Utilization Of Marsh And Seagrass Habitats By Early Stages Of Callinectes-aapidus - A Latitudinal Perspective

Seagrass beds and marshes have been identified as important nurseries for the blue crab, Callinectes sapidus. This nursery paradigm is based on blue crab abundance data from trawl, seine and drop-net sampling that has revealed greater abundances in these habitats than in adjacent unvegetated areas. Recently, more quantitative and intensive sampling in seagrass beds and marshes over broad latitudinal scales, combined with manipulative experiments, indicate that the same habitat may vary in utilization on regional scales. Mechanisms accounting for enhanced abundances in these nursery habitats have not been elucidated from a latitudinal perspective. Regional comparisons of blue crab catch data regressed on habitat area were not significant whereas similar comparisons within the Gulf region showed a significant positive relationship of crab harvest with total vegetated area. Thus, the quantity of habitat may be important over small latitudinal scales but other factors could affect population abundances across broad latitudinal distances. Latitudinal differences in habitat use may result from alternate modes of settlement via megalopae or recruitment by juveniles, active or passive habitat selection, post-settlement mortality and food quality and quantity. Tidal regimes and coastal morphology in relation to physical processes may influence the accessibility of important habitats by settling or recruiting individuals and thus be equally important. These factors are reviewed in an attempt to understand regional differences in the patterns of C. sapidus abundance

Utilization Of A Seagrass Meadow And Tidal Marsh Creek By Blue Crabs Callinectes-sapidus II. Spatial And Temporal Patterns Of Molting

Blue crabs were collected weekly from a lower Chesapeake Bay seagrass meadow and adjacent tidal marsh creek over 2 months (July-August 1987) and molt staged. Molting activity, determined from a total of I ,220 crabs, was greater in the grassbed than in the marsh creek, and greater for small crabs and females. The difference between the two habitats in molting activity decreased from the first to the second month of sampling, possibly in response to seasonal decline in seagrass biomass. The proportion of small (\u3c70 mm) females in both habitats was greatest on full moons. There was a lunar rhythm of molting activity by large crabs (2:70 mm), with peak molting activity on fuIl moons. SmaIl crabs demonstrated a similar, but nonsignificant rhythm of molting. We suggest that blue crabs approaching ecdysis aggregate in seagrass meadows, taking advantage of the refuge from predation that this structurally complex habitat affords. Lunar rhythmicity of molting activity may further reduce predation mortality through a dilution effect

Settlement indices for blue crab megalopae in the York River, Virginia: Temporal relationships and statistical efficiency

The efficacy of artificial settlement substrates in quantifying relative rates of settlement of blue crab, Callinectes sapidus, postlarvae (megalopae) was examined. The technique has been widely used to assess settlement at local (Chesapeake Bay) and broad geographic scales (Atlantic and Gulf Coasts). This analysis examined differences in settlement between two configurations of substrates and two depths of deployment, in relation to lunar day, month, year and hours of flood tide occurring at night. Substrates were deployed daily for four years (1989-1992) during the settlement season (July-November) in the York River, Virginia. Settlement did not differ between substrate configurations (flat and cylindrical) deployed at the same location in the water column. Substrates deployed at the bottom of the water column had higher settlement than substrates at the surface, except during the last lunar month sampled (approximately November), when settlement was higher at the surface. There was a semilunar periodicity in settlement with high settlement following the new and full moon phases. Settlement varied annually and with lunar month. Statistical efficiency was achieved with a minimum of three or four replicate substrates. Cylindrical artificial settlement substrates are efficient, reliable and capable of detecting temporal patterns in settlement