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Everyone to his own.
The bird is in the sky, the stone rests on land, In water
lives the fish, my spirit in God's hand.
–Angelus Silesius
The teeth of predators bounce off of them. The
rough surfaces of the sides of rocks and the branches of coral
skid over them. Dirt and disease-carrying organisms slide
off them. And in some species, their spectacular colors and
patterns provide camouflage; in others, they grow as spines
or daggers.
Fish scales!
And more than 20,000 of the world's 21,000 species of fish
have them. It may not seem that a fish needs more than scales
to protect it, but it has one more important layer—one
that keeps the water-dwelling creature dry.
Whether scaled or scaleless, the fish's skin contains cells
that produce mucus, the substance that gives the creature
its slippery feel and its "fishy" odor. On scaleless
fish, such as sharks, eels, hagfishes, and catfishes, only
the mucus keeps the animal free of disease, dirt, and parasites.
For fish with scales, the mucus serves the same purpose, forming
a slimy protective coating over the rough scales. The slick
mucus also helps to lessen drag, allowing the fish to glide
easily through water.
The Petroleum-Jelly Test
This is an easy way to find out how mucus forms a barrier
between a fish's skin and its environment.
What you need
- petroleum jelly
- pan of water
What to do
- Smear petroleum jelly on the back of one of your hands
- Now dip that coated hand in the pan of water.
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What happens? Even though your hand is
submerged, your skin—protected by the slick coating—is
dry!
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