Hydrodynamic signal perception in the copepod Acartia tonsa

Copepods may remotely detect predators from the velocity gradients these ge nerate in the ambient water. Each of the different components and character istics of a velocity gradient (acceleration, vorticity, longitudinal and sh ear deformation) can cause a velocity difference between the cope pod and t he ambient water and may, therefore, be perceived by mechanoreceptory setae . We hypothesised that the threshold value for escape response to a particu lar component depends solely on the magnitude of the velocity difference (= signal strength) it generates. In experiments we isolated the different co mponents and noted the minimum intensities to which the copepod Acartia ton sa responded. As hypothesised, threshold signal strengths due to longitudin al and shear deformation were similar, similar to 0.015 cm s(-1), and were invariant with developmental stage. The latter implies that the threshold d eformation rate for response scales inversely with size, i.e. that large st ages respond to lower fluid deformation rates than small stages and, hence, may detect predators at longer distances. Signals due to vorticity and acc eleration did not elicit escape responses, even though their magnitude exce eded threshold signal strength due to deformation. We suggest that A. tonsa cannot distinguish such signals from those due to their own behaviour (sin king, swimming, passive reorientation due to gravity) because they cause a similar spatial distributions of the signal across the body. Reinterpretati on of data from the literature revealed that threshold signal strength due to deformation varies by ca 2 orders of magnitude between copepods and exce eds the neurophysiological response threshold by more than a factor of 10. In contrast, threshold deformation rates vary much less, similar to 0.5 to 5 s(-1). Model calculations suggest that such threshold deformation rates a re just sufficient to allow efficient predator detection while at the same time just below maximum turbulent deformation rates, thus preventing inordi nate escapes.