I was surprised to hear of a mud or sand location for Arca.  Those I am
familiar with are always attached to coral or rock here and other
Pacific Locations.  Very juveniles are found moving about, and in my
photo aquarium, some Arca can move quite rapidly, and can make a
reattachement with byssus in a minute or two when they are inclined to
do so.

How many species are usually found in sand/mud?
--
                     Aloha from Wesley M. Thorsson
Editor of Internet Hawaiian Shell News, a monthly Internet Publication
           122 Waialeale St, Honolulu, HI  96825-2020,  U.S.A
       http://www.hits.net/~hsn                 [log in to unmask]