See treasure in jewel boxes from the sea
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 one inch 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. 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.