Hi Dominic,
Well there are some elements of this story that I would consider
suspect.  First of all, the details of the species' introduction into
Europe sound rather ludicrous.  Crepidula fornicata (the common
"slipper limpet" or "boat shell") probably did hitch a ride to Europe
on the bottoms of ships, but that happened a long time before the
1940's.  Linnaeus described the species in 1758, and (someone please
correct me if I'm wrong) I believe he described it from European
specimens, not American ones.  Undoubtedly the presence of Crepidula
in Europe dates back to the first transatlantic ship crossings -
because Crepidula does attach to any hard surface in the water,
including the undersides of ships and boats.  Also, consider that the
"landing craft" which, according to the article, carried the snails
ashore were carried in larger ships, high and dry, and any attached
Crepidula would have become desiccated and died.
As for the incredible reproductive rate described in the article, a
few considerations.  It is certainly true that a recently introduced
species can sometimes go on a reproductive binge in its new habitat,
which would never occur in the original habitat, to which it was well
adapted.  However, since this species has peacefully resided in Europe
for hundreds of years, why is it suddenly overreproducing at this
particular time, and more to the point, in the particular place
described in the article?  The species exists in Europe at least from
Iceland to Portugal (I'm not sure if it gets into the Mediterranean),
and its reproduction doesn't seem to get out of hand anywhere else.
That would cause me to ask what factors in the shallow bay around
Mont St Michel are causing this phenomenon.  Possibly water
pollution, with a great increase in the algae which Crepidula feed
on?  Also, I am rather puzzled about the reference to production of
great masses of slime??  I have seen many areas with heavy growths of
Crepidula (though nothing approaching what the article described), and
there is no slime on or around the snails.  Of course their soft
parts, inside the shell, are slimy like any other mollusk, but they
don't normally excrete it into the water - except - when exposed to a
polluting irritant in the water.  Is the "slime" simply the
decomposing soft parts of millions of animals, resulting from some
mass mortality event?  Keep your eyes open for any followup articles.
 I'd be interested in any additional information.
Regards,
Paul Monfils
Rhode Island, U.S.A.