Spatial and temporal distribution of a deposit-feeding polychaete on a heterogeneous tidal flat

Abstract

The distribution of the deposit-feeding, orbiniid polychaete, Scoloplos fragilis (Verrill, 1873), was examined in relation to differences in elevation along-shore and down-shore from August 1976 to April 1977. The sand flat at Cape Henlopen, Lewes, Delaware U.S.A., is characterized by swash bars extending perpendicular to the shore, which undergo annual vertical and lateral fluctuations and have marked differences in elevations. Six stations were monitored on a single swash bar. Three sites were sampled monthly at an area on the swash bar near the beach berm (high-tide zone) and at an area near the water's edge (low-tide zone). The sites were the swash bar ridge, its depositional slope and the adjacent trough. Distribution patterns of S. fragilis were not associated with elevational gradients or a particular tidal level. Highest numbers were found on the slopes, lowest numbers on the high-tide ridge, and second lowest numbers in the low-tide trough. On the high-tide ridge, the sediments were too unstable for habitation by S. fragilis. In the low-tide trough, predation and other biological interactions apparently keep numbers of S. fragilis low. Wave and wind activity may have concentrated S. fragilis at slope stations, where sediment deposition occurs. Mean body size of S. fragilis varied spatially and temporally. Within each tidal level, mean sizes increased significantly from ridge to trough. Body sizes were significantly larger in the high-tide zone than in the low-tide zone and varied significantly with time. Reproductive maturity appears to occur after two years and at least three age-classes are present.