Many corals have bright BLUE tips...Why?

[ishmael]

I am no scientist, but my thoughts....
Maybe to offer a beacon so that large fish will avoid breaking the new growth?
Or to warn other corals that I am claiming this space?
Some corals do grow with some sort of "awareness" as to what is near them, no? How do they sense that?

[ReefWreak]

Most corals I've seen that were growing have white tips, not blue.

[scaryperson27]

He didn't say most, He said many.

I have no idea BTW.

[greenbean36191]

I wouldn't presume that there's any evolutionary advantage of the blue tips. They're not common in the wild and fish avoid running into the corals just the same. Other corals have no way of knowing that their neighbors have tips of any color. The only awareness corals have of their neighbors is through chemical cues, and touch when things get really close.

I would assume that the blue tips are just another sort of random genetic variation and since they look nice they're selected for frequently for the hobby.

[Bebo77]

i think that most corals are green and or brown....

[Grunt007]

Are we talking coral on natural reefs or aquariums?
In aquariums I'd assume it may have to due w/ the spectrum of the artificial lighting systems being used, as well as being attractive and thus popular.

[goreefer]

And I thought that it was like women wearing eyeliner...

[xtm]

It is to tell the aquarist that he is doing a great job.

[dragonforce]

lol I'm with xtm on this one hehe

[BeanAnimal]

chics with blue eyeliner....

not most, but many? What does it mean? Is it a beacon advertising the worlds oldest profession or just common in the city due to trendy fashion?

[ishmael]

BeanAnimal,
GREAT POST!

[LockeOak]

I may be mistaken, but it's my understanding that the lighter-colored growing tips and edges of scleractinian corals are because those areas have not yet been colonized by zooxanthellae. Zooxanthellae (Symbiodinium spp) are brown to golden brown in color, both as single cells and as massive colonies outside of the host in culture flasks. As the coral grows, the zooxanthellae spread into and through the growing coral tissue. Corals without zooxanthellae look very pale even to white, hence the term coral bleaching. If a tip or edge is colored (blue, for example) the color is due to protective pigments produced by the coral. They're produced everywhere, but in most of the coral structure there are also zooxanthellae present to darken the color. These pigments serve to protect the coral and zooxanthellae from UV radiation and shade the zooxanthellae a bit, full sun is way more than their photosynthetic mechanisms could handle and would only damage them (they get "overloaded" and start producing oxygen radicals and other nasty things that will shred the cells). As for why blue in particular, um... I don't know

[greenbean36191]

What are the most common colors for protective proteins? Green and blue.

[BeanAnimal]

Yes, those colors that fluoresce under UV light are thought (proven?) to be protective.

[rppvt]

LockeOak -- took the words right out of my mouth.
Thought that the lack of algae coupled with thinner, newer layers of calcium/mineral growth might be responsible for the bluish color. More mature areas might be more opaque.

[LockeOak]

rppvt - Right. The living tissue there is very thin over the white skeleton and lacking zooxanthellae, hence a pale color.

As for which colors are protective, Dunlap and Schick (1998), "Ultraviolet radiation-absorbing mycosporine-like amino acids in coral reef organisms: a biochemical and environmental perspective" goes into it pretty well. Mycosporine-like amino acids (named so because they were first identified in a fungal fruiting body) absorb light very strongly in the 310-360nm range, which covers both ultraviolet A and ultraviolet B radiation. They're found not only in corals but also in sponges, anemones, algae, flatworms, fish and others. There are 19 distinct MAA compounds. They also state that blue light and UV induce MAA production but red or green light does not. (As a side note that I found really cool, they note that a possible explanation for nighttime spawning in corals is that the coral sperm are too small to contain enough MAAs to protect them from UV). The source of MAAs, whether the coral or the zooxanthellae, seems unclear from this paper (though I would say it leans toward production by the zooxanthellae) so I may have been mistaken about that earlier when I said they were produced by the coral. If they are produced by the zooxanthellae they are likely transported through the coral tissue. Note that there are other colored proteins aside from MAAs that definitely ARE produced by the coral host, including a green fluorescent protein and pocilloporin, which is pink and found in Pocilloporidae.

These are my own thoughts: Given that they are absorptive to UV, it would make sense that these pigments would generally appear to be blue or green, since they would absorb photons in the 310-360nm range and fluoresce at a slightly longer wavelength, 400-500 is a blue-green color.

[ishmael]

Thanks for the scientific explanations.

I guess where I cause myself problems is this....I think of the tips of the coral as a way for the coral to "sense" what is happening in its environment....where the light is and what direction afford the greatest light strength. I hate to state it in such simple terms, but sort of the "eyes" of the coral. Is this ridiculous? Could the coral somehow derive information by the different reaction that the zooanthellae at the tips recieve than the other parts of the coral?

[ishmael]

Here are two of mine....

[greenbean36191]

The growing tips are just that. They have no specialized sensory function. The whole coral is sensitive to differences in lighting, flow, etc. and the concentrations of things like like antioxidants, heat shock proteins, protective pigments, and zooxanthellae vary across the colony accordingly. The individual polyps also regulate things like the rate of gas exchange and photosynthesis through expansion based on their internal chemistry. If the zooxanthellae are cranking out too much oxygen, the polyps can contract and allow less light to reach them.

MAAs come from the diet and the zooxanthallae, not the coral itself and AFAIK they don't contribute much to the bright coloration of the coral. The bright colors are mostly due to pocilloporins (which includes the fluorescent proteins) that the coral itself makes.

[LockeOak]

Yes, I think you've got it. Maybe not "sensing" exactly but the coral certainly does receive feedback from the zooxanthellae in the form of their photosynthate (product of photosynthesis, sugars and other compounds) production. The zooxanthellae then may do most of the "seeing" for the coral. It may be as simple as growth being upregulated by photosynthate, so that lit areas grow faster than shaded ones which may not grow at all. In addition, individual polyps must be able to sense other polyps of the colony around them in order to coordinate enough to make the regular growth patterns we recognize.

It may work something like this (and this is speculation here, coral growth patterns are not something I've studied): The coral, say Acropora, has a genetic set of rules for growth: once it's encrusted to diameter X start growing upwards, once the main branch is Y centimeters long start to branch into Z different branches, make branches B millimeters thick, etc. The action of all the polyps working together and able to sense the density of polyps around them would be enough to control that. Now if the zooxanthellae near the tip of a growing branch aren't producing anything because they're shaded, the lack of photosynthate may just stop the coral from growing in that area (either because the polyps are food limited or the lack of photosynthate turns off growth genes), the effect being that the coral won't grow into shaded areas but divert its energy into growing into lit areas.

I don't KNOW that this is how it works or anything about the molecular/genetic mechanisms, but it seems plausible to me.

[ishmael]

greenbean36191 and LockeOak,
Thankyou for these explanations . They give me an even greater appreciation of the beauty of nature.

[Grunt007]

That was a good read! Thanks.

[hahnmeister]

Perhaps you just have something for corals with blue tips, and thats why you think there are so many. When I first started reefing, I stocked my tank with all corals that were purple and green I realized...

-Green Slimer
-Purple Digi
-Crocea clams
-GSP
-purple shrooms
-Green zoas
-green milli
-purple milli

It wasnt that there are just so many green and purple corals as much as the fact that I was just buying all green and purple corals.

[bernie lyons]

Changing to different kelvin halide bulbs will manipulate the uv pigment on the sps corals thus giving some high intense colors .
I've used 10k than switched to 20k bulbs which brought out very intense blue's and purple colors on the tips .
bernie lyons

[ishmael]

Yeah, I asked the question wrong. I have a beautiful red branching acro with pure white tips. I should have posed "Why do tips of corals have a tendency to be distinctly colored?" Like I said, I am no scientist......
Tips, being new growth and all, I just wondered why the special characteristics......