Kurt,

Yes, a great two cents!  A nice complication to the habitat choice of Vitta
usnea.  This is the first I have read of an observation of the link between
Vitta usnea habitat, and river topography and the availability of limestone.
But it makes sense and will make a nice quote for the write up for that
species.  Thanks.

In fact, the occurrence of the European nerite Theodoxus fluviatilis (a
species with a proven ability to successfully inhabit areas of a wide range
of salinity from pure freshwater to a salinity of up to 17%) in the U.K. has
much more to do with Ph and the availability of limestone, than salinity or
temperature.  This species requires hard water (20-30 Mg of calcium per
liter) and this seems to be the limiting factor for its lack of appearance
in certain areas.  It is absent from both the Cromwell and Devon areas in
England, areas with soft water (Fretter & Graham, 1978) and its presence in
Ireland almost matches perfectly that country's distribution of
carboniferous limestone (Lucey, et al., 1991).

So our very common Vitta usnea has this really wonderful and complex
ecology!

Tom Eichhorst

> -----Original Message-----
> From: Conchologists of America List [mailto:[log in to unmask]]On
> Behalf Of Kurt Auffenberg
> Sent: Wednesday, August 11, 2004 10:08 AM
> To: [log in to unmask]
> Subject: Re: Freshwater barnacle
>
>
> I may as well throw in my two-cents....
>
> Although I've found usnea in fresh water, there is always some tidal
> influence down near the bottom of the river due to the salt wedge.  Salt
> water is heavier than fresh and as the tide comes in, the salt
> water slides
> in under the fresh water.  So although the water is fresh at the surface,
> it is saltier near the bottom.....where many of the usnea live (but not
> all...as they are often found on tree trunks, etc. above the salt wedge).
>
> I've noticed that the river topography influences how far usnea extends
> inland.  If the land is very flat, the salt wedge extends further inland,
> and so does usnea.  A steeper gradient keeps salt water and usnea confined
> closer to the sea.
>
> Limestone is very shallow along the west coast of north central
> Florida.  In Dixie County the limestone is virtually right at the surface
> and fresh water seeps out of this limestone at low tide.  I have found
> hundreds of usnea lined up along the limestone in the fresh water at low
> tide, with none even a couple meters above the fresh water intrusion....
> This zone is covered with tidal water at high tide.....
>
> Kurtles
>
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