone, yet are part of the diversity of reefs. If nothing else, a comma is desperately needed after the word "communities."

(Also, for the record, polyp is a rather general term, and can refer to an individual anemone, hydroid, or jellyfish; saying "polyp" does not specify coral.)

The other four sub-sections are also frustrating.

"Coral Reefs Are Colorless"
Each of the other sections start with "Coral Reefs" and then say something only (mostly) about coral animals. Here, "reefs" should be removed, and the headings should read Corals blah blah blah...

The article continues: "When you think of coral reefs, you imagine their vibrant colors, but coral polyps are actually translucent animals." Yes, mostly. But some corals have their own pigments as well - for instance the coral Siderastrea siderea appears blue, not white, when bleached.

Belize, October 2005: bleached Siderastrea siderea appeared bright blue on our transects

"Reefs get their wild hues from the billions of colorful zooxanthellae (ZOH-oh-ZAN-thell-ee) algae they host." No. Reefs get their wild hues partially from the zooxanthellae hosted by corals and other organisms (for instance, Tridacna clams and anemones also host "zoox"), but other colors on the reef are thanks to other organisms: sponges, algae, etc. To be accurate, this could read Corals get their wild hues... or Reefs get their wild hues partially...

"These algae use photosynthesis to survive and then this process adds nutrients to the environment which benefits the coral..." Sort of, but photosynthesis does not create nutrients, it uses them up (nitrogen, phosphorus, etc.), and produces organic carbon (sugars). The coral benefits from organic carbon translocated from the algae to the coral fully inside the tissues, not leaked to the environment and then recaptured.

"Coral Reefs Are Carnivores"
Yes, corals are carnivores.

"The stomach cavities corals in a reef system are interconnected. Food obtained by one polyp can be passed to other polyps in the colony." That first sentence is weird...maybe it should have an "of" before "corals"? Even still, it's not accurate: the stomach cavities of corals in a particular colony are not connected, and different colonies in a reef system are definitely not connected at all. Instead, corals have connective tissue called coenosarc between each polyp. Sugars and nutrients can move between coral polyps in a colony through this interconnected tissue.

"Coral Reefs Are a Map to Climate Change"
Ok...I'm not sure what this actually means but it's not hugely weird right off the bat.

"Corals are so sensitive to changes in the world climate that scientists study coral reef fossils to construct highly detailed chronologies of prehistoric climate patterns." Well, yes, that's pretty much true. One way that we do this is by analyzing the chemistry of coral skeletons to reconstruct past environmental conditions. Another way is to actually map the locations of fossil reefs, then date them (using other geochemical techniques) and figure out where that bit of Earth's crust used to be (like by using paleomagnetism of nearby rocks). This can give hints as to what the climate was like in that location during the time those fossil reefs flourished.

"When corals experience increased water temperatures, mass coral bleaching can occur.  When coral polyps, stressed by temperature or a variety of other environment factors, they expel the symbiotic algae that live within their tissues. When the algae are expelled, the coral appears white or “bleached.”" Yes, if you can skip over the grammar issues with the middle sentence, this is on track...but it doesn't really build on that first sentence about fossil corals above. What does coral bleaching have to do with fossil corals? Well, we could use old corals to estimate the occurrence of bleaching in the past...but that would require much more explaining than the article seems to want to provide.

Some non-coral reef inhabitants: sponges (orange) and algae are also colorful and important parts of the reef.

"Coral Reefs Have Showy Sex Lives
Some species of coral reproduce by coral spawning. This means that in unison and in some cases on one particular night a year, the coral eject large quantities of eggs and sperm into the surrounding water. This always happens at night and just after the full moon. Trillions of eggs and sperm are released all at once. When this occurs, the eggs and sperm fertilize in the water and then if the larvae that grow survive, they settle back to the ocean floor, attach themselves to a hard surface and grow."

I actually like this last mini-section. Coral spawning does sound pretty amazing, from my colleague's accounts (do read that article, it is gorgeous). Of course, the article doesn't want to end on a depressing note, since it is geared towards increasing viewership of a particular Nat Geo TV program (which looks amazing). But that last sentence holds a lot of importance - if the larvae that grow survive. That critical stage is one of the most delicate, where environmental impacts that might not kill adult corals can be the most problematic. Then the baby corals have to attach--where they choose to do so can also be critical.


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