Tom:
 
I think you will find a number of families and species that travel in
large numbers at specific times.  In Guaymas, Mexico, we saw at least a
hundred Conus ximines in sand at 10 to 12 feet all heading for shore
while we were swimming to shore ourselves. Other days of the 14 we were
there, we would see one or two deeper.  This was repeated for starfish
and Murex.
 
In Hawaii, one day I sawa great many large Conus pulicarius in the sand
at 3 to 4 feet.  They were spaced not over 1 foot apart.  I never saw
that again.  I also saw at least 50 Costellaria pacificum all eating on
a dead murex.  Normally you wouldn't see more than a very few in a dive.
Many Strombus and Nassarius will be seen in crowds than there is the
right kind of food around.
 
Aloha,
 
Wes