[Coral-List] RE: identifying zooxanthellae

[Andrew Baker]

Dear Mohammad 

Fortunately, you are not alone in being confused about the taxonomy of
Symbiodinium, and I suppose for this reason it's probably a good time to
post some clarification to coral-list in case other researchers out there
find this discussion useful. 

There are eleven named species in the genus Symbiodinium (some of which have
been formally described, others have been less formally described but the
names are still used). All of these "species" have been described from
morphological description of cultured isolates. 

The actual diversity of Symbiodinium in nature is much greater than this,
but all of the data revealing this comes from molecular DNA studies. As a
result, most of the different Symbiodinium that have been documented from
DNA work have no name and no formal description. This is partly because few
people have had the chance to culture all the different types that have been
found in molecular surveys, and partly because there is a real problem
determining which types constitute different species and which ones do not
(sexual reproduction has not yet been directly observed in these organisms,
although there is fairly good genetic evidence indicating that it does
occur). 

In an attempt to come up with a reference system for all these different
types of "zooxanthellae" that have no names, it was recognized that these
different types could be divided into different groups (based on their DNA
sequences). Members of each of these groups are more closely related to each
other than they are to algae in other groups. These groups are the "clades"
that get referred to so often. It's a bit like grouping all if the people
who live in a small town according to which family they belong to. 

(A clade is a monophyletic taxon. What this means is that it is a group of
organisms that includes the most recent common ancestor of all of the
members of the group, as well as all of the descendants of that most recent
common ancestor.)

In Symbiodinium, at least seven different clades have been identified so
far. These clades (groups) have been referred to by different letters of the
alphabet (A, B, C, D, E, F and G). Some of these clades contain one or more
described (named) species, but others have no described species in them -
they are just unusual groups of algae that no-one has yet wanted (or been
able) to culture and describe formally as yet. Although there are lots of
undescribed and un-named types of Symbiodinium in culture around the world,
there is generally a lack of enthusiasm to describe them and give them names
because of the species problem I mentioned above, and also because it's a
terrific amount of work to do properly. 

So the term "clade" has been used as a convenient handle to refer to
different types of "zooxanthellae". However, as a reminder, each clade of
algae is really a group of closely-related (but nevertheless different)
types. So when referring to a particular clade it should be recognized that
this isn't really a complete identification. It just narrows down the
possibilities. 

I think the extensive use of the word "clade" in the literature has resulted
in many coral biologists who do not work specifically in this area to assume
that this term can be used interchangeably with the term "species". This is
not correct. The fact remains that we have no idea how many species are out
there; we just know that the group is very diverse. This should not
discourage us, however, because (beginning in the 1990s) plenty of
interesting findings were made by just identifying different algae to the
level of clade. Since about the year 2000 finer levels of resolution have
been developed that have been equally useful and now there are now many
recognizably distinct types of Symbiodinium within all of the different
clades. Unfortunately there is no agreement yet on how to refer to them all.
But perhaps we'll figure this out in the near future. 

One final thing: It should be emphasized that just because two different
types of algae belong to the same clade, they may not necessarily be closely
similar to one another physiologically or otherwise. Different algae in the
same clade have in some cases been shown to be quite distinct. So we should
be careful about making generalized statements regarding the properties of
particular clades. 

I hope this helps. For those who are interested, there is a Special Session
(Session 1-9) at the 10th ICRS meeting in Japan on Symbiodinium diversity
and its implications for corals and coral reefs. 

A recent review (December 2003) also discusses Symbiodinium diversity and
the issues raised above in more detail:

Baker AC. 2003. Flexibility and specificity in coral-algal symbiosis:
Diversity, ecology and biogeography of Symbiodinium. Annu. Rev. Ecol. Syst.
34: 661-689

I can send a pdf copy to anyone who is interested.

Sincerely

Andrew Baker

___________________
Andrew C. Baker, Ph.D.
Wildlife Conservation Society
Center for Environmental Research and Conservation
MC5557, Columbia University
1200 Amsterdam Avenue
New York, New York 10027, USA
Voice: +1 (212) 854-8184
Fax: +1 (212) 854-8188


-----Original Message-----
From: coral-list-bounces at coral.aoml.noaa.gov
[mailto:coral-list-bounces at coral.aoml.noaa.gov] On Behalf Of Fatemi
Sent: Thursday, January 15, 2004 2:46 PM
To: Coral-List at coral.aoml.noaa.gov
Subject: [Coral-List] identifying zooxanthelea

Sorry for cross posting

Could anybody tell me about how different species of zooxanthelea, Genus
Symbiodinium are recognized? is it according to the structural morphology or
to the genetical characteristics? what is the rule of clades in this
regards? I have read some papers in this regard, especially Baker's but I
could not understand cause none of them have mentioned anything about the
names of different species but only clades! May be my background knowledge
is not enough. Thanks.

Mohammad Reza
Iran
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