Pressure Detection: In Darkness, Freshwater Fish Must Sense Changes to Survive
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The lateral line is a unique sensory system that allows fish to detect patterns of water flow over their bodies and waves of pressure from nearby objects. Sensory cells within the lateral line respond to changes in pressure and to changes in water flow to create a kind of sensory field around the fish, in much the same way that electric fishes detect electric fields.
What do fish use the lateral line for?
Many fish use their lateral line to find their way about, but this becomes even more important when there is no light, meaning that vision is excluded. For blind cave fish, building up a spatial map of their surroundings represents a particular problem, yet it is one that they solve using their lateral line. When they are introduced to new surroundings, they respond by increasing their swimming speed and tilting their body over onto one side. Although speeding up when you are in a strange environment seems an odd thing to do, the purpose of this is to speed up the flow of water over the fish's body, which makes its lateral line extra sensitive to the new aquascape. Tilting the body over to one side is much like turning your ear towards a faint sound. Tests have shown that the cave fish is extremely adept at building up a picture of its environment in its mind using this technique — after all, when did you last see a cave fish bump into something?
Are there fish species that can feel their prey?
When a fly accidentally collides with a spider's web, the spider is alerted by the vibrations that travel throughout the web. A similar thing occurs when insects fall onto the surface of water and begin struggling. The butterflyfish (Pantodon buchholzi) specializes in this type of prey and sits close to the top of the water, waiting for its next meal. If an insect does blunder into the water, the butterflyfish is able to detect its presence using an exquisitely sensitive lateral line and related sensory pits around the head.
Similarly, nocturnal fish are able to hunt at night using their lateral lines to detect the movements of invertebrates such as shrimps. As a fish swims along it creates a wake in much the same way as a boat traveling across the surface of the water. And just as with the boat, the wake of a fish diminishes only gradually, leaving a trail for a short time. Predatory fish, such as large catfish, have been shown to be able to follow the wakes of prey fish using their sensitive lateral lines to track and gradually catch up with their unfortunate target
Do fish use vibrations to communicate?
The bubblenests of fish such as the Siamese fighting fish (Betta splendens) are used both to harbor the eggs and later as a nursery for the young fish during the first few days of their life. Throughout this time the male defends the offspring, which sit at the surface of the water where they are relatively safe. However if danger threatens, the male communicates with the young by agitating his fins to produce V vibrations on the surface of the water. These are detected by the offspring, who respond by moving towards the comparative safety offered by their protective father.