Staying the course: chemical signal spatial properties and concentration mediate cross-stream motion in turbulent plumes.

This study examined the role of broadly distributed sensor populations in chemosensory searching, especially cross-stream heading adjustment. We used three-dimensional laser-induced fluorescence to collect chemical concentration data simultaneously with behavior observations of actively tracking blue crabs (Callinectes sapidus). Our analysis indicates that the spatial distribution of the odorant concentration field is necessary and sufficient to mediate correct cross-stream motion, although concentration provides information that supplements that obtained from the spatial distribution. Crab movement is continually adjusted to maintain an upstream heading, with corrections toward the source modulated only in the presence of chemical cues. Crabs detect and respond to shifts in the position of the center-of-mass (COM) of the odorant concentration distribution as small as 5% of the leg span, which corresponds to ∼0.8-0.9 cm. The reaction time after a 5% threshold shift in the position of the COM is in the range of 2-4 s. Data also indicate that these steering responses are dependent on stimulus history or other characteristics of the plume, with crabs taking longer to respond in conditions with large-scale spatial meanders. Although cross-stream motion is determined by chemical signal inputs to receptors on the walking legs, crabs do make rotational movements in response to chemical signals impinging on the antennules. These rotational movements do not affect the direction of travel, but rather, determine the crab's body angle with respect to the flow. Interestingly, these body angles seem to represent a compromise between reducing drag and obtaining better chemical signal information, and this trade-off is resolved differently under different plume conditions.