bleaching-more food for thought

[Judith Lang & Lynton Land]

Hello everyone,
Holiday greetings! 
Several comments on Julian Sprung's most interesting message.
1. The several meanings of the word bleached.
---I suspect that, by now, it is too late to invent a new term to describe
the loss of the microalgal symbionts like Symbiodinium spp. that are
commonly called zooxanthellae (and/or the loss of photosynthetic pigments
from algal symbionts remaining within host tissues), but we can at least
clearly explain what we mean when we use the word bleaching.

2. The confusion between freshly-exposed white skeletons and bleached corals
(particularly acute when observers have poor eyesight and lack corrective,
underwater lenses).
---My experience at dive shops in the wider Caribbean is that the mass
bleaching events, especially during 1998, were such large-scale phenomena
that many amateurs have learned to tell the difference between bleached
(live but colourless soft tissues, through which the white of the underlying
skeleton is clearly visible) and parts of corals that are very recently dead
(for any reason), particularly the divemasters who watched some bleached
corals die whilst others gradually regained their symbionts.

Divers in general tend not to understand that coral polyps are also killed
by predators, competitors and parasites, in addition to severe bleaching and
biotic diseases.

3. Why do reef corals bleach at high temperatures?
While not disputing the possibility of a bacterial component, it is relevant
to point out that Robert Trench (pers. comm.) and his associates have
demonstrated in the laboratory.that Symbiodinium microadriaticum is less
tolerant of warm temperatures than its host, the jellyfish Cassiopeia
xamachana.

In other words, as temperature is increased, bleaching (loss of
zooxanthellae) may occur without mortality of the animal host, so long as
the thermal tolerance of the latter is not exceeded. Moreover, different
species of Symbiodinium have different temperature tolerances, which may not
 not correlate with their molecular genetic classification into clades. For
example, S. microadriaticum belongs to clade A and is sensitive, but S.
pilosum, which is also clade A, actually tolerates high temperatures
(Trench, pers. comm.).


4. The ability of coral hosts to survive without their algal symbionts.
---Bob and I can both remember when Tom Goreau Sr. maintained live, bleached
corals of several genera (mussids, Meandrina meandrites, Manicina areolata)
in simple aquaria, with regular feedings of exogenous particles, for
months-and even years-at a time.
Judy Lang