A Multidisciplinary Study of Spatial and Temporal Scales Containing Information in Turbulent Chemical Plume Tracking

This report describes the results of a multidisciplinary study of turbulent chemical plume tracking of blue crabs and autonomous agents. The study consists of a coordinated investigation of animal behavior, fluid mechanics, strategy simulations, and chemical sensing. The objective is to provide a comprehensive understanding of chemical plume tracking in a single biological system and to prescribe strategies that are effective for autonomous agents. The consensus of the study is that spatial variation in the plume, measured by sampling at multiple locations simultaneously, yields information that is useful for plume tracking. Behavioral investigations reveal that blue crabs demonstrate the ability to detect the chemical plume and use lateral movements to avoid losing contact with the odor. Blue crabs move rapidly towards the source, strongly suggesting that temporal comparisons of odor properties are not employed during navigation. Analysis of the concentration fields reveals that a spatial correlation between spanwise-separated sensors indicates the relative direction of the plume centerline over short time periods provided the sensor spacing is scaled appropriately relative to the plume. Similarly, simulations of tracking strategies reveal an optimal separation for the sensors at a distance roughly equal to the plume width; both smaller and larger sensor spans degrade tracking performance. The simulations further reveal an optimal sensor size above which the fine details of the concentration distribution are obscured and below which there is insufficient contact with the odor to enable effective navigation. Finally, analysis of the chemical signal shows that the frequency dependent correlation function between two (or more) sensors indicates the relative position of the source.     

Avoided temperature leads to the surface: computer modeling of slime mold and nematode thermotaxis

Summary In thermal gradients, pseudoplasmodia of slime molds and starved nematodes have been reported to move away from a temperature near their acclimation temperature. The consequences of this behavior are not clear in the thermal environment of soil where typically there are waves of alternating high and low temperatures propagating down into the soil with little difference in temperature averaged over time. Computer modeling of this situation demonstrates complex movements with important consequences. The organisms move upward during two intervals during a day and downward during the intervening intervals. The upward and downward movements do not balance. The net movement may be in either direction depending on the specific values of certain physiological parameters. Thus, thermotaxis can lead to a net change in depth even though the average temperature is the same everywhere. Organisms moving faster than the rate of penetration of the thermal wave (2–3 cm/h) will follow it into the interior. Also, organisms that avoid a temperature outside the range of thermal variation, will have a net movement away from the surface. However, slow moving organisms that avoid a temperature near the mean temperature move toward the surface and will reach it in twice the time it would take if always moving toward the surface. The optimal rate of movement to reach the surface in minimum time is about 0.8 cm/h. Thermal acclimation can increase the efficiency of moving toward the surface. At the rate of locomotion of slime molds (0.1–0.2 cm/h), the optimal rate of acclimation is about 0.3/h, which is approximately their actual rate of acclimation. These results suggest that thermotaxis can be used by simple organisms to move toward or away from an interface between media of different thermal properties even though there is no gradient in average temperature.