Predation on zooplankton by fish has a strong effect on major ecological processes, but the hydrodynamic nature of the interaction is poorly understood. The predominant feeding mode used by zooplankton-feeding fish involves the use of suction flows that pull their prey into the mouth. The prey, on the other hand, is well equipped to detect those flows, and other hydrodynamic disturbances generated by the swimming fish. In fact, some planktonic organisms rely completely on this information to evade predators. This raises the question of whether and when does the prey detect the disturbance generated by the suction flows. If those flows are sensed very late in the strike, the fish can be considered an ambush predator, approaching the prey without being noticed. However, if the fish generates a detectable disturbance when swimming towards the prey, the fish can be considered a pursuit predator, and the interaction will depend on the fish’s ability to close the distance to its prey. Our new manuscript describes a detailed investigation of the hydrodynamic disturbance generated by a suction-feeding fish (Bluegill sunfish), obtained using particle image velocimetry (dPIV; see image above). This hydrodynamic disturbance is compared to other biologically-relevant flow sources induced by swimming fishes and active suspension feeders, and to the disturbance generated by artificial sources used to study the escape response of zooplanktonic animals. Our inference shows that the hydrodynamic signal is detected by common prey animals (copepods and rotifers) when suction flows are fully-developed, already exerting forces that drags the prey into the mouth. Therefore, we conclude that within this interaction, fish can be considered ambush predators. Our study identifies the ability to forward-protrude the jaws while opening the mouth as the critical mechanism that enables fishes to reduce the disturbance by several orders compared to fixed-aperture suction devices seen in bivalves, sponges and other active suspension feeders.

Holzman, R. & Wainwright, P. C. 2009. How to surprise a copepod: Strike kinematics reduce hydrodynamic disturbance and increase stealth of suction feeding fishes. Limnology and Oceanography 54:2201–2212. download pdf.

posted October 2, 2009

Are zooplankton-feeding fishes ambush or pursuit predators?

PIV image

Figure 1. The image sequence above shows the hydrodynamic disturbance generated by suction-feeding fish during a feeding strike. This disturbance is measured as “strain rate”, a term describing the deformation of a fluid particle as it moves, which was calculated from velocity fields obtained using particle image velocimetry (dPIV).