ORIGINAL PAPER David M. Fields Æ Marc J. Weissburg Rapid firing rates from mechanosensory neurons in copepod antennules Received: 17 November 2003 / Revised: 18 June 2004 / Accepted: 19 June 2004 / Published online: 20 July 2004 Ó Springer-Verlag 2004 Abstract Small detection distances coupled with rapid movements require copepods to respond to stimuli with behavioral latencies on the order of milliseconds. Receiving adequate sensory information in such a short time necessitates extremely rapid firing rates of the efferent receptors. Here we show that copepod me- chanoreceptors can fire at frequencies up to 5 kHz in response to fluid mechanical stimuli. Neural activity at these frequencies enables these animals to code for a range of fluid velocities thus providing important information regarding the nature of different fluid dis- turbances. Introduction Whether escaping predators, pursuing mates or attack- ing potential prey, rapid behavioral responses often are critical to the survival of individual animals. As natural selection hones an animal’s ability towards shorter behavioral latencies, there must be a concomitant in- crease in either the transmission speed for sensory data or a decrease in the amount of sensory information necessary on which to base these behavioral responses. The biological and physical environment of pelagic co- pepods requires highly discriminate and yet rapid behavioral responses. Living at low Reynolds numbers, chemical stimuli are transported to animal sensors lar- gely through the slow process of laminar fluid dis- placement and Fickian diffusion (Moore et al. 1999). Similarly, mechanical stimuli are attenuated quickly by viscous dampening causing fluid velocity to decrease with distance cubed (Fields and Yen 2002). As such, copepods often do not detect other individuals until they are within a few body lengths of each other and yet these animals still elicit rapid directed behavioral responses. For example, Euchaeta rimana, a predatory copepod, detects and identifies the fluid motion (Fields and Yen 2002) created by prey and initiates an attack within 5 ms. Similarly, the tandem hops during the ‘‘mating dance’’ of some copepod species occur within 1 ms of each other (M. Doall, personal communication) and escape responses from other fluid mechanical stimuli are initiated within 1.5 ms (Lenz and Hartline 1999). In addition to the need for rapid reactions, each of these scenarios requires a unique behavioral response that necessitates accurate discrimination of the stimuli. Responding in an ecologically appropriate manner with a 1- to 5-ms behavioral latency presents a unique challenge. Rapid and accurate behavioral responses of copepods require that three fundamental conditions be met. First, the neural impulses must have time to travel from the receptor site to the target motor region. Sec- ond, the neural impulses must contain adequate infor- mation to differentiate stimuli from each other and, third, they must provide information to determine the 3- D location of the stimulus source. How does the neural system in copepods generate enough information in such a short time interval? Calanoid copepods possess numerous antennal sensors (Strickler and Bal 1973; Friedman and Strickler 1975; Yen et al. 1992; Fields et al. 2002). Depolarization of the nerve cell results from the deflection of the associated mechanosensory hair. Disturbance size is encoded by the number of cells simultaneously stimulated, whereas fluid velocity, at the level of the individual sensor, is encoded by the firing rate (Adrian 1928). The rate and angle of deflection are likely to provide the animal with information regarding the velocity and potentially the acceleration of the fluid signal. Compared to other crustaceans (Wiese 1976), copepod mechanoreceptors are at least an order of magnitude more sensitive, which allows them to detect fluid velocities as small as 20 lms 1 with a hair tip displacement of only 10 nm at 1 kHz (Yen et al. 1992) D. M. Fields (&) Æ M. J. Weissburg School of Biology, Georgia Institute of Technology, 310 Ferst Drive, Atlanta, GA 30332, USA E-mail: david.fields@biology.gatech.edu Tel.: +1-404-8948430 Fax: +1-404-8940519 J Comp Physiol A (2004) 190: 877–882 DOI 10.1007/s00359-004-0543-2