Rigid pen shell

The rigid shell has a pair of thick spiny valves held together by ligaments that run along the entire dorsal side of the bivalve. The bivalve is triangular with 15 to 25 low ribs radiating from the pointed anterior end (or umbo) to the large posterior edge. The exterior of the shell is usually a dull brownish color with many small tube-like spines along the crest of its radiating ribs. Its anterior end is typically buried in fine substrate and attached by byssal threads, with its wider posterior gaping end extending just above the sea bottom surface to facilitate filter-feeding. Algae (e.g. Lobophora variegata) and invertebrates such as sponges and encrusting corals tend to grow on the exposed part of the shell and may camouflage it very well.