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Biogenic Sedimentary Structures Produced by Crabs in Lagoon Margin and Salt Marsh Environments Near Beaufort, North Carolina
Elizabeth A. Allen (2), H. Allen Curran
Intertidal burrowing crabs produce distinctive biogenic sedimentary structures in protected lagoon margin and salt marsh environments in the vicinity of Beaufort, North Carolina. Distribution of crabs is determined primarily by substrate characteristics, salinity, and vegetation cover in the intertidal zone. Protected sand beaches and flats are heavily burrowed by the sand fiddler, Uca pugilator, which produces gently curved J-shaped and sharply curved L-shaped burrows 1 to 2 cm in diameter. Larger Y- and U-shaped burrows are formed by the ghost crab, Ocypode quadrata, in the backshore and foredune ridge zones above the high tide line.
Salt marshes influenced by normal to near normal marine salinities are burrowed by U. pugilator, U. pugnax, and Sesarma reticulatum. U. pugnax and S. reticulatum burrow in muddy substrates, particularly along tidal creek banks, whereas U. pugilator is restricted to sandy substrate areas. U. pugnax burrows are 1 to 2 cm in diameter and have complex twisting and turning forms. The larger communal burrows of S. reticulatum consist of a complex of interconnected shafts with several oval-shaped surface openings. Panopeus herbstii is often found inhabiting these burrows. In salt marshes where the influence of fresh water is great, U. minax and U. pugnax are the dominant crabs. U. minax forms burrows 2 to 5 cm in diameter with ho ded entrances and long vertical shafts.
Intertidal crabs are important agents of bioturbation in the environments that they inhabit. Their burrows have excellent potential for preservation in the sedimentary record. Recognition of the record of biogenic sedimentary structures produced by intertidal crabs would be of significant value in identifying paleoenvironments, fixing ancient lagoon margin and estuarine shorelines, and determining paleosalinities.
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