Brachiopod orientation to current direction and substrate position (Terebratalia transversa)

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Abstract
Brachiopods orient to ambient water currents to passively augment active filter feeding. Although T. transversa does not actively reorient to current flow under laboratory conditions, in situ observations show that they are oriented in a highly specific manner in nature. Approximately 70% of observed animals were oriented with the direction of incurrent pumping parallel to current, the remaining 30% with incurrent pumping perpendicular to current. In all animals the direction of excurrent pumping was perpendicular to current. Factors other than current direction combine to further limit the observed habitat of T. transversa. In the areas sampled they were found only on 2 of 5 possible substrate positions with respect to current direction. Areas without current and areas where current velocity exceeds about 3 knots (4.8 km/h) were devoid of brachiopods. Current within a certain velocity range not only augments normal active pumping but may be an essential requirement for survival.