Brachiopod orientation to water movement: functional morphology

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Abstract
Previous work has shown that Lagueus californianus, Terebratulina unguicula, and, to a lesser extent, Hemithyris psittacea will actively reorient to water currents in the laboratory; Terebratalia transversa will not. This active reorientation is effected primarily by the dorsal adjustor muscles; the ventral adjustors function to depress and tilt the shell. The torque around the pedicle generated by drag on the shell is low at moderate current speeds. The dorsal adjustor muscles are competent to resist these torques up to current speeds of 35 cm/s for L. californianus and H. psittacea and 56 cm/s for T. unguicula. Shell shape, gross pedicle form, and forms of the pedicle foramen are unreliable indicators of a brachiopod''s ability to actively reorient. Shape of the pedicle bulb and position and size of the pedicle connective are better indicators but, on functional grounds, the dorsal adjustor muscle morphology is the best indicator of active reorientation. An angle .alpha. (the maximum possible rotation of the shell that can be produced by contraction of one of the dorsal adjustor muscles) correlates well with both the maximum observed rotations and the exhibition of active reorientation to currents. For fossil material, the existence of dorsal adjustor muscle scars that tough medially should also be a good indicator of active reorientation ability.