Fish choose a collision course
Flying insects steer away from objects to avoid collisions, but fish swim toward them at full speed.
Insects pass through the air with enormous speed. It is a wonder that they don't crash into things.
The reason is that they have a warning system that prevents collisions. This helps them keep track of the distance between them and the objects around them.
From the perspective of a bumblebee, the world moves while the bumblebee itself remains stationary.
If objects seem to move quickly, they are close and it's a good idea to change course.
Similarly, when we humans sit on a moving train, it can seem like the landscape outside is moving, not us. A tree that is moving past us quickly, must be close.
Fish are full of surprises
Fish and insects experience some of the same challenges.
In particular, it is challenging to keep track of speed and direction when winds and water currents are involved. Fish, for instance, have to swim to stay in the same place in the stream.
Biologists at Lund University in Sweden sent bumblebees and zebrafish through corridors filled, respectively, with air and water.
On the walls, the experimenters projected an optical illusion that fooled the test animals into believing that one wall was closer than the others, because it appeared to move faster.
“We thought the fish would behave in the same way as insects, but we were surprised”, says postdoctoral researcher Christine Scholtyssek.
The zebrafish did exactly the opposite of the bumblebees.
Full stop without problems
Bumblebees kept far away from the wall that appeared to be close. They also slowed down because they perceived a danger of collision in the narrow tunnel.
The fish, however, kept going at the same speed and headed straight for the nearest wall.
“Interestingly, they did not crash more often than the bumblebees”, says Scholtyssek.
The explanation is probably that it is easier to stop in water than in air. Moreover, since a fish is floating, it can stop without being overwhelmed by gravity and thus falling.
Unlike bumblebees, fish are not afraid of nearby objects. The fish may need to get close to the wall to know how fast they are swimming, the researchers concluded.
Zebrafish have poorer visibility in water than bumblebees have in air. They live in murky water where visibility is about 30 centimetres.
In order to monitor movement, they rely on reference points. Since visibility is low, they need to get closer.
Deprived of reference points
“It is safest for the fish to stick close to visible points”, Scholtyssek says.
The fish became clearly distraught when they could no longer see their surroundings, which allowed the researchers to confirm their hypothesis.
Now Scholtyssek wants to investigate other species of fish. They may display other types of behaviour, particularly if they live in clear water. The study on zebrafish suggests that fish cling to walls because they are accustomed to low visibility.
Translated by: Lars Nygaard
- Control of self-motion in dynamic fluids: fish do it differently from bees. Biology Letters. Mai 2014, vol. 10, nr. 5
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