Hi Caroline,

Arca is a genus of bivalved molluscs commonly known as Ark Shells.  The
Latin noun "arca" actually means a box or chest, the genus name derives from
the resemblance of the closed shell to a ship (Noah's Ark).  In fact there
is one species named Arca noae, meaning Noah's Ark.  Species of Arca are
found in most temperate to subtropical areas, including the Caribbean,
Mediterranean, and both sides of the central Pacific.  These bivalves attach
themselves to hard surfaces by means of byssal threads, a tuft of tough
fibers which the animal secretes.  Some of them are shallow water species,
others deep water.  Probably the commonest species is the Florida/Caribbean
zebra ark, scientific name Arca zebra.

Paul M.