1. Theory, laboratory behavior, and field orientations

(1) Hydrodynamic principles and experiments with empty shells predict that pressure distributions around brachiopod shells generated by ambient currents should, depending on the orientation of the shell relative to the current, either augment or oppose the ciliary-driven flow of water through the lophophore. For living articulate brachiopods with plectolophes or spirolophes, orientations where the anterior-posterior axis of the shell is parallel to the current direction should result in pressure distributions which oppose active pumping. This effect should be strongest when the excurrent region of the shell faces into the current. Orientations where the anterior-posterior axis is perpendicular to the current direction should result in pressure distributions which act in concert with active pumping, most strongly when one of the incurrent regions is directed into the current. These effects are independent of specific shell shape.(2) Laqueus californianus and Terebratulina unguicula actively reorient to currents in the laboratory, preferring orientations where the anterior-posterior axis of the shell is perpendicular to the current and the right-left axis is parallel to the current. Both species may traverse an arc as great as 120° to achieve their final orientation. Hemithyris psittacea also will actively reorient to currents, moving towards orientations where the anterior-posterior axis is perpendicular to the current. The maximum rotation observed for H. psittacea was 45°. Terebratalia transversa never reoriented in the laboratory.(3) Using epifaunal hydroid colonies as indicators of current direction, both Hemithyris psittacea and Terebratalia transversa are oriented in nature with the anterior-posterior axis of the shell perpendicular to the prevailing currents. While scuba diving, I confirmed this orientation phenomenon for T. transversa by direct measurement of the orientation of the brachiopods relative to prevailing currents.(4) Larval Terebratalia transversa avoid areas with current speeds greater than about 0.2 cm/s during metamorphosis and show no orientation to the ambient currents immediately after metamorphosis. Post-metamorphic T. transversa can actively reorient on the pedicle. The orientation observed in adults is probably achieved by active reorientation to local currents of post-zygolophe juveniles.(5) Threshold current speeds for reorientation in Laqueus californianus and Terebratulina unguicula are low and approximately equal to the excurrent pumping speeds of each species; dynamic pressure rather than viscous entrainment is probably the relevant factor determining reorientation behavior.