Brachiopods look like clams but lost the ancient seas to them
Pick up a brachiopod shell and you would swear it came from a clam. It did not. Brachiopods evolved their two-part shell independently, hinged top and bottom rather than left and right, and for hundreds of millions of years they outnumbered clams in variety. Then two mass extinctions knocked them back for good.
Brachiopods appeared in the Early Cambrian, probably from creatures known as tommotiids, and flourished through the Paleozoic, when they were more diverse than bivalves. The end-Capitanian and end-Permian extinctions hit them hard, their diversity never recovered, and bivalves took over the seafloor. Today around 400 brachiopod species survive, compared with roughly 9,200 bivalves, and most live in cold, dim water. Some ancient giants, Gigantoproductus and Titanaria, reached 38 centimetres across; modern ones are mostly 10 to 30 millimetres.
The name joins Greek words for arm and foot, and the common name lamp shell comes from one group whose curved shells resemble old pottery oil lamps. The arms are the lophophore, a U-shaped crown of tentacles whose beating hairs pull water in so food particles can be filtered out. It cannot be withdrawn and fills up to two-thirds of the shell, while the body itself takes only about a third, tucked near the hinge. Many species anchor to the seabed with a fleshy stalk through a hole in the larger valve.
Two traditional groups differ at the hinge. Articulate brachiopods lock their valves with teeth and sockets; inarticulate ones rely on muscles alone. The closing muscles hold both quick fibres for emergencies and slow catch fibres that keep the shell shut for long periods. Tiny extensions of the mantle poke through the shell, perhaps for storage or breathing: smear petroleum jelly on the outside and oxygen use drops.
Fish and crustaceans seem to find them distasteful and seldom attack. Individuals live from three to over thirty years, and inarticulate larvae look like miniature adults, feeding and swimming for months before settling. Only lingulids have been fished commercially, and on a tiny scale.
Source: Brachiopod
