Is the meander growth in the Brazil Current system off Southeast Brazil due to baroclinic instability?

Temporally-growing frontal meandering and occasional eddy-shedding is observed in the Brazil Current (BC) as it flows adjacent to the Brazilian Coast. No study of the dynamics of this phenomenon has been conducted to date in the region between 22 degrees S and 25 degrees S. Within this latitude range, the flow over the intermediate continental slope is marked by a current inversion at a depth that is associated with the Intermediate Western Boundary Current (IWBC). A time series analysis of 10-current-meter mooring data was used to describe a mean vertical profile for the BC-IWBC jet and a typical meander vertical structure. The latter was obtained by an empirical orthogonal function (EOF) analysis that showed a single mode explaining 82% of the total variance. This mode structure decayed sharply with depth, revealing that the meandering is much more vigorous within the BC domain than it is in the IWBC region. As the spectral analysis of the mode amplitude time series revealed no significant periods, we searched for dominant wavelengths. This search was done via a spatial EOF analysis on 51 thermal front patterns derived from digitized AVHRR images. Four modes were statistically significant at the 95% confidence level. Modes 3 and 4, which together explained 18% of the total variance, are associated with 266 and 338-km vorticity waves, respectively. With this new information derived from the data, the [Johns, W.E., 1988. One-dimensional baroclinically unstable waves on the Gulf Stream potential vorticity gradient near Cape Hatteras. Dyn. Atmos. Oceans 11, 323-350] one-dimensional quasi-geostrophic model was applied to the interpolated mean BC-IWBC jet. The results indicated that the BC system is indeed baroclinically unstable and that the wavelengths depicted in the thermal front analysis are associated with the most unstable waves produced by the model. Growth rates were about 0.06 (0.05) days(-1) for the 266-km (338-km) wave. Moreover, phase speeds for these waves were low compared to the surface BC velocity and may account for remarks in the literature about growing standing or stationary meanders off southeast Brazil. The theoretical vertical structure modes associated with these waves resembled very closely to the one obtained for the current-meter mooring EOF analysis. We interpret this agreement as a confirmation that baroclinic instability is an important mechanism in meander growth in the BC system. (C) 2008 Elsevier B.V. All rights reserved.

Duration of spermatogenesis in the bullfrog (Lithobates catesbeianus)

The bullfrog (Lithobates catesbeianus) has substantial economic importance and has also been used as an experimental model for biological studies in the fields of pharmcology, medicine, and reproductive biology, especially studies addressing gametogenesis. However, there is a lack of comprehensive information in the literature regarding testis structure and function in this amphibian. The main objective of the current study was to estimate the duration of the various phases of spermatogenesis in this vertebrate. Sixteen sexually mature bullfrogs received an intracoelomic administration of tritiated thymidine. Testes were analyzed at various times between I h and 33 d after administration to detect the most advanced germ cell types labeled at each interval, as well as labeled preleptotene spermatocytes, which presumably originated from spermatogonial stem cells. The duration of the spermatogonial, spermatocytic, and spermiogenic phases of spermatogenesis in the bullfrog were approximately 18, 14, and 8 d, respectively. Thus, the total duration of the spermatogenesis process from early spermatogonia through to spermatozoa was 40 d in this species, similar to that of most previously investigated mammalian species. To our knowledge, this is the first reliable report on the duration of the full spermatogenic process in any amphibian species. These findings will be very useful for tracking the pace of germ cells in studies involving spermatogonial transplantation in lower vertebrates. (C) 2009 Elsevier B.V. All rights reserved.