Crustaceans such as crabs, lobsters and crayfish use dispersing odorant molecules to determine the location of predators, prey, potential mates and habitat. Odorant molecules diffuse in turbulent flows and are sensed by the olfactory organs of these animals, often using a flicking motion of their antennules. These antennules contain both chemosensory and mechanosensory sensilla, which enable them to detect both flow and odorants during a flick. To determine how simultaneous flow and odorant sampling can aid in search behavior, a 3-dimensional numerical model for the near-bed flow environment was created. A stream of odorant concentration was released into the flow creating a turbulent plume, and both temporally and spatially fluctuating velocity and odorant concentration were quantified. The plume characteristics show close resemblance to experimental measurements within a large laboratory flume. Results show that mean odorant concentration and it's intermittency, computed as dc/dt, increase towards the plume source, but the temporal and spatial rate of this increase is slow and suggests that long measurement times would be necessary to be useful for chemosensory guidance. Odorant fluxes measured transverse to the mean flow direction, quantified as the product of the instantaneous fluctuation in concentration and velocity, v′c′, do show statistically distinct magnitude and directional information on either side of a plume centerline over integration times of <0.5 s. Aquatic animals typically have neural responses to odorant and velocity fields at rates between 50 and 500 ms, suggesting this simultaneous sampling of both flow and concentration in a turbulent plume can aid in source tracking on timescales relevant to aquatic animals.

Crustaceans contain a great variety of sensilla along their antennules that enable them to sense both hydrodynamic and chemical stimuli in aquatic environments, and can be used to inspire the design of engineered sensing systems. For example, along the antennule of the freshwater crayfish, Procambarus clarkii, four predominant mechanosensory sensilla morphologies are found. To study their response to upstream flow perturbations, atomic force microscopy was utilized to determine P. clarkii sensilla bending in response to an applied force and a mean torsional stiffness, k(t) = 1 × 10(-12) N m degree(-1) was found. A numerical model was developed to quantify the deformation of the four sensilla morphologies due to flow perturbations within their surrounding fluid. These flow perturbations were intended to mimic predator and ambient fluid movements. Results show that upstream fluid motion causes alterations in velocity near the sensilla, accompanied by corresponding variations in pressure along the sensilla surface. The feathered and filamentous sensilla, which are hydrodynamic sensilla, were found to be highly sensitive to flow perturbations. The beaked and asymmetric sensilla, which are bimodal chemo-mechanoreceptors, were found to be much less sensitive to hydrodynamic disturbances. Results also show that sensilla are most sensitive to fluid movement in the along-axis plane of the antennule, with a sharp drop in sensitivity perpendicular to this axis. This sensitivity agrees well with neural responses measured directly from the paired sensory neurons associated with each sensillum. Greater along-axis sensitivity is likely beneficial for determining the direction of fluid movements, which may be important for both aquatic organisms and biomimetic sensing systems.

A numerical model was developed to determine advective-diffusive transport of odorant molecules to olfactory appendages of the crayfish, Procambarus clarkii. We tested the extent of molecule transport to the surfaces of aesthetasc sensilla during an antennule flick and the degree of odorant exchange during subsequent flicks. During the rapid downstroke of a flick, odorant molecules are advected between adjacent aesthetascs, while during the slower return stroke, these odorants are trapped between the sensilla and molecular diffusion occurs over a sufficient time period to transport odorants to aesthetasc surfaces. During subsequent flicks, up to 97.6% of these odorants are replaced with new odorant molecules. The concentration of molecules captured along aesthetasc surfaces was found to increase with increased gap spacing between aesthetascs, flick speed, and distance from the proximal end of the aesthetasc, but these changes in morphology and flicking kinematics reduce the animal's ability to take discrete samples of the odorant-laden fluid environment with each flick. Results suggest that antennule flicking allows discrete sampling of the time- and space-varying odorant signal, and high concentration odorant filaments can be distinguished from more diffuse, low concentration filaments through changes in both the timing and the encounter rate of odorant molecules to aesthetasc surfaces.

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The olfactory sense organs of crayfish are aesthetasc sensilla, arrayed along the distal half of the lateral antennular flagella on each side of the animal. The sensillar array is sparse at its proximal origin, where each annulus houses only a single aesthetasc, and it is most dense distally, with occasionally up to six aesthetascs residing on each antennular annulus. Previous studies have tacitly assumed that the aesthetascs are co-equal in their functional properties. We restricted exposure of small zones of aesthetascs to odorant along the array, from near its proximal origin, its midpoint, and its termination near the tip of the lateral flagellum, while recording neural responses within the ipsilateral olfactory lobe of the brain. Simultaneous combinations of zonal exposure to odorant gave proportionally larger central responses, indicative of spatial summation of peripheral inputs. Surprisingly, however, zonal effectiveness was not equal; stimulating even small numbers of aesthetascs near the proximal origin of the array was far more excitatory to local deutocerebral interneurons than stimulating greater numbers of aesthetascs at the tip of the flagellum. The results are discussed in terms of continuing growth and attrition of the antennular segmentation and associated olfactory receptor neurons.