Now, Lets Talk More in Depth About Coral Bleaching

[Wy Renegade]

Now that we've taken the time to learn about Zooxanthellae, feeding, color and bleaching in marine host organisms; https://www.reef2reef.com/forums/general-reef-discussions/57712-lets-talk-about-zooxanthellae.html Lets look in a little more detail at coral bleaching.

Borneman, in his book Aquarium Corals; Selection, Husbandry, and Natural History describes bleaching as "probably the most likely problem to be encountered with corals, both in in the wild and in aquariums . . ." As we already learned, bleaching is the mass expulsion of zooxanthellae or the pigments within zooxanthellae from coral, which leaves the coral appearing white because their tissue is mostly translucent and their calcium carbonate skeletons are white. Please realize that just because a coral has bleached, it is not dead. In fact, Borneman states that "bleaching is a regular occurrence on natural reefs, and some level of bleaching is almost always present in some species . . . the expulsion of old, degenerate, or extra algae cells is always happening as part of the natural growth and regeneration of a coral." In fact, up to a 50 percent loss of zooxanthellae can occur without any visible change occurring in the appearance of the coral. Those with a higher loss of zooxanthellae may appear pale yellow or green and generally appear less healthy and exhibit poor tissue expansion according to Delbeek and Sprung (The Reef Aquarium vol. 1). Further, it has been proposed that bleaching in nature may give corals a competative advantage as they swap out and aquire new zooxanthellae which may be better adapted to the existing conditions. According to Borneman, If the bleaching is not too severe and the causative agents are removed zooxanthellae populations can be regained in a few weeks to a few months or longer. Even after recovery however, lost tissue replacement will take time, and growth rates may be slowed for a considerable time as a result.

There is no guarantee that a coral will regain its lost pigments or zooxanthallae however, as the bleaching event seriously compramises the integrity of the coral. Without their full complement of symbiotes, they begin to starve and become more vulnerable to disease.

 

[Wy Renegade]

What Causes Bleaching to Occur?

So specifically then, what causes bleaching? Borneman lists the following causes;

bacteria
chemicals
darkness
fungi
heavy metals
higher temperature
hypersalinity
hyposalinity
lack of water movement
light
medications
noxious agents
physical stress
red spectral light
sedimentation
starvation
stress
temperature change
as well as a few others

Delbeek and Sprung add the subsequent reduction of iodine and other trace elements as a result of the excessive use of carbon.

 

[Wy Renegade]

How Can We Prevent Bleaching From Occurring?

So how can we prevent a bleaching event from occuring? Delbeek and Sprung advise avoiding temperature extremes, being careful when moving corals into higher light intensity, and when adding new lighting fixtures or when replacing old lamps. Further, the authors recommend the following interventions in regards to water flow (keeping in mind that not all genera of corals tolerate high water flow);

"1. When changing lights or when moving to a much brighter lighting scheme, increase the water flow in the aquarium;"

"2. Increase the water flow when you expect temperature increases such as during the warmer months of the year or if you like to mimic nature and increase wate temperature during certain months;"

"3. Increase water flow if your cooling system fails and;"

"4. Increase water flow if introducing new specimens from dimmer aquaria or place these in areas with greater water flow."

 

[Wy Renegade]

How Do We Treat Corals that have Bleached?

So how can we treat or care for corals that have bleached? Delbeek and Sprung advise moving those in dim light slowly into brighter lighting over a period of several weeks and moving those that bleach from too much light to a slightly shadier region. They also advise adding iodide regularly to the aquarium as it appears to assist corals in adapting to strong illumination. If properly cared for, most corals can recover completely from bleaching.

 

[1Der9_3hy]

Wow tons of good info. Will have some nighttime reading ahead of me :)

 

[Wy Renegade]

Thanks. Again, I'm no expert, so if anybody has anything to add, please feel free to post it up.

 

[Troylee]

nice article randy i would replace temperature changes with temp being to hot.... if your tank runs warm or gets hot the corals seem to bleach from my exp. the colder the water the better the colors again just from my exp.im not a expert like borneman but temp swings don't cause bleaching from what i have seen it's just when they get warm...

 

[Wy Renegade]

nice article randy i would replace temperature changes with temp being to hot.... if your tank runs warm or gets hot the corals seem to bleach from my exp. the colder the water the better the colors again just from my exp.im not a expert like borneman but temp swings don't cause bleaching from what i have seen it's just when they get warm...

Hey Lee, appreciate the input. Check my recent thread; https://www.reef2reef.com/forums/zo...gades-bleached-polyp-recovery-experiment.html . Just my own experience of course, and the drop in temperature was certainly not the only contributing factor. The polyps were already showing signs of bleaching from the higher lighting, despite my having used the screen method to acclimate the corals. I do think that the sudden drop when the heater failed was the final straw. Interestingly enough all the other corals (primarily LPS) which were showing signs of stress due to the lighting change have recovered, only the polyps remain bleached.

 

[Alpha Aquaculture]

I think temp swings could cause bleaching if there are more stressors and the swing is larger.

I think the real issue is stress related to stability. They are used to the stable ocean. Corals once acclimated to the ocean have a lot of changing to do in captivity in order to survive. Thats why its such a good thing to buy aquacultured. Once a coral and its zooxanthallae are adapted to captivity the chances of a bleaching event are reduced considerably even after shipping and having to acclimate to a new captive environment. The ocean to aquarium jaunt for wild corals is a huge part of bleaching as we see it in captive reefs ime.