Ichthyological ecoregions of Argentina

The Argentine Republic is situated in the southernmost portion of the American continent, occupying over 2,785,600 km2 not including the Antarctic territory. The country ranges from subtropical areas (21º46’S) to subantarctic regions (55º03’S), extending latitudinally over about 4,000 km. It possesses significant latitudinal and altitudinal variation (33º of latitudinal range, and heights from Bajo de San Julián in Santa Cruz province at 105 m below sea level, up to Mt. Aconcagua, 6,959 m over sea level), as well as two gradients of physical variability, extending in north-south and east-west directions. Owing to these features, the country presents a wide range of climates and soil types, being one of the countries with greatest diversity of biogeographical units (Lean et al., 1990, In: Bertonatti & Corcuera, 2000). There are four main hydrographic systems: Río de la Plata basin, the Atlantic and Pacific drainages, and several endorrheic systems. Within these basins, the ichthyofaunistic assemblage is well represented, with different magnitude in accordance with the different taxonomic groupings and regions considered. From an ichthyogeographic standpoint, and according to the works of Ringuelet (1975) and Arratia et al. (1983), Argentina is included in the Brasilic and Austral Subregions. The first of these is represented by two domains: the Andean Domain, comprising the southernmost portion of Titicaca Province, and the Paranensean Domain, including part of Alto Paraná and Paranoplatensean Provinces. The Austral Subregion is represented in Argentina by the Subandean-Cuyan and Patagonian Provinces. The present survey indicates that there are about 441 fish species in Argentina, distributed throughout the country; this number represents less than 10% of the total fish species occurring in the Neotropical Region. There is a recognizable trend of faunal impoverishment, both in North-South and East-West direction, reaching its maximum expression in the provinces of Tierra del Fuego (situated at approximately 52º30’S to 55ºS, and 65ºS to 68º50’W) and San Juan (approximately 28º50’S and 67ºW to 70º45’W), which have 4 and 5 fish species respectively. In north-south direction, one of the regional indicators of this phenomenon is the Salado river basin in Buenos Aires province, which constitutes the southern distributional boundary for the majority of the paranoplatensean ichthyofauna; 12 of the families occurring in the Paraná-Plata system are absent from this pauperized paranensean ichthyofaunal assemblage. Most of the continental fish fauna of Argentina belongs to the primary division of Myers (1949), while some elements are included in the secondary division and others in an amphibiotic or ‘marine penetration’ category. This ichthyofaunistic scope encompasses a wide range of morphological, biological, ecological and ethological types (benthic and pelagic, migrating and sedentary, haematophagous or parasites, annual species, inhabitants of plains or heights, estivation-adapted, etc.) inhabiting different regions within the national territory

Ordering in the dilute weakly-anisotropic antiferromagnet Mn(0.35)Zn(0.65)F2

The highly diluted antiferromagnet Mn(0.35)Zn(0.65)F2 has been investigated by neutron scattering in zero field. The Bragg peaks observed below the Neel temperature TN (approximately 10.9 K) indicate stable antiferromagnetic long-range ordering at low temperature. The critical behavior is governed by random-exchange Ising model critical exponents (nu approximately 0.69 and gamma approximately 1.31), as reported for Mn(x)Zn(1-x)F2 with higher x and for the isostructural compound Fe(x)Zn(1-x)F2. However, in addition to the Bragg peaks, unusual scattering behavior appears for |q|>0 below a glassy temperature Tg approximately 7.0 K. The glassy region T<Tg corresponds to that of noticeable frequency dependence in earlier zero-field ac susceptibility measurements on this sample. These results indicate that long-range order coexists with short-range nonequilibrium clusters in this highly diluted magnet.Comment: 7 pages, 5 figure

Effects of Liposomes Contained in Thermosensitive Hydrogels as Biomaterials Useful in Neural Tissue Engineering

Indexación: Scopus.Advances in the generation of suitable thermosensitive hydrogels for the delivery of cells in neural tissue engineering demonstrate a delicate relationship between physical properties and capabilities to promote cell proliferation and differentiation. To improve the properties of these materials, it is possible to add liposomes for the controlled release of bioactive elements, which in turn can affect the physical and biological properties of the hydrogels. In the present investigation, different hydrogels based on Pluronic F127 have been formulated with the incorporation of chitosan and two types of liposomes of two different sizes. The rheological and thermal properties and their relation with the neurite proliferation and growth of the PC12 cell line were evaluated. Our results show that the incorporation of liposomes modifies the properties of the hydrogels dependent on the concentration of chitosan and the lipid type in the liposomes, which directly affect the capabilities of the hydrogels to promote the viability and differentiation of PC12 cells. © 2017 by the authors.http://www.mdpi.com/1996-1944/10/10/112

Chemistry in isolation: High CCH/HCO+ line ratio in the AMIGA galaxy CIG 638

Multi-molecule observations towards an increasing variety of galaxies have been showing that the relative molecular abundances are affected by the type of activity. However, these studies are biased towards bright active galaxies, which are typically in interaction. We study the molecular composition of one of the most isolated galaxies in the local Universe where the physical and chemical properties of their molecular clouds have been determined by intrinsic mechanisms. We present 3 mm broad band observations of the galaxy CIG 638, extracted from the AMIGA sample of isolated galaxies. The emission of the J=1-0 transitions of CCH, HCN, HCO+, and HNC are detected. Integrated intensity ratios between these line are compared with similar observations from the literature towards active galaxies including starburst galaxies (SB), active galactic nuclei (AGN), luminous infrared galaxies (LIRG), and GMCs in M33. A significantly high ratio of CCH with respect to HCN, HCO+, and HNC is found towards CIG 638 when compared with all other galaxies where these species have been detected. This points to either an overabundance of CCH or to a relative lack of dense molecular gas as supported by the low HCN/CO ratio, or both. The data suggest that the CIG 638 is naturally a less perturbed galaxy where a lower fraction of dense molecular gas, as well as a more even distribution could explain the measured ratios. In this scenario the dense gas tracers would be naturally dimmer, while the UV enhanced CCH, would be overproduced in a less shielded medium.Comment: Letter accepted for publication in A&

Curvature and Acoustic Instabilities in Rotating Fluid Disks

The stability of a rotating fluid disk to the formation of spiral arms is studied in the tightwinding approximation in the linear regime. The dispersion relation for spirals that was derived by Bertin et al. is shown to contain a new, acoustic instability beyond the Lindblad resonances that depends only on pressure and rotation. In this regime, pressure and gravity exchange roles as drivers and inhibitors of spiral wave structures. Other instabilities that are enhanced by pressure are also found in the general dispersion relation by including higher order terms in the small parameter 1/kr for wavenumber k and radius r. These instabilities are present even for large values of Toomre's parameter Q. Unstable growth rates are determined in four cases: a self-gravitating disk with a flat rotation curve, a self-gravitating disk with solid body rotation, a non-self-gravitating disk with solid body rotation, and a non-self-gravitating disk with Keplerian rotation. The most important application appears to be as a source of spiral structure, possibly leading to accretion in non-self-gravitating disks, such as some galactic nuclear disks, disks around black holes, and proto-planetary disks. All of these examples have short orbital times so the unstable growth time can be small.Comment: 30 pages, 5 figures, scheduled for ApJ 520, August 1, 199