Deep skeletal refugia for zoox

J. Charles Delbeek delbeek at
Mon Sep 18 16:15:08 EDT 2000

On Mon, 18 Sep 2000, Andrew Baker wrote:

> Yes, there are a couple of places I would like to study zoox repopulation 
> following bleaching. My main project with regards to this (with Peter 
> Glynn) is looking at zoox repopulation in communities of reef corals in 
> Panama and the Galapagos. These communities showed quite high incidences of 
> bleaching, but not as much mortality as we might have expected (contact 
> Peter for more details). I have tagged colonies in both areas that we can 
> re-sample to see how zoox communities changed. These tagged colonies 
> include some colonies that were extremely bleached and other adjacent 
> colonies (of the same species) which appeared entirely unaffected (and - 
> yes - its because they appear to have contained different zoox). But we 
> don't know yet how many of the tagged colonies survived, or what zoox they 
> have now if they did. We'll find out in the spring, although (with the 
> intervening time period) interpretation might be difficult (but it usually 
> is anyway...!)

A couple of observations that may be of use:

In aquaria one of the big differences in light regiemes with that in
nature is the directionality of the light source in aquaria. As a result
it is not uncommon to see gradual lightening of the undersides of coral
colonies, not only branching species like Acropora but also plating
species such as Echinopora. We do not see this as much in our aquaria that
also get natural sunlight or are outdoors. Now if the zooxanthellae on the
undersides of branches/plates etc are supposedly low-light adapted, then
why do corals in artificially lit systems exhibit apparent zooxanthellae
loss in these shaded areas? I assume it is because the light levels fall
too low even for these zooxanthellae.

I recently returned from a week in Majuro and Arno atolls (Sept 1-9th) in
the Marshall Islands. In Majuro lagoon there is a small patch reef lying
just south of the deepwater channel into the lagoon. This reef is
dominated by Acropora species. There was no bleaching apparent except on
the very top of the reef where the water felt warmer and was about 1-2'
deep. What was striking was that on vertical branches of Acropora
(gemmifera?) one side of the branch appeared bleached but the other side
still had zooxanthellae, but there were also adjacent colonies that were
totally bleached and algal growth was already apparent. This reef is
rarely dived since seas are usually too high for the small dive charter
boats to cross the nearby channel, however, during this week westerly
winds provided calm waters and I assume, higher temps and lower water
flow, resulting in almost 90% bleaching of Acroporids in the shallowest
sections of this reef. Now, assuming that vertical branches get equal
amounts of lighting over the course of a day, why would only one side of
the branches on these colonies bleach? It seems counter-intuitive that the
zoonxanthellae populations would be different on opposite sides of
vertical branches, the undersides of horizontal branches ... I can see the
logic, but vertical branches??

J. Charles Delbeek
Aquarium Biologist
Waikiki Aquarium

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