Previous estimates of suspended food resources have not incorporated the hydrodynamics of species-specific food entrainment. To assess food resources available to the siphonate bivalve Mercenarja mercenarra, we developed a n in situ sampling protocol utilizing realistic incurrent siphon characteristics (e.g diameter, sampling height and pumping rate) with simultaneous measurements of the current veloclty profile. Microalgal food resources, a s estimated by clam-blased chlorophyll a measurements, varied 10-fold over both temporal (hourly and seasonally) and spatial (centimeter and meter) scales in estuarine vegetated and adjacent unvegetated habitats. For the near bottom (1 cm height) samples, chlorophyll a concentrations and variations were greater in a seagrass bed than those measured in the adjoining bare sand habitat. Samples collected at 5, 15, and 45 cm above the bottom did not exhibit any habitat-specific pattern. U p to 90 ' ?L of the near-bottom microalgal count in the seagrass bed was pennate diatoms suggesting that much of the chlorophyll a is derived from the bed. Current speeds measured just above the bottom in the seagrass bed were considerably lower ( c 1 cm S -' ) than elsewhere. These results suggest a relatively concentrated, locally-generated near-bottom food resource for M. mercenaria within the vegetated habitat. Therefore, resource depletion (i.e. suspension feeders reducing the food level for others downstream) may not be relevant to hard clam populat~ons at this site.

Rather than relying on chance delivery of larvae to experimental field substrata, we developed a field caging method to expose lab-reared crab megalopae to natural cues that could affect timing of molting to the first crab stage. Uca pugnax megalopae were placed in plastic mesh cages containing freshly collected sediments from a marsh occupied by U. pugnax, or similar sediments that had been combusted to remove organic material. Cages of each sediment type were placed at identical tidal elevations in a Spartina alterniflora marsh for 1 or 3 d of exposure to natural seawater. Significantly more megalopae molted in cages containing fresh than combusted marsh sediments (90% vs 32%. ANOVA F,,,= 53.95, p < 0.001) after a 3 d penod of exposure. Three days after the 1 d exposure period. nearly all megalopae from cages of both sediment types had molted. Furthermore, all sibling megalopae remaining In the lab during the field experiment fa~led to molt. High rates of molting of megalopae caged in the marsh w~t h natural and combusted sediments ind~cate that chemical substances both in the water overlying the marsh and associated with marsh sediments stimulated molting of megalopae.