DISTRIBUTION OF DIOPATRA-CUPREA ACROSS MODERN TIDAL FLATS - IMPLICATIONS FOR SKOLITHOS

Skolithos is the dominant sedimentary structure in Cambrian foreshore/ shoreface deposits. Diopatra cuprea, a predaceous, tube-building polyc haete, has been cited as a posible modern analog for the Skolithos org anism. This investigation examines the spatial distribution of D. cupr ea across several tidal flats and evaluates how well D. cuprea populat ion parameters parallel those of some Cambrian and Silurian Skolithos. A sampling network was constructed encompassing a tidal pool and chan nel on an ebb-tidal delta shoal (ETDs) in Chincoteague Inlet, VA, and traverses were surveyed from the high- to low-tide line in Tom's Cove of Assateague Island, VA. The number and position of worm tubes within a square meter grid were recorded for each station. The number of tub es per grid varied from 0 to 83. Fewer numbers of D. cuprea were prese nt at higher intertidal reaches, whereas lower intertidal areas had po tentially higher numbers of organisms. Densities were at a maximum nea r mean low-water and decreased into the subtidal zone. Analysis of the worm distribution using the Clark and Evans nearest neighbor techniqu e within each grid indicated significant clustering of D. cuprea at hi gh densities and a highly variable distribution at low densities. R va lues (mean observed/mean expected for a random population) indicated c lustering in 69% of the ETDS grids and 72% of the Tom's Cove grids. Co efficients of dispersion of 5.1 to 24.6 corresponded to significant ag gregation of the D. cuprea population about the low-water line. Observ ed densities of D. cuprea were an order of magnitude lower than densit ies reported for Cambrian and Silurian Skolithos. This discrepancy may be related to the relatively instantaneous nature of observations wit hin the D. cuprea population versus a time-averaged picture showing mu ltiple colonization of a particular substrate through time in the case of Skolithos. The vertical change in Skolithos abundance in Cambrian, tidal-flat deposits in Virginia is comparable to the reduction in num bers of observed D. cuprea with height above low water on a modern tid al flat. The clustered distribution of D. cuprea contrasts with the un iform distribution of Cambrian Skolithos in the Bradore Formation and the random distribution of Silurian Skolithos in the Thorold Formation . Different spacing patterns may be attributable able to dissimilar fe eding modes. Both Skolithos and D. cuprea show aggregated populations that are concentrated near the low-water line.