Black ghost knifefish in a strange angle

ResearchBlogging.orgI bet you never wondered why the black ghost knifefish hunts at an uncomfortable angle of -30°! Prepare to take a journey on the intersection of animal behaviour, neurobiology and biomechanics!

Suppose you’re one of your animal ancestors, swimming around in one of the warm and shallow Cambrian seas 500 million years ago. You’re a small predator and the struggle of life is as fierce as it ever was. To increase your chances of survival, would you invest your energy in larger eyes and brains so that you can discover prey quicker and more effective than your competitors? Or would it be wiser to increase your muscle power, allowing you to cover a larger area in your hunt for prey?

The example above illustrates some of the trade-offs between optimizing a sensory system and movement performance. Considering the fact that neuronal tissue consumes 20 times more energy than muscle tissue, this is not a trivial dilemma! Humans and some other animals are lucky to have stumbled upon movable heads and eyes, so we don’t have to turn our entire body if we want to look somewhere. Other species are less lucky though…

The black ghost knifefish (A). The fish typically hunts for prey under an angle of -30 degrees (B). Picture from reference.

One such species is a fish with a badass name: the black ghost knifefish. They spend their nights hunting for insects in the Amazon. With such a name come cool abilities: they use an electric field around their body to find their prey. As soon as an insect enters the field, the fish detect the disturbance of the field and can locate their prey. It is this South-American knifefish that is used to illustrate the the conflict between movement and information processing, reported by MacIver and colleagues in PLoS Computational Biology.

Obi-Wan can sense disturbances in the force, black ghost knifefish can sense disturbances in electric fields

When they search for prey, the fish hold their head and body down in a -30° angle. Such a posture does not make sense from an aerodynamic (hydrodynamic?) perspective, since it increases water resistance and makes movement more costly. An experimental model of the fish experienced more than 2 times as much drag at an angle of -45° and a speed of 15 cm/s, compared to a neutral angle.

The advantage of swimming at such an angle becomes apparent when you consider the space that the knifefish can cover and hunt in. Take a look at the figure below, and imagine the knifefish is travelling forward. The area that the electrical field (the grey blob around the fish) covers, is two times as much under the -30° angle as compared to the neutral body pitch. This effect is caused by the increased absolute height of the electrical field.

The black ghost knifefish with its electrical field. As the fish travels forward, it detects insects that enter the field. Under an angle, the height of the scanned area becomes larger, so the fish can scan more water for bugs.

The authors were able to estimate the energy required to encounter prey using a formula I won’t pretend to understand. This formula predicted an optimal pitch of -50°, which is steeper than the observed angle of -30°. The authors think this difference arises because their model doesn’t include the effect of the angle on the propulsive power of the ribbon fin on the belly of the fish (see picture on top), which drops by 25% at an angle of -50°.

The researchers finish their paper with two thought experiments. In the first scenario, they analyzed what would happen if the knifefish used its eyes instead of it’s fancy electric powers. In the second scenario they investigated whether the knifefish would be better off decoupling its electrical sensing from its body movement (much like our eyes: we can rotate our eyes in our sockets and turn our heads, without turning our bodies). You can see the results in the graph below. As you can see, swimming at an angle doesn’t make much sense if you’re using your eyes. However, decoupling the electrical field from body movement seems like a smart thing to do! Whether this is biologically and physically possible is a whole different question altogether of course.

Energy needed per prey when using a visual, an electric and a decoupled electric sensory system, based on the pitch angle.

I don’t usual read papers on animal behaviour and biomechanics, but this one was well-written and interesting to boot. I keep thinking that this principle must also hold up in the smaller worlds of bacteria and protists, I wonder if anyone did any research on that specifically. At least it would be more simple to do controlled experiments and find traces of information/movement constraints on a genomic level. Ah, cross-pollination between different spheres of science.. A subject worthy of a blogpost on itself! Be aware of standard repetition and direct application though. As the Black Ghosts (how appropriate!) sing: repetition kills you!


MacIver, M., Patankar, N., & Shirgaonkar, A. (2010). Energy-Information Trade-Offs between Movement and Sensing PLoS Computational Biology, 6 (5) DOI: 10.1371/journal.pcbi.1000769


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5 comments to Black ghost knifefish in a strange angle

  • I wonder if this swimming position is fulfilling a dual purpose. The fish is using vision and electrosense to search for prey. But using the increase in ‘electrospace’ to also detect potential predators or conspecifics coming from above. Just a thought?

  • That certainly sounds like a reasonable suggestion. I don’t know whether the range of the electric field (3 cm) is large enough to react quickly enough to a predator suddenly appearing above.. As an aside, in the same paper the authors calculate that increasing the range to 6 cm, would increase energy demands by 16 times! I did find that Knifefish also use their electrosensing for navigation and communication, but body pitch doesn’t play a role in these behaviours it seems.

  • Electrosense came from the same ancestral sensory system as the lateral line. The lateral line responds to prey up to about a body length and this is certainly used for predator avoidance in fish. It would not surprise me if electrosense was also. If you have to shove your head in the substrate then your limited for options in detecting predators behind you, and it would seem this fish is trying to maximise its electrospace. My hunch would be its doing this to either avoid predators or be alerted to conspecifics.

    • Thanks for your insights Daniel, you’ve obviously got a greater knowledge of fish biology than I have =). You’re logic sounds entirely plausible to me.. A larger disturbance of the field would simply mean that it’s better to get the hell away from where you’re swimming!

  • Well non-visual sensory systems of fish is my area of expertise. Which is why I found this post very interesting!!!

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