As far as I know, all reported cases of paralytic shellfish poisoning (PSP)
have involved bivalve molluscs.  Gastropods generally do not become
infected, nor do Cephalopods (squid, octopus), Crustaceans (lobsters, crabs,
shrimp) or finfishes.  This is because bivalves are filter feeders, passing
large volumes of water through their bodies, and straining out any suspended
particles.  Such particles can include the organisms that produce saxitoxin,
primarily dinoflagellates of the genus Gonyaulax and a few other associated
forms.  These organisms are always present in sea water in small numbers,
and normally cause no problems.  But occasionally, for reasons not well
understood, they undergo a rapid and massive increase in number, a
phenomenon known as a "bloom" (the "red tide" being the best known example).
During a bloom, dinoflagellates may be present in concentrations of many
thousands of individuals per cubic centimeter of seawater.  A bivalve may
pass thousands of cc's of water per day through its feeding mechanism,
thereby concentrating millions of dinoflagellates in its visceral mass.
Gastropods are not usually a threat because they are not filter feeders, so
they do not tend to concentrate the organisms.  There could be some
theoretical danger from Gaspropods which feed on bivalves.  However, most of
the gastropods that are commonly used as human food are not carnivores; and,
the viscera of gastropods, unlike that of bivalves, is not usually consumed.
The economic impact of a major PSP outbreak is compounded because people who
read in the paper that "shellfish" are infected and dangerous, stop
purchasing not only bivalves, but also lobsters, crabs, shrimp, squid,
gastropods, and in some cases even finfish.  (It's a great time to purchase
lobsters, which are usually expensive.  The seafood stores drop the prices
drastically in an effort to get rid of them.)  A friend told me he had
passed up the escargot at a fancy buffet he attended.  I pointed out that
escargot are land snails which could not possibly be infected with a marine
organism.  But many people just react, rather than thinking it out.  Many
folks will not even buy canned clams or mussels during a local red tide
outbreak, even though these were imported from another country.  Of course,
all of this is not an absolute guarantee that all Gastropods are safe to eat
at all times.  They could be toxic due to some cause other than PSP.
Paul M.