H. van der Meer

INTERSPECIFIC VARIATION OF PHOTORECEPTORS IN FOUR CO-EXISTING HAPLOCHROMINE CICHLID FISHES

The photoreceptors of four co-existing haplochromine cichlids have been studied morphometrically and microspectrophotometrically, and the results were interpreted in terms of resolution and sensitivity. Among the three species, Haplochromis argens, H. piceatus and H. ishmaeli, the cone dimensions and the visual pigments differed only slightly, in spite of differences in vertical distribution and feeding behaviour between these species. The fourth species H. pyrrhocephalus showed distinctly different visual properties, with very large double cones at low densities, and its spectral sensitivity shifted to significantly longer wavelengths. To some degree, the photoreceptor morphometrics and visual pigments appeared to be attuned to the photic conditions of the specific habitats. The two species H. piceatus and H. pyrrhocephalus, occupying a dimmer habitat with less penetrating blue light, had reduced blue sensitive single cones, whereas the surface dwelling species H. argens had relatively small red/green sensitive double cones. Also an attempt has been made to relate the visual capacity of the cichlids to their behavioural patterns, focussing on prey detection. The greatest visual differences were observed in the two species H. argens and H. pyrrhocephalus, with resources coincidence and with the highest encounter frequency, which may indicate that visual segregation contributes towards reducing interference competition among co-existing species with a similar diet.

Retinal Resolving Power and Sensitivity of the Photopic System in Seven Haplochromine Species (Teleostei, Cichlidae)

Retinal parameters of the photopic system are determined in seven adult haplochromine cichlids. Resolving power is calculated as a function of the density of the twin-cones and the size of the eye-lens. The size of the twin-cones is considered to be a useful criterion to compare their sensitivity. Intra-retinal variation of the resolving power shows a negative correlation with the variation of the sensitivity of the photopic system. Interspecific variation of the resolving power and the sensitivity both show a positive correlation with the absolute eye-size. A maximum packing of the cones is approached in the examined species, except in Haplochromis microdon. Variation of the resolving power is discussed in relation with the theoretical value of the optical resolving power as calculated by OTTEN (1981). The significance of this variation is correlated with ecological data.

Ontogenetic Change of Visual Thresholds in the Cichlid Fish Haplochromis sauvagei

The ontogenetic alterations of visual sensitivity thresholds and acuity of the cichlid Haplochromis sauvagei were quantitatively predicted from the developmental change of size and convergence of the photoreceptors and measured behaviourally using optomotor response. The observed enlargement of the cone-photoreceptors was considered to improve their photon catching ability. Accordingly, the ontogenetic change of the photopic threshold was accurately predicted by the increase of the cone size during growth. The measured scotopic threshold decreased to a greater degree during growth than was predicted by the increasing number of rods connected to a ganglion cell. Additionally, the resolution and the flicker fusion frequency were derived from histological and behavioural measurements, respectively. The behaviourally measured resolution was much higher than expected, which was probably due to the method used. In conclusion, the ontogenetic addition and enlargement of photoreceptors in the eyes of fishes lead to improved visual properties in general, which enable a fish to perform new visual tasks.

Visual Resolution during Growth in a Cichlid Fish: A Morphological and Behavioural Case Study

The spatial resolution of the visual system during growth of the cichlid fish Haplochromis argens was deduced from the cone density according to two models of possible connectivity patterns. The model

An insert for single‐molecule magnetic‐resonance spectroscopy in an external magnetic field

We describe an insert for optical and magnetic‐resonance experiments on single molecules in a solid matrix at liquid‐helium temperatures. The experimental arrangement allows in situ adjustment of the focusing lens and of the sample. A parabolic mirror serves to collect the fluorescence emission and to direct the light onto a photodetector. Microwaves can be irradiated through a coil around the sample while a superconducting magnet provides the possibility of a stationary magnetic field.

The Influence of Light Deprivation On the Development of the Eye and Retina in the Cichlid Sarotherodon Mossambicus (Teleostei)

Postbuccal specimens of Sarotherodon mossambicus were raised under different photic conditions. Light deprivation resulted in an enlargement of the pupil, an enlargement of the double cones, a diminution of the single cones, a moderate decrease of cone density and a substantial decrease of postsynaptic cell densities. Dim light only affects the single cone size. The results are discussed from a functional and a morphogenetic point of view and are compared with the results of other investigations which relate to the retinal variations with different environmental light conditions.

Continuous-wave EPR at 275 GHz: Application to high-spin Fe3+ systems

The 275GHz electron-paramagnetic-resonance spectrometer we reported on in 2004 has been equipped with a new probe head, which contains a cavity especially designed for operation in continuous-wave mode. The sensitivity and signal stability that is achieved with this new probe head is illustrated with 275GHz continuous-wave spectra of a 1mM frozen solution of the complex Fe(III)-ethylenediamine tetra-acetic acid and of 10mM frozen solutions of the protein rubredoxin, which contains Fe(3+) in its active site, from three different organisms. The high quality of the spectra of the rubredoxins allows the determination of the zero-field-splitting parameters with an accuracy of 0.5GHz. The success of our approach results partially from the enhanced absolute sensitivity, which can be reached using a single-mode cavity. At least as important is the signal stability that we were able to achieve with the new probe head.

A low-temperature insert for a 95 GHz electron-spin-echo spectrometer

The construction is described of an insert that allows electron-spin-echo experiments to be performed on photo-excited paramagnetic states in single crystals at liquid helium temperatures at a resonance frequency of 95 GHz.

A loop-gap resonator for microwave-induced optical nuclear polarization, electron spin-echo experiments, and dynamic nuclear polarization

Abstract A special double loop-gap resonator for CW and pulsed EPR experiments at X band is presented. It allows for simultaneous optical access, high uniaxial pressure on the sample, and NMR and EPR experiments at liquid helium temperatures. It is used for microwave-induced optical nuclear polarization in semiconductors.

Continuous-wave EPR at 275GHz: Application to high-spin Fe3+ systems

The 275GHz electron-paramagnetic-resonance spectrometer we reported on in 2004 has been equipped with a new probe head, which contains a cavity especially designed for operation in continuous-wave mode. The sensitivity and signal stability that is achieved with this new probe head is illustrated with 275GHz continuous-wave spectra of a 1mM frozen solution of the complex Fe(III)-ethylenediamine tetra-acetic acid and of 10mM frozen solutions of the protein rubredoxin, which contains Fe(3+) in its active site, from three different organisms. The high quality of the spectra of the rubredoxins allows the determination of the zero-field-splitting parameters with an accuracy of 0.5GHz. The success of our approach results partially from the enhanced absolute sensitivity, which can be reached using a single-mode cavity. At least as important is the signal stability that we were able to achieve with the new probe head.