Shells of many species congregate for different reasons.
 
In Mexico, we usually saw 0 to 2 of one species of cone.  One evening
swimming to shore we saw hundreds.  I have seen Conus pulicarious, all
much larger than normal in shallower than normal water off Diamond Head
almost every foot.  Going back weekly for a month I never saw a very
few, and they were smaller.  I had reported in Kaneohe ban Conus
quercinus being grouped in a number of circles (about 5 foot diameter),
all pointing inward.  In Mexico, we saw Murex of one species in piles of
10 or more not too far separated.  These were undoubtedly mating
atractions.
 
I have also seen about 50 Costellaria pacificum all feeding on one dead
Murex.  Put some meat down in many places you will see a number of
Olives arrive.  Same for other species that are naturally
scattered grouped for food.
 
In Hawaii Oliva paxillus generally live in tribes in a small area as do
Oliva porphyria in Mexico.  This is generally a sign of rapid settlement
of veligers, tending to make specimens in the tribe all more similar
than you would expect.
 
Aloha,
 
Wes