Trophic ecology of yellownose skate Zearaja chilensis, a top predator in the south-western Atlantic Ocean.

The diet and trophic level (TL ) of the yellownose skate Zearaja chilensis in the south-western Atlantic Ocean (35°-54° S), and how these varied in relation to body size, sex, maturity stage, depth and region were determined by analysis of stomach contents. From 776 specimens analysed, 671 (86·5%) ranging from 180 to 1190 mm total length (LT ) had prey in their stomachs. The diet was dominated by fishes, mainly the notothenioid Patagonotothen ramsayi and the Argentine hake Merluccius hubbsi. The consumption of fishes and crabs increased with increasing predator size, and these preys were more important in the north than in the south. Isopods and other crustaceans were consumed more in the south and their consumption decreased as the size of Z. chilensis increased. The TL of Z. chilensis increased with LT from 4·29 to 4·59 (mean 4·53), confirming their ecological role as a top predator. The small and large size classes exhibited a low diet overlap and the highest spatial segregation, whereas medium and large specimens had higher co-occurrence and dietary overlap indices. A clear distinction in tooth shape was noted between sexes in adult specimens, with males having longer cusps. This sexual heterodonty may be related to reproductive behaviour, increasing the grasping ability of males during courtship, because there were no differences in diet between the sexes.

Composition dependence of the elastic constants of beta-phase and (alpha+beta)-phase PdHx.

We have measured the composition and temperature dependence of the shear moduli C' and C(44) for two-phase (alpha+beta)- and single-phase beta-PdH(x). In the two-phase region, the alpha- and beta-phases are coherent. Here, the composition dependence of C(44) and C' deviate negatively from a Vegard-type volume average. We attribute the deviations to two effects: (1) the partly in-series arrangement of the precipitate and matrix phases, relative to the externally applied stress, and (2) thermally activated anelastic relaxations involving the rapid motion of H interstitial atoms, leading to slight changes in the shape of coherent precipitates. The first effect is present for both C' and C(44) and is temperature-independent, whereas the second is present only for C' and is strongly temperature-dependent.