Temporal resolution of birth rate analysis in zooplankton and its implications for identifying strong interactions in ecology.

Expanding on Haeckel's classical definition, ecology can be defined as the study of strong and weak interactions between the organism and the environment, hence the need for identifying strong interactions as major drivers of population and community dynamics. The solution to this problem is facilitated by the fact that the frequency distribution of interaction strengths is highly skewed, resulting in few or, according to Liebig's law of the minimum, just one strong interaction. However, a single strong interaction often remains elusive. One of the reasons may be that, due to the ever-present dynamics of ecological systems, a single strong interaction is likely to exist only on relatively short time intervals, so methods with sufficient temporal resolution are required. In this paper, we study the temporal resolution of contribution analysis of birth rate in zooplankton, a method to assess the relative strength of bottom-up (food) versus top-down (predation) effects. Birth rate is estimated by the Edmondson-Paloheimo model. Our test system is a population of the cladoceran Bosmina longirostris inhabiting a small northern lake with few planktivorous predators, and thus likely controlled by food. We find that the method's temporal resolution in detecting bottom-up effects corresponds well to the species' generation time, and the latter seems comparable to the lifetime of a single strong interaction. This enables one to capture a single strong interaction "on the fly," right during its time of existence. We suggest that this feature, the temporal resolution of about the lifetime of a single strong interaction, may be a generally desirable property for any method, not only the one studied here, intended to identify and assess strong interactions. Success in disentangling strong interactions in ecological communities, and thus solving one of the key issues in ecology, may critically depend on the temporal resolution of the methods used.

Updated functional segregation of retinal ganglion cell projections in the tectum of a cyprinid fish-further elaboration based on microelectrode recordings.

Single-unit responses of retinal ganglion cells (GCs) were recorded extracellularly from their axonal terminals in the tectum opticum (TO) of the intact fish (goldfish, carp). The depths of retinal units consecutively recorded along the track of the microelectrode were measured. At the depth of around 50 µm, the responses of six types of direction-selective (DS) GCs were regularly recorded. Responses of two types of orientation-selective (OS) GCs and detectors of white and black spots occurred approximately 50 µm deeper. Responses of GCs with dark- and light-sustained activity were recorded deeper than all others, at about 200 µm. The receptive fields of consecutively recorded units overlap, so they analyze the same fragment of the visual scene, focused by eye optic on the photoreceptor raster. The responses of pairs of DS GCs (ON and OFF units that preferred same direction of stimulus movement) and OS GCs (detectors of vertical and horizontal lines) were often simultaneously recorded at one position of the microelectrode. (The paired recordings of certain units amounted about fourth part of all recordings.) This suggests that their axonal arborizations are located close to each other in the tectal retinorecipient layer. Electrophysiological method, thus, allows to indirectly clarify and make precise the morphology of the retino-tectal connections and to establish a morpho-physiological correspondence.

Color properties of the motion detectors projecting to the goldfish tectum.

Interactions between color channels (long-wave (L), middle-wave (M) and short-wave (S)) in the receptive field of direction-selective (DS) and orientation-selective (OS) ganglion cells (GCs) were investigated with combined selective stimulation of pairs of cone types (L and M, L and S, M and S). In the experiments with DS GCs of both ON and OFF types, it was shown that: (1) M and S channels were synergistic relative to each other and opponent to L channel. (2) Three-parameter signal (from L, M and S cones) is transformed to one-parameter signal at the output of DS GC, thus illustrating the principle of univariance. (3) In the experiments with OS GCs, it was shown that L and M channels were synergistic in the OFF-pathway, while the S channel was opponent to them. Our results suggested that photoreceptor synaptic connectivity of the bipolar cells hypothetically involved in the goldfish OS circuitry substantially differs from connectivity of bipolar cells presumably targeting DS GC. (4) To sum up, the results obtained on DS GCs confirmed the plausibility of proposed DS GC wiring diagrams; as to the OS circuitry of fish retina it still remains unclear and needs further investigation.

Colour cues proved to be more informative for dogs than brightness.

The results of early studies on colour vision in dogs led to the conclusion that chromatic cues are unimportant for dogs during their normal activities. Nevertheless, the canine retina possesses two cone types which provide at least the potential for colour vision. Recently, experiments controlling for the brightness information in visual stimuli demonstrated that dogs have the ability to perform chromatic discrimination. Here, we show that for eight previously untrained dogs colour proved to be more informative than brightness when choosing between visual stimuli differing both in brightness and chromaticity. Although brightness could have been used by the dogs in our experiments (unlike previous studies), it was not. Our results demonstrate that under natural photopic lighting conditions colour information may be predominant even for animals that possess only two spectral types of cone photoreceptors.