Micromitra, a Burgess Shale brachiopod. Although looking like a clam, its internal anatomy is very different from any mollusk. There is a large organ called a lophophore that, in shower-curtain fashion, separates the interior of the shell into two compartments. As water is forced over this net-like structure, both food and oxygen are extracted. Thus, all brachiopods are examples of creatures using the pump-gill respiratory system. The spines on the outside of this species are not part of the respiratory system.

lophophore, composed of a large loop with numerous long, thin fingers producing a delicate fan-like shape in the shell. This organ filters seawater for food, and, as it is filled with a body fluid and is very thin, it also serves as an exquisite respiratory organ. Like the body of an echinoderm, the interior of a brachiopod shell is almost all water. There is very little flesh, which is in contact with a steady flow of seawater.

The brachiopod lophophore creates several currents of seawater that pass into the sides of the shell, move across the lophophore, and are then sent out the front of the shell. This constant stream of new water entering a brachiopod has the same effect as the current passing through a sponge. The small volume of flesh to great surface area of the lophophore, coupled with the steady flow of water (many times the volume of the interior of the shell), makes the brachiopod consummately adapted for a world of low-oxygen. While the lophophore is explained as primarily an adaptation for feeding, its respiratory function may, in fact, have been a primary aspect of its evolution and shape in the first place. By building a shell and then creating the water currents passing over the lophophore, the brachiopods appear to have been among the first examples of an animal using a “pump gill.” Sponges also pump but they do not have an internal organ (nor indeed



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement