LAMPSHELLS (Brachiopods)
A brachiopod (lampshell) looks like a small clam or mussel, but the resemblance is only superficial. Each brachiopod has a ventral and a dorsal shell rather than the right and left shells of a bivalve mollusc. Also, unlike clams and mussels, virtually all brachiopods are attached to something solid via a peduncle (stalk, featured in the photograph of a transverse lamp shell, BR1), through either a hole in the ventral shell or a gap between the two shells. Oddly, a lampshell often rotates upon its stalk, orienting itself “upside down” so that the ventral shell is uppermost. Finally, a brachiopod’s internal anatomy is based on a large filter-feeding organ called a lophophore. This structure, powered by the movement of cilia (hairs), rests on a delicate calcareous “tray” supported by the ventral shell. A lampshell’s anatomy is therefore fundamentally different from that of a clam.
Exclusively marine and solitary, lampshells were dominant in Cretaceous seas some millions of years ago, but today are relatively few in number.
Sixteen species of brachiopods have been documented from the Pacific Northwest. A majority live at great depths and are seldom encountered.
Each of the lampshells listed above (BR1–5) has two shells that have hinge teeth and sockets that form a joint, allowing for articulation. A second group, known as inarticulate brachiopods, have no such joints and their shells are held together only with body tissues. The flat brachiopod, Crania californica, lives in the Pacific Northwest. Because its lower shell is cemented directly to hard substrate, it superficially resembles the green false-jingle (MC54) rather than an articulate brachiopod. It usually lives affixed to large boulders in deep water and is unlikely to be encountered by most naturalists.