The first step in processing olfactory information, before neural filtering, is the physical capture of odor molecules from the surrounding fluid. Many animals capture odors from turbulent water currents or wind using antennae that bear chemosensory hairs. We used planar laser-induced fluorescence to reveal how lobster olfactory antennules hydrodynamically alter the spatiotemporal patterns of concentration in turbulent odor plumes. As antennules flick, water penetrates their chemosensory hair array during the fast downstroke, carrying fine-scale patterns of concentration into the receptor area. This spatial pattern, blurred by flow along the antennule during the downstroke, is retained during the slower return stroke and is not shed until the next flick.

Laboratory investigations of a turbulent scalar plume are performed to investigate the relationship between instantaneous scalar structure and the resulting mean scalar statistics. A planar laser-induced fluorescence technique is used to image two-dimensional instantaneous spatial plume structure at various locations and in three orthogonal planes. Long image sequences are used to calculate time-averaged scalar statistics (concentration mean, variance and intermittency), and the relationship between these statistics and the observed instantaneous scalar structure is discussed. We present both snapshots and animations of instantaneous scalar structure at various locations within the boundary layer. As with all boundary layer phenomena, the structural variation is greatest in the vertical direction (normal to the bed). The existence of a persistent, relatively uniform layer of dye within the viscous sublayer is identified. In this layer, instantaneous concentrations are moderate, but the persistence of the dye produces a relatively high mean concentration. Above this layer, stronger fluctuations and higher peak concentrations are present, but lower values of the intermittency produce lower mean concentrations. It is argued that a combination of three time-averaged statistics (mean, variance and intermittency) is required to deduce meaningful information about the nature of the instantaneous scalar structure.

Many marine crustaceans use water-borne chemical cues in ecologically critical activities such as finding food, mates and suitable habitat, detecting predators and communicating with conspecifics (Caldwell, 1979(Caldwell, , 1982Ache, 1982;Atema and Voigt, 1995;Zimmer-Faust, 1989;Weissburg and ZimmerFaust, 1993, 1994;Weissburg, 2000). The act of following an odor plume to its source is called 'plume tracking'. In turbulent environments, odor plumes consist of fine filaments containing high concentrations of odor molecules interspersed with the surrounding fluid (Murlis and Jones, 1981;Moore et al., 1994;Weissburg, 2000;Crimaldi and Koseff, 2001;Crimaldi et al., 2002). Crustaceans sample the fine structure of the odor filaments by moving their antennules through the filaments . We are interested in discovering how mantis shrimp (and by extension, other crustaceans) use the information contained in odor filaments to find the source of the odorant.Initial studies of plume tracking relied on descriptions of plumes as slowly diffusing clouds of chemicals rather than as filamentous, intermittent and dynamic structures. By recording at a point, later investigators showed that odor plumes are intermittent (Zimmer-Faust et al., 1988a,b, 1995Moore and Atema, 1988, 1991;Atema et al., 1991;Moore et al., 1994;Consi et al., 1995;Dittmer et al., 1995). More recently, planar laser induced fluorescence (PLIF) techniques have shown that odor plumes in water are filamentous Webster and Weissburg, 2001;Crimaldi et al., 2002). Koehl et al. (2001) have shown how odor filaments are encountered by a real antennule swept through a realistic plume by computer-driven motor attached to a stationary lobster carapace. Our study examines odor encounter by antennules of a live, odor-plume-tracking stomatopod.In addition, earlier plume-tracking investigations focused on unidirectional flow. While some species inhabit environments exposed to unidirectional flow (e.g. crabs and crayfish), many others live in coastal habitats and thus experience wave-affected flow. Understanding plume-tracking algorithms requires accurate information about the odor signal encountered by the animal's sensors as it tracks a plume in an environmentally relevant flow field. Our goal in this paper is to answer, for the first time, the following question: what is the instantaneous, fine-scale chemical signal encountered by the mantis shrimp as it tracks an odor plume in wave-affected and in unidirectional flow? This information, correlated with behavior, is the critical first step in deducing the algorithms used by odor-plume-tracking animals. Mantis shrimp as model systemsWe use mantis shrimp (also called stomatopods) as a model