Coral exhibits a range of colors, most commonly some shade of brown, red, orange, yellow, green, blue or purple. The specific color depends on the coral species, as well as factors like water depth and lighting conditions. While green and blue are less common coral colors, some species do display these hues.
What Causes Coral Color
Coral is made up of hundreds to thousands of tiny living coral polyps. Coral polyps produce a hard external skeleton made of calcium carbonate. The coral skeleton provides structure and protection for the soft, living coral tissue.
The colors seen in coral come from three main sources:
– Coral tissue – Coral polyps contain pigments like greens, browns, and reds that can be seen through the tissue. Darker, more saturated tissue typically overlays the skeleton and obscures it.
– Zooxanthellae – These are tiny, symbiotic algae that live within the coral tissues. Zooxanthellae provide corals with energy through photosynthesis and come in shades of yellow, brown and green.
– Coral skeleton – The calcium carbonate skeleton is naturally white. But it can take on the color of the tissue covering it. Skeletal structures like septa and corallites influence how much of the skeleton is visible.
Common Coral Colors
Most corals display warm, saturated colors like browns, yellows, oranges and reds. Shades of green and blue are less common, but some coral species do exhibit these cooler hues.
Brown Coral
Brown is the most ubiquitous coral color. It results from a blend of tissue pigments, zooxanthellae and skeletal color. Shades range from light tan to virtually black. Here are some examples of brown coral species:
– Brain corals – Named for their rounded, brain-like shape, these large corals can be shades of yellowish to dark brown.
– Star corals – Branching star corals often take on tawny browns. Encrusting star corals may show concentric brown and white banding.
– Boulder corals – Massive, rounded boulder corals are frequently dark brown to black in color.
– Finger corals – Branching finger corals may be light tan to golden brown.
Red Coral
Vibrant reds come from reddish pigments in the coral tissue as well as reddish zooxanthellae. Red is commonly seen in:
– Mushroom corals – Solitary mushroom corals are often bright red discs.
– Bubble corals – These contain extended red polyps that make them resemble bubbles.
– Tube corals – Branching tube corals have prominent red polyps that are often extended nightly.
– Trumpet corals – Solitary trumpet corals have elongated skeletons that can display rich red hues.
Orange and Yellow Coral
Shades of orange and yellow appear primarily due to golden-brown zooxanthellae and tissue pigments overlaying the white skeleton. Examples include:
– Elegance coral – Branching elegance coral takes on vivid orange.
– Cactus coral – Round lobes appear orange with lighter ridges.
– Cup corals – Solitary cup corals often show off yellow-orange hues.
– Cabbage coral – Boulder-shaped cabbages exhibit buttery yellows to oranges.
Purple Coral
Purple coral is quite rare. But some coral colonies can take on bluish to purplish hues, including:
– Acanthastrea coral – Can form purple encrusting colonies.
– Montipora coral – Some display blue or purple polyps.
– Dendronephthya soft coral – Branching soft corals sometimes called carnation corals for their purplish color.
Green Coral
While less common, green is seen in these coral species:
– Lettuce coral – Green leaf-like lobes make it resemble lettuce.
– Cauliflower coral – Has round green heads like its namesake vegetable.
– Tubastrea coral – Bright green polyps extend at night from these stony corals.
– Goniopora – Colonies of flowerpot-shaped polyps can be shades of dark green.
– Zoanthids – These colonial, anemone-like animals contain zooxanthellae that lend green color.
Blue Coral
Blue is perhaps the rarest coral color. But a handful of coral types can display blue hues, including:
– Heliofungia coral – Solitary mushroom coral that can be blue in deep waters.
– Pocillopora coral – Sometimes called cauliflower coral, branches can take on blue in deep water.
– Millepora or fire coral – Resembles blue-gray branched twigs. Despite the name, it is not a true stony coral but a hydrocoral.
What Makes Coral Blue or Green
Most coral color comes from brownish to reddish pigments in their tissue and zooxanthellae. So what makes the occasional coral exhibit rare blue and green hues? There are a few possible explanations:
Water Depth
Some corals can change color based on water depth. Shallow water coral may appear more vibrant and warm-colored due to exposure to full sunlight. But in deeper, dimmer waters, the same coral species can shift to cooler blues and greens.
Why does this happen? Dimmer lighting in deep water allows more light energy to get through the tissue and reflect off the white skeleton, giving some blue color. And with less light, the coral needs fewer pigments to protect from UV radiation. So less pigment means we see more of the underlying white skeleton.
Dense Tissue and Distinct Polyps
For blue or green to show, the coral skeleton has to be somewhat visible through gaps in the tissue. Corals with very dense tissue obscuring the entire skeleton tend to be more solidly brown or red.
But in some corals, the polyps are spaced apart enough for patches of skeleton to show through the thinner tissue between polyps. This can lend rare hints of green or blue especially if in deeper water.
Novel Pigments
A few coral species have pigments that lend blue, purple or green color that persists even in shallow water. This includes species like:
– Heliofungia actan – Contains an unusual pigment called actinioverdin that bestows blue.
– Dendronephthya spp. – Have pigments called dendronphthins that can create purple.
– Zoanthids – Contain zoanthoxanthin, giving some colonies a greenish tint.
Lighting Effects on Coral Color
External lighting conditions can also affect the perceived color of corals:
Sunlight
Full-spectrum natural sunlight elicits the most accurate coral colors. Lower light underwater shifts them to more muted or bluish tones.
Artificial Lighting
The type of artificial lighting used on aquarium corals also impacts their colors:
– Blue light enhances blues and greens.
– White light provides a balanced full-spectrum effect.
– Purple or ros