by PhilipJ on 20 December 2007
The November issue of PLoS Biology has a very neat article (open access, of course) on the mechanism by which the South American black ghost knifefish Apteronotus albifrons senses prey. A self-generated electric field surrounding the fish is coupled with some 14,000 electrically-sensitive organs, which can measure perturbations in voltage induced by local changes in electrical conductivity in the water, such as when prey enter what is called the sensory volume, or where this electric field is active.
For A. albifrons, it was found that the sensory volume is actually roughly a cylinder which encompasses the entire body of the fish, shown in Figure 3 from the paper and reproduced here:
This is in contrast to most other active sensing mechanisms, such as echolocation by bats and the use of sonar by dolphins, as the sensory volume is more often roughly cone-shaped and necessarily in the forward direction. Unlike dolphins or bats, for which changing direction quickly is not necessarily easy, A. albifrons has, along with pectoral fins, a long ribbon fin that runs the length of it’s body (See above, in figure A), which allows the fish to swim forward, backward, or upward, and change pitch or roll its body. It has also been observed that the fish comes to a near stop when consuming or “handling” its prey, so the combination of a versatile swimming mechanism and observed behaviour suggested a motor volume, or roughly, the volume through which the fish can react to a stimulus in its sensory volume, that should match the sensory volume. This makes some intuitive sense: there is no point sensing for prey in areas that are too far away for you to catch them given the energy costs of creating this electric field.
The determine motor volumes (not shown here, see the text for details) are a good match to the sensory volumes, or, related to known behaviour, the sensory volume is roughly equal to the volume required for the fish to come to a stop, and that these volumes changes as the ambient conductivity in the water changes.
That the sensory volume encompasses the entirety of the fish also implies that it isn’t solely for the search of prey. Analysis of the “mouth” motor volume was (not surprisingly) biased towards the front of the sensory volume, indicating where in the search volume prey can be sensed and caught. That the sensory volume is larger than this suggests that sensing could also be used to detect other obstacles in the fish’s environment.
To read the rest of the study (and see some neat interactive, 3d renderings of the sensory and motor volumes), click here.