The answer may be more temperature than depth, since many molluscs live in
deeper (50-100') water in the Gulf of Mexico than in the Keys and south
(intertidal even).
Peggy
Shell Collecting Trips Around the World
Visit my website at http://www.mindspring.com/~shellelegant
Peggy Williams
Shell Elegant
PO Box 575 * Tallevast FL 34270
(941) 355-2291 * [log in to unmask]