- Scientists think they have finally identified
one of the main reasons why so many deformed frogs have been turning up
in the US over recent years.
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- Two new pieces of research strongly suggest
that parasites are to blame. The issue has been a major concern since a
group of schoolchildren in Minnesota discovered a strikingly high rate
of limb deformities in local frogs in the mid-80s.
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- Many suspected pesticide pollution was
the cause. In particular, environmental campaigners pointed to a certain
class of powerful chemicals known as retinoids. These are capable of interfering
with the growth of limbs in amphibians.
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- But a study by Stanley Sessions and colleagues
at Hartwick College in Oneonta, New York, shows the type of deformities
being seen in the wild are more characteristic of the damage done by small
parasitic flatworms called Riberoria trematodes.
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- Rearranged bud cells
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- These creatures burrow into the hindquarters
of tadpoles where they physically rearrange the limb bud cells and thereby
interfere with limb development.
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- "It's about as close to using an
egg beater on the limb bud cells as you can get," says Sessions. His
team studied five species of frogs from 12 different locations in California,
Oregon, Arizona, and New York.
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- In a separate study by Pieter Johnson
and his colleagues at Stanford University in California, tadpoles were
exposed to trematodes under carefully controlled laboratory conditions.
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- The tadpoles grew into frogs with deformed
hind legs that closely matched those found in the wild. In contrast, tadpoles
raised without any exposure to parasites developed free of deformities.
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- "It worked much better than I had
expected," says Johnson. "We found a high frequency of deformities
even [under conditions with] low parasite density."
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- Other deformities
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- Both teams stress this is not the end
of the story. Other kinds of frog deformities are being reported, including
missing or malformed limbs and eyes, which may be caused by something other
than parasites.
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- "We've chipped off the most mysterious-looking
piece of the deformed limb puzzle," says Sessions. "That's how
we have to do it. First we have to figure out what's causing these deformities.
Then we can roll up our sleeves and look at what's happening over time.
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- "We need to ask whether the rate
of deformities has been increasing, and if so, why?"
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- And both men suggest chemicals may still
have an influence, even if only indirectly. It is possible, they say, that
chemical leaching from fertiliser-runoff is spiking water courses with
excess nutrients.
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- This would boost the numbers of aquatic
snails which play host to trematodes at an earlier stage in parasites'
development.
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- The research is published in the journal
Science.
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