| Popis: |
Azimuth measurements on commissures of the oyster Crassostrea virginica and the mussel Geukensia demissa reveal preferred alignments by these animals to tidal current flow. In beds along tidal creeks, where ebb-current velocities exceed flood-current velocities most oysters orient parallel to channel trend and exhibit a slight preference for the ebb direction; in contrast, mussels nestling among these oysters tend to orient transverse to channel trend. Small clumps of oysters and mussels on the salt marsh surface also exhibit discernible orientations; most tend to be aligned with tidal flood, which is stronger than tidal ebb across the marsh. Locally, however, these trends may be difficult to relate to large-scale components of marsh hydrography, because of pronounced microtropography and its influences on current flow. Overall configuration of oyster biostromes evidently is controlled by a combination of slope, areal extent, and elevation of habitable substrates, and competition among individual oysters. Valve orientations per se seem to be governed primarily by hydraulic pressure gradients, including adaptations by the animals to both drag forces and passive disposition of waste materials. The latter also is considered to be the most important stimulus affecting the rheotaxis of salt marsh oysters and mussels. |
