Pharmacological modulation of fish-induced depth selection in D. magna: the role of cholinergic and GABAergic signalling

Animal behaviour is closely related to individual fitness, which allows animals to choose suitable mates or avoid predation. The central nervous system regulates many aspects of animal behaviour responses. Therefore, behavioural responses can be especially sensitive to compounds with a neurodevelopmental or neurofunctional mode of action. Phototactic behavioural changes against fish in the freshwater crustacean Daphnia magna have been the subject of many ecological investigations. The aim of this study was to identify which neurotransmitter systems modulate phototactic behaviour to fish kairomones. We used a positive phototactic D. magna clone (P132,85) that shows marked negative phototactism after exposure to fish kairomones. Treatments included up to 16 known agonists and antagonists of the serotonergic, cholinergic, dopaminergic, histaminergic, glutamatergic and GABAergic systems. It was hypothesized that many neurological signalling pathways may modulate D. magna phototactic behaviour to fish kairomones. A new custom-designed device with vertically oriented chambers was used, and changes in the preferred areas (bottom, middle, and upper areas) were analysed using groups of animals after 24 h of exposure to the selected substance(s). The results indicated that agonists of the muscarinic acetylcholine and GABAA receptors and their equi-effective mixture ameliorated the negative phototactic response to fish kairomones, whereas antagonists and their mixtures increased the negative phototactism to fish kairomones. Interestingly, inhibition of the muscarinic acetylcholine receptor abolished positive phototaxis, thus inducing the phototactic response to fish kairomones. Analysis of the profile of neurotransmitters and their related metabolites showed that the D. magna behavioural responses induced by fish depend on changes in the levels of acetylcholine, dopamine and GABA.

Apparent Phototaxis Enabled by Brownian Motion

Biomimetic behaviour in artificially created active matter that allow deterministic and controlled motility has become of growing interest in recent years. It is well known that phototrophic bacteria optimize their position with respect to light by phototaxis. Here, we describe how our magnetic, photocatalytic microswimmers apparently undergo phototactic behaviour. Since there is no obvious reason for the particles to do so, we analyze different influences and elucidate through experiments and theoretical considerations from which physical circumstances this behaviour originates.

Apparent Phototaxis Enabled by Brownian Motion

Biomimetic behaviour in artificially created active matter that allow deterministic and controlled motility has become of growing interest in recent years. It is well known that phototrophic bacteria optimize their position with respect to light by phototaxis. Here, we describe how our magnetic, photocatalytic microswimmers apparently undergo phototactic behaviour. Since there is no obvious reason for the particles to do so, we analyze different influences and elucidate through experiments and theoretical considerations from which physical circumstances this behaviour originates.

Phototactic Behaviour of Active Fluids: Effects of Light Perturbation on Diffusion Coefficient of Bacterial Suspensions

Active fluids refer to the fluids that contain self-propelled particles such as bacteria or micro-algae, whose properties differ fundamentally from the passive fluids. Such particles often exhibit an intermittent motion; with high-motility “run” periods separated by low-motility “tumble” periods. The average motion can be modified with external stresses, such as nutrient or light gradient, leading to a directed movement called chemotaxis and phototaxis, respectively. Using cyanobacterium Synechocystis sp.PCC 6803, a model micro-organism to study photosynthesis, we track the bacterial response to light stimuli, under isotropic and non-isotropic conditions. In particular, we investigate how the intermittent motility is influenced by illumination. We find that just after a rise in light intensity, the probability to be in the run state increases. This feature vanishes after a typical time of about 1 hour, when initial probability is recovered. Our results are well described by a model based on the linear response theory. When the perturbation is anisotropic, the characteristic time of runs is longer whatever the direction, similar to what is observed with isotropic conditions. Yet we observe a collective motion toward the light source (phototaxis) and show that the bias emerges because of more frequent runs towards the light.

Negative phototactic behaviour of crystals on a glass surface

We demonstrate that visible light irradiation can drive negative phototactic behavior of azobenzene crystals, which have an amoeba-like crawling motion.

Phototactic behaviour and the role of light in host transmission of Bursaphelenchus xylophilus

Summary The pine wood nematode (PWN) Bursaphelenchus xylophilus is the causal agent of pine wilt disease (PWD). To understand the light influence on PWN, we investigated its phototactic behaviour. Our data indicated the mixed population of propagative PWN had a positive response to red, orange, yellow, green, blue and white lights, but a weak negative response to violet. For age-synchronised propagative nematodes, however, phototactic behavioural features changed with development. Interestingly, the dispersal fourth-stage juveniles (JIV) showed negative response to all tested lights, which was almost completely the reverse of the propagative fourth-stage juveniles (J4). Further bioassays proved that green, blue and white lights suppressed the host transmission of dispersal JIV from vector beetle to healthy pine branches. Our results revealed that night could be the peak of host transmission. With the results of previous studies, we speculate volatiles from the host tree and light may play pull-and-push roles to accelerate the host transmission of B. xylophilus.