ABSTRACT - Fields and Jiang Proposal Number: 0718832 - Collaborative Research: From structure to information in mechanosensory systems. The role of sensor morphology in detecting fluid signals.
Copepods present a spectacular diversity of antennule and setal morphologies, orientations and degree of ornamentation. The causes and consequences of this diversity remain unexplored, but the staggering degree of morphological variation suggests structure-function relationships between mechanosensor properties and their sensory roles. Using copepods as a model system, this work will address the relationship between the shape of mechanosensory structures and their movement under non steady-state fluid conditions. Planktonic copepods provide a unique model system for mechanoreception because: 1) copepods show a variety of behavioral responses to fluid signals; 2) the basic properties of copepod's mechanosensory systems are easy to identify and are likely conserved across a diverse range of species; 3) because of their size, copepods operate at low Reynolds numbers where fluid motion remains comparatively coherent and easy to quantify and model. Also, one of the truly unique characteristics of copepods is their extremely rapid response times (milliseconds). Living at low Reynolds numbers, mechanical stimuli are attenuated quickly by viscous dampening causing fluid velocity (driven by steady swimming) to decrease with distance squared (Fields and Yen 2002). Consequently, copepods often do not detect other individuals (including predators) until they are within a few body lengths of each other. Because of the close proximity and their ability for rapid movement, behavioral latencies of milliseconds are required. Collecting enough information within this short latency period is an extraordinary challenge for the copepod and requires rapid firing rates and the spatial integration of numerous signals from their antennae. To determine the relationship between fluid motion, setal motion, nerve firing rates and behavior, the dynamics of all components must be simultaneously known. Using analytical solutions for monopole or dipole movement, previous studies have calculated what the fluid characteristics should be at the sensor under steady state conditions. However, the mechanical devises generating the fluid signals and the sensor responding to the signal require 100 - 1000 ms to reach steady state. Yet it is during these crucial first few milliseconds that copepods (or any other organism with short behavioral latency) gather the pertinent information. Therefore steady state solutions are appropriate for organisms that integrate information over long time periods but are not applicable for organisms such as copepods with short behavioral latencies. Numerical treatments for quantifying fluid motion are available but rarely applied. A second consideration is how the receptors are modeled. Due primarily to analytical tractability, most current models of setal motion characterize the hair as a rigid cylinder of uniform diameter and rely on spatially homogeneous-steady state flow over the entire hair to move it. However, anatomical features, such as non-uniform cuticular thickness, asymmetry in cross-sectional diameter, setal geniculations and fine hair-like projections from the setae differ between seta and likely affect the transduction of fluid motion to setal bending. Furthermore, mechanical features such as the degree of setal arcing may give rise to large changes in the relationship between angular deflection and fluid velocity. For setae immersed in water, as opposed to air (where many of the models have been applied) these effects may be even more pronounced. The goal of this project is to quantify the relationship between fluid motion and sensory morphology. SEM measurements of the size and width of different setae and TEM measurements of cuticular thickness and the extent of the dendritic penetration up the shaft of individual mechanoreceptors will be made for three species of copepods. The force required to bend the seta and the physiological response of individual hairs to the well described flows created in the lab will be quantified with respect to the physical characteristics of the receptor. The empirical data will be used to build two interacting models: Finite Element Method (FEM) and Computational Fluid Dynamics (CFD). The FEM will be used to model the motion of individual seta with known morphology and bending characteristics while the CFD will be used to calculate the hydrodynamic force and torque applied to each region of the seta and the influence of the seta on the surrounding flow. These data are fundamental to understanding how these small, neurologically simple organisms can distinguish from the myriad of biologically and physically induced fluid movements.
