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Coral lighting

How Corals Grow

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Coral lighting

175g Reef Tank

Vince Motte
By Don Carner

At the end of Chapter 3 I mentioned SPS corals and the necessity for proper CRI and intensity. First and foremost, most corals can be placed into three major categories: Stony (exterior skeletons), Soft (no calcium-based skeleton), and SPS (Short/Small Polyp Stony). Naturally, the scientific world can categorize these animals down to the "inth" degree, but for our purposes these three categories will suffice.

When it comes to how corals grow, each category of animal requires its own unique lighting requirements, but all share one common trait; photosynthesis in order to survive. Just as plants convert sunlight to produce chlorophyll, marine animals survive similarly by converting light energy into "food". Actually, this energy is consumed by zooxanthellae algae that produce by products that the corals need to survive; a true symbiotic relationship.

How Changes In Lighting Effects Corals

Coloration of coral polyps and tissues is dictated by these zooxanthellae. By differing the spectral output of our tank lighting we can actually influence the ultimate color/shading of our corals. How? Let's say that we have been using a 5500K VHO fluorescent setup at 220 watts. We get that itch to spend money and help our tank and animals by installing a 250 watt metal halide with a 10,000K lamp. Aside from the aesthetics of the rippling light show these lamps provide, we have suddenly changed the frequency of light that all the animals in our system have grown accustomed to. I use the term "frequency" to describe the change in CRI or spectral output.

Often, the corals will shrink up, close their polyps, or otherwise show their displeasure at this sudden and drastic change in their energy source. It's actually the billions of symbiotic algae that are recoiling, sending shock waves through their host and causing this rapid change in appearance. Within days, and sometimes even hours, the zooxanthellae will adapt to this new frequency and intensity by changing their absorption capabilities or their overall color. That's right, the coral's color is actually that of their hitch-hiking algae, adapting to the increases or decreases of ultraviolet and other energy-source factors.

Have you ever looked at a Tridacna clam from the top of the tank, then lower your gaze to a sideways view, only to be disappointed? Strange how dull the clam's color appears from the side, while from above all those rich and vibrant colors seem to shout at the sky? Well, that's the clam's zooxanthellae algae, doing their thing, protecting the clam's delicate tissues from sunburn!

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