1498 INTRODUCTION In turbulent flow conditions (i.e. chaotic and unpredictable flow), organisms such as decapod crustaceans and flying insects rely on the filamentous nature of chemical plumes to find odorant sources (Vickers, 2000; Weissburg, 2000). Turbulent stirring has dramatic effects on the structure of chemical plumes, creating filaments of highly concentrated odorant interspersed with regions of low or zero concentration that vary greatly in space and time (Murlis and Jones, 1981; Webster and Weissburg, 2001; Crimaldi et al., 2002). The spatial and temporal characteristics of the fine-scale structure of chemical plumes provide information for olfactory-mediated behaviors (Moore et al., 1991; Mafra-Neto and Cardé, 1994; Finelli et al., 1999), and variation in the plume structure influences how organisms perceive and respond to chemical signals in their environment (Willis et al., 1994; Jackson et al., 2007). The importance of instantaneous plume properties to successful chemosensory search of rapidly tracking organisms has been widely reported (e.g. Vickers and Baker, 1994; Webster and Weissburg, 2009). Reiterative contact with discrete odorant filaments is necessary to sustain up-current progress. Both aquatic (Mead et al., 2003; Keller and Weissburg, 2004) and terrestrial arthropods (Mafra-Neto and Cardé, 1995a; Willis and Avondet, 2005) display straighter search trajectories and improved source localization with increasing filament contact frequency. In some moths, in-flight arrestment occurs in homogenous pheromone plumes (e.g. Willis and Baker, 1984; Justus and Cardé, 2002). This leads to the speculation that the frequency resolution of the pheromone-sensing elements sets an upper limit; frequencies exceeding a critical value cause fusion of incoming pulses such that animals perceive a single continuous pulse that stops upwind progress. However, not all moths seem to show in-flight arrestment in putatively continuous pheromone sources (Justus and Cardé, 2002), making the role of high contact frequencies unclear. An alternative explanation is that organisms stop once they experience high odorant concentrations. Indeed, evidence from moths (Baker et al., 1988; Baker and Haynes, 1989) suggests that adaptation of antennal neurons associated with in-flight arrestment is frequently related to differences in the concentration of incoming pheromone pulses, and that adaptation as a result of high pulse rates is weaker than that due to concentration differences. Our understanding of odor-guided navigation is based largely on experiments where behavioral strategies are examined in specific odor environments, allowing us to interpret behavioral responses in light of the general properties of the odorant signal (e.g. Moore et al., 1991; Keller et al., 2003). However, the chaotic and unpredictable nature of turbulent plumes makes it impossible to precisely define the temporally varying signals experienced by a given searcher. Only a single study has directly resolved the relationship between odorant signal structure and behavior (Vickers and Baker, 1994) by simultaneously recording physiological responses of isolated insect antennae mounted to moths flying through a plume. In another study, Mead et al. examined the effect of waves on odorant filament The Journal of Experimental Biology 214, 1498-1512 © 2011. Published by The Company of Biologists Ltd doi:10.1242/jeb.049312 RESEARCH ARTICLE Getting ahead: context-dependent responses to odorant filaments drive along- stream progress during odor tracking in blue crabs Jennifer L. Page 1 , Brian D. Dickman 2, *, Donald R. Webster 2 and Marc J. Weissburg 1,† 1 School of Biology, Georgia Institute of Technology, Atlanta, GA 30332-0230, USA and 2 School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, USA *Present address: Department of Mechanical Engineering, Colorado State University, Fort Collins, CO 80523, USA † Author for correspondence (marc.weissburg@biology.gatech.edu) Accepted 24 January 2011 SUMMARY The 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. Key words: chemical plume, chemical sensing, odor-guided navigation, rheotaxis, turbulent plume. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY