SUMMARYThe chemosensory signal structure governing the upstream progress of blue crabs to an odorant source was examined. We used a three-dimensional laser-induced fluorescence system to collect chemical concentration data simultaneously with behavior observations of actively tracking blue crabs (Callinectes sapidus) in a variety of plume types. This allowed us to directly link chemical signal properties at the antennules and legs to subsequent upstream motion while altering the spatial and temporal intermittency characteristics of the sensory field. Our results suggest that odorant stimuli elicit responses in a binary fashion by causing upstream motion, provided the concentration at the antennules exceeds a specific threshold. In particular, we observed a significant association between crab velocity changes and odorant spike encounters defined using a threshold that is scaled to the mean of the instantaneous maximum concentration. Thresholds were different for each crab, indicating a context-sensitive response to signal dynamics. Our data also indicate that high frequency of odorant spike encounters terminate upstream movement. Further, the data provide evidence that the previous state of the crab and prior stimulus history influence the behavioral response (i.e. the response is context dependent). Two examples are: (1) crabs receiving prior odorant spikes attained elevated velocity more quickly in response to subsequent spikes; and (2) prior acceleration or deceleration of the crab influenced the response time period to a particular odorant spike. Finally, information from both leg and antennule chemosensors interact, suggesting parallel processing of odorant spike properties during navigation.

SUMMARY

Blue crabs (Callinectes sapidus) and other aquatic organisms locate food and mates by tracking turbulent odorant plumes. The odorant concentration fluctuates unpredictably due to turbulent transport, and many characteristics of the fluctuation pattern have been hypothesized as useful cues for orienting to the odorant source. To make a direct linkage between tracking behavior and odorant signal properties, we developed a laserinduced fluorescence measurement system to quantify the instantaneous three‐dimensional (3D) concentration field surrounding actively tracking blue crabs. The data suggest a connection between upstream walking speed and bursts of odorant concentration arriving at the antennule chemosensors, which are located near the mouth region. Specifically, we note rapid upstream walking speed when high concentration bursts arrive at the antennules location and a decrease in upstream walking speed in the absence of odorant filaments near the antennules. Additionally, we note transverse crab movements in apparent response to the transverse distribution of the odorant concentration field. Specifically, asymmetry of the odorant concentration distribution at the elevation of the leg chemosensors is associated with subsequent crab position adjustments in the transverse direction. The methods described here allow for powerful and direct tests of chemosensory navigation strategies in blue crabs, as well as other macroscopic aquatic animals.