La Jolla, CA (03/16/04)- The traditional classification system used
for determining which families coral belong to is out, and DNA taxonomy is
in.
DNA analyses of dozens of corals in both the Atlantic and Pacific oceans
show that many have been misclassified and the old system was wrong.
Indeed,
some corals found in both oceans thought to be closely related are actually
distinct from each other, while others are more closely related
than previously believed. These findings, by an international team of researchers,
have turned coral taxonomy completely on its head.
Not only that but the study, published in Nature, shows that conservation
efforts for coral need to focus on a wider number of reef areas if distinct
families are to be protected. Traditionally, corals have been classified
by looking at specific morphological features, but this is now inadequate,
said Nancy Knowlton, PhD, from the Scripps Institute of Oceanography where
she is director of the Center for Marine Biodiversity and Conservation. "Our
results show that morphological studies, as conventionally done, give very
misleading versions of the coral family tree," she said.
The study began as an attempt to determine the closest relatives of a few
corals from the Caribbean that she and her colleagues were studying. But
right from the start they found that DNA analysis showed lineage classifications
that differed from what conventional taxonomy and morphology showed. The
study was expanded to include corals from Brazil, Japan, Taiwan and Palau.
Entire lineages had been misclassified.
Traditionally, features called septal
teeth were used to classify some families
of reef-building
coral. Tiny creatures called polyps secrete a calcium compound
that, over time, build not only the cup-shaped abodes the polyps live in
but entire reefs. The septa are spoke-like calcareous structures that provide
supports to the skeletal-cup the polyps live in.
This study shows, for the first time, that there is a family of corals unique
to the Atlantic and that there is no pattern for septal tooth length in terms
of determining which corals are related to which. It had been assumed that
big-toothed coral in the Atlantic were related to big-toothed coral in the
Pacific, and
small with small. Researchers, however, also identified features other than
septal teeth of the coral skeleton that show promise for helping identify
families. This is important because it can help with classifications of fossil
corals which lack DNA.
Researchers focused their studies on the Faviidae and Mussidae families
which comprise a third of all reef-building coral in the Atlantic. Samples
were taken from dozens of types of corals within those two families and submitted
to detailed DNA analyses. DNA was taken from three genes: two from the mitochondria
and one from the nucleus.
"It's important to have them from the nucleus as well as from the
mitochondria because they're inherited separately. It gives you two completely
independent
sources of evidence for evolutionary relationships," Dr. Knowlton said.
Researchers selected these specific genes for study because they were known
to mutate
at a pace that matches the evolutionary history of coral families.
"Some genes change really fast, almost too fast. They keep changing
and changing, sort of overwriting the evolutionary signal. In some cases
genes
change so slowly that even things that have been separated for a long time
haven't accumulated enough mutations to tell them apart. We looked long
and hard for genes that were changing at the appropriate rate," Dr.
Knowlton said.
The large number of samples studied provided an impressive amount of data
to support the fact the existing family trees for coral were wrong. Another
problem with using the septal teeth as a classification tool is that they
are a feature that changes fairly rapidly.
"They keep getting invented over and over again," she said. Rapidly
changing features make it difficult to do such detective work. The researchers
estimate that the Atlantic and Pacific corals diverged about 34-million years
ago.
An important consequence of this work ties in with conservation efforts
of coral reefs. When threatened hotspots were identified and ranked a
number of years ago by environmentalists, the rankings were based on the
known coral species, ignoring family relationships. This new research
shows that the patterns for priority need to be changed now that the family
trees have been reorganized and new, unique members identified.
"There weren't supposed to be any major groups of coral that were only in
the Atlantic. Our study showed that this assumption was wrong," she said.
In order to preserve biodiversity, Atlantic corals need protecting too.
end
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