Ross
If your friends put any pond weeds, even purchased ones in the tank at the
same time then that wuld be my guess as the likely source of the snails.
However, I believe some freshwater snails can aestivate for considerable
periods of time.  I had an experience where I was using a basin for cleaning
winter collected shells from streambanks.  In all cases, a few of the
beached and presumably frozen critters would miraculously come back to
life!  But the most amazing thing was that this basin was left dry for
several months and after soaking my orchids in it I noticed a small snail
stuck to the side happily crawling around!  There's no way it came from the
orchids - it was one of the species I had collected months before and only a
couple of millimeters in diameter.

Nora
Calgary, Alberta
CANADA

Ross Mayhew wrote:

> I habitutally give inexpensive or "commercial" shells to pretty well
> everyone i know, but the greatest surprise that has ever come from this,
> came last week, when a friend came up and said "You know those big
> shells you gave us a while ago?  Well, we put them in the kid's
> aquarium, and now we have a pile of snails crawing around the place!"
> Ok, i thought - what the heck is going on?
> These were an ancient Charonia and Cassis cornuta from the Phillipines,
> so i was puzzled in the extreme, until i saw the critters in question:
> Fresh-water snails of the genus Physa.  However, i had had these shells
> for a few years, and they had come from an old collection before that -
> so assuming that once upon a time, they had been left in a stream for a
> brief period of time, and had picked up either "hitch-hikers" or the
> eggs of same, it had still been quite a number of years since this was
> possible.  Oddly enough, the wee beasties cam in two distinct flavors -
> completely grown (over 20mm, after having been in the tank for less than
> 2 weeks), and rather small (less than 10mm), which indicates that both
> eggs and adults had survived in an aestivated form for at least 10
> years!! (None have passed on yet, so i can't get a photo, but i don't
> recall having seen this species in Nova Scotia, and in any case, Halifax
> has notoriously acidic soil and water, being built upon granite and
> slate: so, non-marine shell-makers are uncommon, except in gardens and
> lawns, where a man-made alkaline condition exists.  So, they could not
> have come accidentally from the stream which runs past their house,
> which has 75mm spiders, but no Physa that i have ever phound.
>
> I'd like to know whether it is an uncommon occurance for fresh-water
> snails to survive in a dormant form for many years, and what is the
> longest period known for the revival of same: has anyone ever left an
> amphora from King Tut's tomb, for example, in a wash-basin, and found
> 3000 year-old snails the  next day?
>
> Yours in The Wilderness of the Great White North,
> Ross.