Not quite marine snail herds but I have watched Pomacea bridgesi swarm from
all ends of the tank (I have about 19 in the "Mollusk friendly.. all mollusks,
all the time" tank) to a single medium piece of apple within about 5 min.
(they like the Galas the best.. as do I *grin*) They move very fast (well for
a snail) and if one is somewhat slower than the others, they will crawl right
over the top of it. They then crowd out the others for the little buffet and
when it is gone, they all swarm away at the same time to wherever they were.
The whole process takes about 30 min. It's the most hysterical thing I have
ever witnessed (except for that darned octopus who pulled me into the tank...
long story)  Whoever said snails were slow!!!!!!!
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Sarah Watson