Rates of predation on juvenile blue crabs Cahnectes sapidus (12 to 64 mm carapace width) in eelgrass Zostera marina, sea lettuce Ulva lactuca, and paired adjacent unvegetated substrates, and a Spartina marsh creek were compared using 24 h tethering experiments from July to October 1987. Vegetation provided the best cover from predation, with mean predation rates in sea lettuce of 9 O/O and in eelgrass of 20 %, compared with mean rates of 40 % or greater in unvegetated controls and the marsh creek Seasonal changes in rates of predation on crabs occurred in eelgrass and sea lettuce habitats indicating that the refuge value of these macrophytes varied seasonally. Sea lettuce, previously considered a poor habitat for decapod crustaceans, provided effective refuge for blue crabs and should be considered a potential nursery habitat.

Effects of tethering on predatory escape by juvenile blue crabs Callinectes sapidus Rathbun were examined. Trials were conducted in a large flume to simulate the natural sedimentary and hydrodynamic properties of open sand flats in low-energy estuarine environments. Behavioral interactions between juvenile (prey) and adult (predator) blue crabs were video recorded, then quantified by cornputervideo motion analysis. Sig~uficantly more tethered juveniles (70%) were captured by adults than non-tethered juveniles (10%). Kinematics of the prey swimming escape response changed little as a result of the tether. Mean velocity, maximum velocity, and maximum acceleration did not differ significantly between tethered and non-tethered juveniles. But tethered juveniles traveled shorter distances during bursts of sw~rnrning to escape predatory adults, and they could not evade persistent attackers. Whereas tethering is used to assess relative rates of predation, it increases vulnerability of prey organisms to attack. Predation rates will be most inflated for highly mobile prey that effectively evade capture when they are not restrained. Investigators are strongly encouraged to quantify the impacts of tethering on prey escape and survivorship.

No abstract

ABSTRACT. From 1989 to 1990, we studied a previously undescribed mussel Modiolus americanus (Leach) habitat (with densities as high as 2000 ind. m-2) within the seagrass meadows of St. Joseph Bay, Florida, USA. Using suction sampling, we compared macroinvertebrate abundance, biomass, and annual production estimates in mussel habitat with 3 other habitats: monotypic stands of Thalassia testudinum (Banks ex Konig), Halodule wrightii Aschers and unvegetated sand flats. Previous studies have positively correlated macrofaunal densities and species number with measures of vegetation abundance or architecture. This relationship was not found to be strong in our study. Macroinvertebrate abundance, biomass, and annual production were usually equivalent or higher in mussel than in the other 3 habitats despite aboveground seagrass biomass usually being intermediate between pure stands of T testudinum and H. wrightii, and leaf surface area lowest. Of particular interest was annual macroinvertebrate production whlch ranged from 228.77 to 428.59 g ash-free dry mass (AFDW) m-2 in mussel habitat, followed in descending order by pure stands of T. testudinum (144.90 to 245.44 g AFDW m-'), H. wrightii (83.98 to 180.44 g AFDW m-2), and sand flats (19.83 to 42.47 g AFDW m-').These production estimates illustrate the positive effect that mussels can have in elevating already high levels of macroinvertebrate production in seagrass meadows.

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.