Why do big fish let little fish clean their mouths?

Think of a time a crumb got stuck between your teeth. You could feel it with your tongue, and it bothered you until you brushed it away. Now picture a big fish in the sea, with no toothbrush at all.

Down on a warm reef, a big fish opens its wide mouth. A tiny fish swims straight inside, and the big fish does not bite! So why do big fish let little fish clean their mouths?

Big fish carry parasites, which means tiny creatures that hold on to another animal and feed on it. Parasites cling to a fish's skin, its gills and the inside of its mouth. They make the big fish itchy and sore, and a fish has no hands to pick them off.

Here is where the little fish come in. They are cleaner fish, which means small fish that eat parasites off bigger fish. For a cleaner fish, a mouthful of parasites is a tasty dinner.

Cleaner fish live at a cleaning station, which means one special spot on the reef where they wait for visitors. Big fish remember that spot. When they feel itchy, they swim back to the cleaning station, just like you might walk to the sink to wash.

At the cleaning station, the big fish slows down and floats very still. It opens its mouth wide and spreads its gills, as if it is saying, come on in.

The cleaner fish swims right in! It nibbles parasites off the big fish's teeth, tongue and gills, one tiny bite at a time. Soon the big fish feels clean and comfy again. Why do you think the big fish does not gulp its little helper down?

A cleaner fish is worth much more to a big fish as a helper than as one small snack. It can clean the big fish again and again, every time the big fish feels itchy. The big fish waits until the little fish swims out, and both of them get something good: one gets clean, and one gets fed.

Parasites, cleaner fish, cleaning station. Big fish let cleaner fish into their mouths because the little fish eat the itchy parasites, so the big fish feels better and the cleaner fish gets a meal.

Sleep well, little explorer, and may your dreams be full of friendly fish helping each other under the waves.