Some ecological observations on necton and benthon of Guatemalan mountain streams (Central America), with special reference to the temporary Río Cuxjá

The benthos and necton of several mountain streams in central Guatemala were examined during two expeditions (July - August 1974 and 1975). One of the streams examined is temporary, the others permanent. The benthos was collected by use of a drift net and a SURBER sampler. The fish were caught with the aid of Rotenone. The size, biomass and, to an extent, the condition of the gonads and stomach content of the fish were ascertained. The different aspects of the results are discussed, inclucling trophic situation, diversity of species and zoogeography

Rotifers from Ecuadorian waters

195 species, including preservation artifacts which were identified to genus level, were found in 18 samples collected in 12 different waters of Ecuador during September 1988. 178 belong to the order Monogononta and 17 to the order Bdelloidea (Digononta). The highest diversity was in the Amazonian lowland with 161 species; in waters of the pacific coastal area we found 81 species and in the Andes only 63. The poorest Rotifer fauna was in Lake Mojanda in the high mountain region at 3,700 m above sea-level with only 9 species. Some remarks were made on 12 particular species: Collotheca tenuilobata, Floscularia melicerta, Hexarthra intermedia brasiliensis, H. mira, Manfredium eudactylotum, Monommata grandis, Octotrocha speciosa, Ptygura elsteri, Henoceros falcatus, Macrotrachela multispinosa and Mniobia russeola

Relationship between solidification microstructure and hot cracking susceptibility for continuous casting of low-carbon and high-strength low-alloyed steels: A phase-field study

© The Minerals, Metals & Materials Society and ASM International 2013Hot cracking is one of the major defects in continuous casting of steels, frequently limiting the productivity. To understand the factors leading to this defect, microstructure formation is simulated for a low-carbon and two high-strength low-alloyed steels. 2D simulation of the initial stage of solidification is performed in a moving slice of the slab using proprietary multiphase-field software and taking into account all elements which are expected to have a relevant effect on the mechanical properties and structure formation during solidification. To account for the correct thermodynamic and kinetic properties of the multicomponent alloy grades, the simulation software is online coupled to commercial thermodynamic and mobility databases. A moving-frame boundary condition allows traveling through the entire solidification history starting from the slab surface, and tracking the morphology changes during growth of the shell. From the simulation results, significant microstructure differences between the steel grades are quantitatively evaluated and correlated with their hot cracking behavior according to the Rappaz-Drezet-Gremaud (RDG) hot cracking criterion. The possible role of the microalloying elements in hot cracking, in particular of traces of Ti, is analyzed. With the assumption that TiN precipitates trigger coalescence of the primary dendrites, quantitative evaluation of the critical strain rates leads to a full agreement with the observed hot cracking behavior. © 2013 The Minerals, Metals & Materials Society and ASM International

Doping, density of states and conductivity in polypyrrole and poly(p-phenylene vinylene)

The evolution of the density of states (DOS) and conductivity as function of well controlled doping levels in OC_1C_10-poly(p-phenylene vinylene) [OC_1C_10-PPV] doped by FeCl_3 and PF_6, and PF_6 doped polypyrrole (PPy-PF_6 have been investigated. At a doping level as high as 0.2 holes per monomer, the former one remains non-metallic, while the latter crosses the metal-insulator transition. In both systems a similar almost linear increase in DOS as function of charges per unit volume c* has been observed from the electrochemical gated transistor data. In PPy-PF_6, when compared to doped OC_1C_10-PPV, the energy states filled at low doping are closer to the vacuum level; by the higher c* at high doping more energy states are available, which apparently enables the conduction to change to metallic. Although both systems on the insulating side show log(sigma) proportional to T^-1/4 as in variable range hopping, for highly doped PPy-PF_6 the usual interpretation of the hopping parameters leads to seemingly too high values for the density of states.Comment: 4 pages (incl. 6 figures) in Phys. Rev.

Model-based Comparison of Cell Density-dependent Cell Migration Strategies

Here, we investigate different cell density-dependent migration strategies. In particular, we consider strategies which differ in the precise regulation of transitions between resting and motile phenotypes. We develop a lattice-gas cellular automaton (LGCA) model for each migration strategy. Using a mean-field approximation we quantify the corresponding spreading dynamics at the cell population level. Our results allow for the prediction of cell population spreading based on experimentally accessible single cell migration parameters

Water mites from stagnant waters of Paraguay

Near to the Paraguayan town Concepción water mites were sampled in one shallow lake and four ponds, so-called Tajamares. These waters are characterized briefly in their abiotic aspects. The six net catches had as a result 29 species out of 13 genera and 5 families. Six species are new to science. Just a single species is found in 7 out of 13 genera. The genus Koenikea is the one with the largest species number, namely 7. Only a few species show greater abundances. Many species have markedly inferior abundances: of 10 species only a single individual could be caught. Not a single species occurred in all five waters

Rotifers from Ecuadorian waters II

In 1990 the second author made a second expedition to Ecuador, where, together with a small team, studied waters in the three regions of this country: Oriente, Sierra and Costa. Beside geographical studies, the group had taken physical and chemical data of wateranalyses in running and still waters. To complete our knowledge of the rotatoria fauna, samples of different visited streams, lakes and swamps were collected. 257 species (including preservation artifacts) were identified by the first author. Now, including the results of the first investigation of 1988, 300 rotifers are known from Ecuador. Some remarks are made of Floscularia melicerta, Lecane (s. str,) calcaria, L. murrayi, L. pusilla, Lepadella quinquecostata, Monommata phoxa, Platyias leloupi and P. quadricornis brevispinus. As nov. spec. and nov. subspec. are described: Aspelta ecuadoriensis, Cephalodella tincaformis, Lecane eupsammophila, Lepadella elongata; Lepadella latusinus striata. The diversity of Ecuadorian rotifer species as a whole (in its entirety) has surely been recorded in its essential features with these two present studies. We do not know any other aquatic group of invertebrates in Ecuador of which samples were taken in such a number of different types of water and which simultaneously was evaluated taxonomically and systematically by means of ecological data

Scaling of THz-conductivity at metal-insulator transition in doped manganites

Magnetic field and temperature dependence of the Terahertz conductivity and permittivity of the colossal magnetoresistance manganite Pr_{0.65}Ca_{0.28}Sr_{0.07}MnO_3 (PCSMO) is investigated approaching the metal-to-insulator transition (MIT) from the insulating side. In the charge-ordered state of PCSMO both conductivity and dielectric permittivity increase as function of magnetic field and temperature. Universal scaling relationships between the changes in permittivity and conductivity are observed in a broad range of temperatures and magnetic fields. Similar scaling is also seen in La_{1-x}Sr_xMnO_3 for different doping levels. The observed proportionality points towards the importance of pure ac-conductivity and phononic energy scale at MIT in manganites.Comment: 5 pages, 4 figure

Optical absorption and activated transport in polaronic systems

We present exact results for the optical response in the one-dimensional Holstein model. In particular, by means of a refined kernel polynomial method, we calculate the ac and dc electrical conductivities at finite temperatures for a wide parameter range of electron phonon interaction. We analyze the deviations from the results of standard small polaron theory in the intermediate coupling regime and discuss non-adiabaticity effects in detail.Comment: 7 pages, 8 figure