How can a shark feel a fish it cannot see?

Have you ever swished one hand through the bath and felt the water push against your other hand? Far out in the deep, dark sea, a shark glides along. Somewhere nearby, a small fish is swimming where the shark cannot see it.

So how can a shark feel a fish it cannot see? Let's swim along with the shark and find out together.

Each time the fish wiggles its tail, it pushes the water around it. That push makes small ripples, like the rings that spread when you drop a pebble in a pond. The ripples travel out through the water, all the way to the shark.

Along each side of its body, the shark has a lateral line, which means a row of tiny holes that feel the water moving. Inside the holes sit little hairs in soft jelly. When the ripples arrive, the water nudges those hairs and they bend.

The bending hairs in the lateral line send a message to the shark's brain. Now the shark knows something is moving close by, even in the dark.

The ripples feel stronger on the side of the shark that is closer to the fish. That tells the shark which way to go. So the shark swishes its tail and turns toward the fish.

Just then, the fish dives down and lies very still under the sandy sea floor. Now it makes no ripples at all, and the lateral line has nothing to feel. What do you think the shark could still feel from a fish lying quiet under the sand?

Even a fish lying still has a beating heart and gills moving to breathe. That busy body makes a very faint electric buzz in the water around it. The buzz is far too weak for you to feel, but a shark can feel it.

On the shark's snout are lots of dark dots, and each one is a hole filled with jelly. These holes feel the faint electric buzz. So the shark sweeps its snout over the sand, feels the buzz, and finds the hidden fish right underneath.

Lateral line. A swimming fish makes ripples that the shark's lateral line feels, and even a still fish makes a faint electric buzz that the shark's snout feels.

Sweet dreams, and may you drift off to sleep as calmly as a shark gliding through the quiet sea.