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Information concerning the way juvenile crabs choose their diet from a variety of prey types can be useful for a better understanding of community dynamics, as well as for the adequate management of natural resources. Prey size and species selection by juvenile Carcinus maenas (15-35 mm carapace width, CW) and Cancer pagurus (20-40 mm CW) feeding on four bivalves of contrasting shell morphology were investigated. When offered a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas, and Cerastoderma edule presented individually, crabs generally showed evidence of size-selective predation. Cancer pagurus selected larger mussels relative to the size of their chelae (relative prey size, RPS) than did Carcinus maenas of similar and even larger carapace width. However, the RPS of selected O. edulis and Cerastoderma edule were similar for all crabs, suggesting that certain prey features constitute effective barriers even to the powerful chelae of Cancer pagurus. When offered a wide size range of mussels and oysters simultaneously, all crabs consistently selected mussels. When offered O. edulis and Crassostrea gigas, crabs consumed both these oyster species in similar numbers. Carcinus maenas consumed similar numbers of mussels and cockles; Cancer pagurus, however, showed no preference for either prey in the smaller size classes but selected more mussels than cockles as prey increased in size. Although previous studies report that adult Carcinus maenas select prey species according to their profitability (amount of food ingested per unit of handling time, milligrams per second), consumption rates of the size classes of prey selected by juvenile shore crabs did not always parallel prey value. Although variations in crab strength can account for many of the differences between the foraging strategy of juvenile and adult C. maenas, our results suggest that juvenile crabs are less species selective than adults as a result of the restrictions imposed on small individuals that have limited access to larger prey.

We report on measurements of the easy-plane anisotropy field of MgO-based magnetic tunnel junctions. The free layer easy-plane anisotropy field was measured by perpendicular field thermal noise-based ferromagnetic resonance spectroscopy (T-FMR) and by tunneling magnetoresistance (TMR). A model was derived to extract the saturation field of the free layer from perpendicular TMR-based resistance-field data. Both measurements confirm that for 20 Å CoFeB free layers, the measured effective easy-plane anisotropy field of 4πMeff equal to (5.3±0.6) kOe is well below the bulk value based on demagnetization. T-FMR measurements show a base resonant mode with a linear field dependence in accordance with the Kittel formula. Higher-order modes are also observed, suggesting the presence of confined spin-wave excitations. The mode frequency spacing, however, shows significant device-to-device variations whose origin is not yet uniquely determined. [ABSTRACT FROM AUTHOR]

Magnetoresistance measurements at 300 K and 1.7 K are used to quantify the vortex state magnetic field duration in a series of Ni80Fe20 ring structures with two nm-sized gaps. The measured gap sizes are from 40 nm to 510 nm. The vortex magnetic field range of stability reduces on increasing gap size but then stabilizes and is predominantly determined by extrinsic domain wall pinning defects. Virtual domain wall mediated switching with the applied magnetic field direction parallel to the gap, reduces a stochastic switching behavior in these structures, compared to a continuous ring. A comparison of experiment with the predicted switching behavior from a micro-magnetic simulation is made and the stray fields are calculated in the vortex remanent state. The reduction in the vortex field duration with increasing gap size is more gradual than that predicted by the micro-magnetic simulation up to a gap size of ∼100 nm. [ABSTRACT FROM AUTHOR]