Hello Carlos,
 
I don't know how "exact" I can be on the range of Siphonaria pectinata, but I
do know it is found on both sides of the Atlantic.  On your side it occurs in
the Mediterranean, the Azores, the Cape Verde Islands, and west Africa at
least as far south as Liberia, but I don't know how much farther south it
might extend.  I don't think it is found in South Africa proper.  I once read
an item suggesting that Siphonaria capensis, from South Africa, might be a
form of S. pectinata, but I have specimens of both, and they are quite
different in my opinion.
In the western Atlantic the species is found in Florida and on the Gulf Coast,
including Texas and presumably beyond.  Also in many of the Caribbean Islands,
and in Brazil, at least south to Rio de Janeiro.  Again, south of that I don't
know.
Over here, S. lepida Gould, S. brasiliana Reeve, and S. lineolata Orbigny are
synonyms.  Over there S. grisea (Gmelin) is a synonym, and I'm not sure about
S. algesirae (Quoy & Gaimard).  The S. algesirae I've seen were bigger and
heavier than any S. pectinata I've seen from anywhere else, but they could be
conspecific.  S. algesirae is common in your area, so you may have an opinion
on that - if so, I would appreciate hearing.
 
Regards,
Paul Monfils