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SHELL SEEKER: Jewel Boxes From The Sea

By Staff | May 20, 2010

The Florida spiny jewelbox (Arcinella cornuta) is a bivalve.  Like most

bivalves, it is a filter feeder, feasting on organic particles strained from

the sea.  The body of a bivalve is surrounded by the mantle folds and the

two shell plates it produces. A ligament holds the two shell halves tightly

together.  The hinge area is where the shell begins to grow.  Bivalves are

important food sources for gastropods, fish, shorebirds like sandpipers, and

humans.

The Florida spiny jewelbox is about 1 1/2 inches long.  It is a thick,

sturdy shell that is off-white in color with an interior that may resemble

the china used for Thanksgiving dinner, shiny and mildly tinted with a pink

to purplish hue.  The origin of the name becomes readily apparent with the

discovery of your first specimen. They are shaped like a little jewelry box,

the kind diamond rings are delivered in.  Their most distinguishing feature

is the frilly, hollow, spines that protrude from the seven to nine ribs that

cross the shell, originating at the beak (bump above the hinge.)  Jewel

boxes are found in warm seas.

Though impressive, their spines aren’t as dramatic as the thorny oyster.

Shells found in deep water, not damaged by strong currents or the impact of

being tossed around by waves, have longer, sharper spines.  Often those

found on the beach have smooth, worn-down spines. Flat artificial structures

like concrete slabs often serve as a home for mature specimens that haven’t

been damaged and are wonderful additions to a collection.

One of the fascinating aspects of the spiny jewel box is that when younger

they are fastened to a rock, sea wall, coral, or even another shell

somewhere between the intertidal area (exposed to air during low tide) and

deep water but later in life become detached. The right and left valve are

the same size; in fact the valves are mirror images of each other.