Originally Posted by A&A2
How can you tell it has been bleached? Any how do you know the original color?
One look tells that it is somewhat bleached. I will provide a google search on zooxanthellae below. In an animal such as an anemone this zooxanthellae is usually a yellowish tan to brown color, but depending on nutrients and photosynthisis you can also see reds, blues and other hues. But for the most part tan to brown.
Zooxanthellae- are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans. They are members of the phylum Dinoflagellata and are typically dinoflagellate algae, although algae such as diatoms can also be zooxanthellae. They may be acquired by direct ingestion, and subsequently reproduce by splitting apart; a process known as budding. In other cases, zooxanthellae may be transmitted by the coral eggs and planulae. Most are autotrophs and provide the host with energy in the form of translocated reduced carbon compounds derived from photosynthesis. In return, the coral provides the zooxanthellae with protection, shelter, and a constant supply of CO2, required for photsynthesis. Their population in the host tissue is limited by available nutrients and incident light, and by expulsion of excess cells. However, zooxanthellae do not appear to be digested by their hosts.
Hermatypic (reef-building) corals have zooxanthellae and are largely dependent on them, limiting their growth to the photic zone. The symbiotic relationship is probably responsible for the phenomenal success of corals as reef-building organisms in tropical waters. However, when corals are subjected to high environmental stress, they can lose their zooxanthellae by either expulsion or digestion and die. The process known as coral bleaching occurs when the zooxanthellae densities within the coral tissue become low or the concentration of photosynthetic pigments within each zooxanthella decline. Color loss is also attributed to the loss or lowering of concentrations of Green Fluorescent Proteins (GFP) from the cellular pigments of the cnidarian itself. The result is a ghostly white calcareous skeleton, absent of zooxanthellae, with the inevitable death of the coral unless conditions improve, allowing for the zooxanthellae to return.
Coral are under constant disturbance, which is ultimately felt by the zooxanthellae living within their tissues. Exposure to air during extremely low tides or damage from intensifying solar radiation in shallow water environments are some of the ecological stressors zooxanthellae face. Temperature changes have provided the most stress to the zooxanthellae-coral relationship. A rise in temperature of 1-2 degrees Celsius for 5-10 days or a decline in temperature of 3-5 degrees Celsius for 5-10 weeks has resulted in a coral bleaching event. Strong temperature changes shock the zooxanthellae and cause them to suffer cell adhesion dysfunction which sees the detachment of the cnidarian endodermal cells from the zooxanthellae.
Other organisms which may have zooxanthellae include jellyfish, clams, sea slugs, and radiolaria. There are several different species of zooxanthellae, typically grouped together as the genus Symbiodinium, which appears to be monophyletic.
