Wednesday, November 21, 2007

I've heard about omnipresent but omnidirectional?

PLoS Biology brings us the news about a discovery of an omnidirectional fish. The black ghost knifefish (Apteronotus albifrons) uses a weak electric field to actively monitor its surrounding. This sort of monitoring is different from the ones that we usually see in that it's active, rather than passive (which we humans engage in).

PLoS Biology has been nice enough to write an synopsis about this discovery. That makes it a lot easier for us non-biologists to understand the meaning of the original article (excellent as that might be). The articles own authors' summary isn't too bad though

Most animals, including humans, have sensory and motor capabilities that are biased in the forward direction. The black ghost knifefish, a nocturnal, weakly electric fish from the Amazon, is an interesting exception to this general rule. We demonstrate that these fish have sensing and motor capabilities that are omnidirectional. By combining video analysis of prey capture trajectories with computational modeling of the fish's electrosensory capabilities, we were able to quantify and compare the 3-D volumes for sensation and movement. We found that the volume in which prey are detected is similar in size to the volume needed by the fish to stop. We suggest that this coupling may arise from constraints that the animal faces when using self-generated energy to probe its environment. This is similar to the way in which the angular coverage and range of an automobile's headlights are designed to match certain motion characteristics of the vehicle, such as its typical cruising speed, turning angle, and stopping distance. We suggest that the degree of overlap between sensory and movement volumes can provide insight into the types of control strategies that are best suited for guiding behavior.


So, we're dealing with a fish that can sense in all directions, and then move in any direction where it might locate prey. That sounds like a pretty good advantage to have while hunting (or escaping for that matter). So, why doesn't all, or at least a large number, of animals have this ability? Well, first of all, they would have to been maritime, as they need water for the electric field. Second of all, such things comes at a cost. As the summary clearly states.

Although it's certainly useful to be able to sense in all directions, active sensing comes at a cost; energetically, it's very expensive to generate a good-sized electric field, since the signal falls off rapidly with distance.


Obviously, this leads to a shorter detection range - according to the findings, the black ghost knifefish had a averagee estimated prey detection distance of about 3.5 cm from the fish's body. Hardly a substitute for good eyes in areas with sparse food resources.

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