Coral Bleaching

[Dustin L. Laurence]

>Anecdotal reports of maximum acceptable temperatures vary considerably. 
>I normally avoid temperatures above 26 C, but believe I'm acting 
>conservatively. 

If you are interested in amateur anecdotes, I have one, if not, feel 
free to ignore this and I'll return to lurking. 

I'd say you are acting extremely conservatively, if you can control 
the temperature with reasonable accuracy.  This summer we have been 
running around 27C--this would have been a bit higher, perhaps 28C, 
but the temperature was adjusted low following a bleaching episode 
which I will describe.  I have not dialed the temperature up since 
then because I have not been here to observe the tank continuously.  We 
see better growth as temperatures increase, even up to within a degree 
or so of what we regard as maximum (and it's cheaper to run the chiller 
too, a useful bonus on a rather limited budget). 

Common wisdom among advanced amateurs is to expect bleaching around 29 
or 30C, which is what we observed.  Early this summer a thermometer 
failed, reading about 2 degrees low, and our tank was running around 
30C instead of an intended 28C.  This persisted for perhaps three weeks, 
since lighting was changed to higher Kelvin-rated metal halides around 
the same time and I was looking at possible UV-induced bleaching instead 
of temperature (and since the temperature read OK).  I think the 
lighting was a contributing factor, at least as a trigger, but I don't 
think it could have happened without the elevated temperature. 

After about a week of these elevated temperatures we began to notice 
bleaching.  The hardest hit were small fragments.  We lost two fragments 
of Seratiopora hystrix (I'm spelling these things from memory, and these 
are amateur NOT professional ids so beware) which had also been recently 
shipped, one unidentified Acropora fragment, and probably a couple of 
other that have slipped my mind.  A larger Stylophora pistilata fragment 
bleached completely but survived azooxanthellate for a month or two 
before dying.  I feel particularly bad about this specimen because if I 
had been here enough I think I would have had a good chance of saving it 
by feeding it in that critical month. 

Another unidentified Acropora sp. fragment from the same parent colony as 
the dead one just mentioned bleached but did not show tissue loss and has 
made a complete recovery, and a third partially bleached but survived car 
transport from Los Angeles to Sacramento and has reportedly also completely 
recovered.  A fourth, the largest, did not bleach and was also transported. 
Two unidentified Acropora sp. fragments from a different parent (of a 
different species) which were fragmented at the same time as the earlier 
four also did not bleach and easily survived transport.  Both parent 
colonies showed no obvious signs of stress. 

An Acropora humilis suffered total bleaching but only about 50% of the 
tissue subsequently died--it has since completely recovered its symbionts 
on the remaining tissue and is doing well.  A Millepora sp. suffered maybe 
90% tissue loss--I believe there is a small amount of still living tissue 
still but I doubt it can recover. 

The larger and better established Acropora and the other non-Acroporids 
were not noticably affected.  In addition to the two parent colonies 
mentioned above, another small Acropora colony and an Acropora fragment 
(both unidentified) survived without problems, as did two sibling Pavona 
cactus fragments, a Porites cylindrica, and a Montipora digitata. 

There were only two large polyped corals in the tank at the time, both 
Catalyphilia.  The larger and long-established one was damaged, but I 
think would have survived if I had been here to keep an eye on it and 
if it had not been subsequently heavily harassed by a Z. veliferum tang. 
This coral was on the extreme opposite end from the halide fixture that 
was upgraded and should have received very little from that light, so I 
think this must have been a purely temperature effect.  A small 
Catalyphilia that would have received some light from the upgraded bulb 
was unaffected. 

>I'd appreciate receiving actual observed critical temperatures from 
>various parts of the world. 

It would also be interesting to me to know if the critical temperature 
is significantly different in captivity. 

Dustin