Coral reefs are among the most fascinating environments we know. Talking about barrier reefs, immediately brings to mind a colorful world full of constructions of the most disparate shapes and inhabited by all kind of animals. But, if we observe corals displayed on the shelves of shops, museums or collectors, we find them all white or, at most, veiled brownish or pink.
Why?
Corals are invertebrate marine animals whose single individuals, called polyps, form colonies supported by a limestone skeleton built by the animals themselves. Polyps have a body equipped with tentacles that surround a mouth connected to an internal gastric cavity (Celenteron, hence the name of their group, coelenterata, which also include jellyfish). There are many types of corals of various shapes and sizes, but not all of them form reefs. Reef forming corals are know as calcareous corals or “hermatypic” corals. The reefs generally grow in clear, warm and shallow waters, up to a few ten of meters. This notable limitation is imposed by the presence, in the tissues of the polyps, of specific unicellular algae that live in symbiosis with them.
These are the zooxanthellae (dinoflagellated protozoa belonging to the Symbiondinium genus) which need sunlight to perform photosynthesis. And this is where the colors of corals come from, nothing less than the various photosynthetic pigments that absorb the different light radiations of sunlight.
This symbiosis is essential because the entire ecosystem of the coral reef is based on it and allows corals to have high growth rates in an environment that is otherwise rather poor in nutrients. During the day, the coral polyps are retracted within the skeleton and exploit the association with algae to receive oxygen and nutrients produced during photosynthesis (up to 90% of energy resources come from this source). During the night, the corals extend the polyps which feed on instead by catching particles of organic matter present in the water. In exchange, zooxanthellae obtain protection, shelter, nutrients (mostly coral waste materials) and carbon dioxide useful for photosynthesis processes. It has also been documented that the presence of symbiotic algae catalyzes and favors the calcium deposition process for the construction of the skeleton by the polyps, although the molecular mechanism is still being studied today. Note that corals can host a few million zooxanthellae per cm2, usually they inherit them from the parent and maintain this association for life.
BIOLUMINESCENCE
In recent years, many diving centers have been offering night dives to admire the bioluminescence of corals. In fact, in addition to photosynthetic pigments, the zooxanthellae of many corals have pigments which, when stimulated by ultraviolet light, return fluorescence of various colors. This phenomenon is still poorly studied but according to the latest research, the function of these fluorescent pigments is to protect algae and polyps from excess solar radiation, given that in latitudes where barriers develop, the sun’s radiation is very intense. Basically, excess sunlight inhibits photosynthesis and ultraviolet rays can damage the tissues of algae and corals. These pigments absorb UV rays and return them in the form of fluorescence. During the day it cannot be noticed due to the light but at night they offer a fascinating spectacle.
Then why do some corals appear white?
We have seen that barriers survive in rather limited conditions of temperature and brightness. When these conditions fail for a prolonged time, the corals are subjected to stress and eject the symbiotic algae from their tissues, revealing the white color of the calcareous skeleton. If conditions do not normalize for too long, the coral will no longer retake the zooxanthellae and will die. This is why we find white corals (“coral bleaching” or “bleaching”) in the middle of the reefs in a phenomenon that unfortunately is becoming more and more frequent due to climate change.
“So, the precious coral necklace I inherited from my grandmother isn’t red actually!”
Actually, not all corals get their color from zooxanthellae. The Mediterranean red coral (Corallium rubrum Linneaum, 1758), for example, is part of another group of corals and does not build reefs but lives in poorly lit environments such as walls and caves below 20 m depth. Since not enough sunlight reaches these environments, symbiosis with algae (which could not carry out photosynthesis) cannot be exploited to obtain nourishment and in fact the extended polyps appear very white. The branches of the coral instead have a characteristic bright red color. But what is it due to? The composition of its skeleton is still subject of studies but, according to the most recent data, the coloring derives from the presence of carotenoids plus other molecules whose function is not entirely clear: they could have a protective role, or, they could simply be the accumulation of what they eat (like flamingos that take on a pinkish hue due to the shrimp they eat). The fact is that, due to its coloring, this animal, exclusive to the Mediterranean Sea, has been widely exploited in the jewelry industry. Unfortunately, it is an organism with very low reproductive and growth rates and for this reason we are witness of the progressive impoverishment in our seas of the presence of this unique and particular animal. Today, red coral’s fishing is regulated throughout the Mediterranean basin, but there are still numerous episodes of illegal fishing.
The presence of zooxanthellae is also found in other marine organisms such as jellyfish, flatworms, sponges and molluscs. A noteworthy example are the majestic Tridacne, bivalve molluscs of spectacular size (the largest species Tridacna gigas reaches one and a half meters) of which many of you coud have seen the shell used as a holy water stoup in some churches in Europe (such as Basilica of San Nicola di Bari), and in whose mantle they host numerous symbiont algae that give it a strong color ranging from green to yellow and from blue to purple.
Finally, let me share with you a thought which I am forced to by moral and professional duty. It would be better to avoid buying or collecting coral skeletons of any kind. As I have already written, the ability of barriers to grow in nutrient-poor environments is amazing and this ability is based on a very fragile and precarious balance that we already affect in several ways. Therefore, removing the skeletons from the water removes a large reserve of calcium from the sea that cannot be reused by marine organisms. The same thing goes for shells and should be taken as a general rule. I also struggle to resist the human kleptomaniac instinct that develops in front of these beautiful natural masterpieces but the return of minerals to their environment is certainly a positive fact that can avoid disastrous effects on the balance of the ecosystem.
Published on IACC Italia (International Association of Colour Consultants and Designers) Link
References:
- Buck, Bela Hieronymus, Harald Rosenthal, and Ulrich Saint-Paul. “Effect of increased irradiance and thermal stress on the symbiosis of Symbiodinium microadriaticum and Tridacna gigas.” Aquatic Living Resources 15.2 (2002): 107-117.
- Cattaneo‐Vietti, Riccardo, et al. “Illegal ingegno fishery and conservation of deep red coral banks in the Sicily Channel (Mediterranean Sea).” Aquatic Conservation: Marine and Freshwater Ecosystems 27.3 (2017): 604-616.
- Cicogna, Fabio, and Riccardo Cattaneo-Vietti. Il corallo rosso in Mediterraneo: Arte, Storia e Scienza. CLEM, 1993.
- Cvejic, Jelena, et al. “Determination of canthaxanthin in the red coral (Corallium rubrum) from Marseille by HPLC combined with UV and MS detection.” Marine Biology 152.4 (2007): 855-862.
- Cvejic, Jelena, et al. “Reply to the comment of Fritsch and Karampelas on “Determination of canthaxanthin in the red coral (Corallium rubrum) from Marseille by HPLC combined with UV and MS detection” by Cvejic et al.(2007) Mar Biol 152: 855–862.” Marine Biology 154.5 (2008): 931-932.
- Eyal, Gal, et al. “Spectral diversity and regulation of coral fluorescence in a mesophotic reef habitat in the Red Sea.” PloS one 10.6 (2015).
- Fritsch, Emmanuel, and Stefanos Karampelas. “Comment on: Determination of canthaxanthin in the red coral (Corallium rubrum) from Marseille by HPLC combined with UV and MS detection (Cvejic et al. Mar Biol 152: 855–862, 2007).” Marine Biology 154.5 (2008): 929-930.
- Gianfranco Rossi, Le scogliere Coralline. Biologia ed Ecologia degli Ecosistemi delle Scogliere Coralline. Reef Check Italia Onlus, 2012. ISBN 8890678305
- Guidance For CITES Scientific Authorities: Checklist to assist in making non-detriment findings for Appendix II exports. IUCN, 2002.
- Salih, Anya, et al. “Fluorescent pigments in corals are photoprotective.” Nature 408.6814 (2000): 850-853.
- Smith, Edward G., et al. “Acclimatization of symbiotic corals to mesophotic light environments through wavelength transformation by fluorescent protein pigments.” Proceedings of the Royal Society B: Biological Sciences 284.1858 (2017): 20170320.
- Streamer, M., D. J. Griffiths, and Luong-van Thinh. “The Products of Photosynthesis by Zooxanthellae (Symbiodinium microad, riaticum) of Tridacna gigas and Their Transfer to the Host.” Symbiosis (1988).
