Our Leptastrea coral recently started turning lighter and more brown in color and we became concerned for its health. We started researching what causes corals to turn a lighter color and found that coral bleaching may be what is going on here.
As you can see in the above photo’s, our Leptastrea coral has been turning lighter in color, and the white skeletal base is more prominent than it was before when the polyps are not extended.
Our first instinct was that it was getting too much light, so we moved it to a more shaded location. Within a few days it started to darken again and seems to be ok in this new location.
As it turns out, coral colors fading in home aquariums is mainly due to lighting. In the wild however, corals losing their color completely (also known as bleaching) is due to high water temperature, pollutants, and disease. When this happens, the zooxanthellae (which is a symbiotic algae) is expelled from the coral and turns it completely white.
From my reading, what may be happening to our Leptastrea is this: the zooxanthellae produce chlorophyll which then reacts with the aquarium lighting to produce food for the coral, otherwise known as photosynthesis. Too much light can cause an increase in oxygen from the photosynthesis process and can be toxic at high levels. The coral is then expelling some of the zooxanthellae to compensate for the increased production of oxygen. The coral turns lighter and browner because zooxanthellae are yellow and brown in color. The loss of color is causing the underlying coral skeleton to become more visible.
Something else we noticed was a dark, almost black spot. On closer inspection, a piece of sand had fallen in one of the mouths of the Leptastrea. We believe this is what caused this particular location to turn dark. We removed the sand and the dark spot is starting to return to a more normal color.
In conclusion, I don’t think that our Leptastrea’s health is failing, it is simply adjusting to the new environment. And since we like the darker more vibrant colors, we will keep it in a more shaded location. With the exception of the black spot from trying to ingest sand, the coral seems to be quite healthy. In fact, the rock where the Leptastrea was perched has a small patch of new growth where the coral was spreading!