If you take a close look at some pond water, you just might see a hydra (HI-dra)--or lots of them. Don't worry, they won't stare back at you with beady eyes. They have no eyes. In fact, a hydra hardly looks like an animal at all. It looks more like a tiny tree with bare branches. But it's an animal, all right--a close relative of jellyfish and sea anemones.
Up close you can see that a hydra has a long body with a mouth on top. Its mouth is surrounded by six or seven tentacles (TEN-tuh- kulz), or arms. To see the hydra's tentacles even better, look through a magnifying glass.
Hydras are attracted to light. So when they move around, it's usually toward a brighter place. But they don't move fast.
One way a hydra moves is by trapping a bubble of air with its bottom end, called its foot. It then rides the bubble to the surface of the water. There it hangs onto the bubble and waits for prey to come its way.
And what about those cartwheels? Yes, the hydra really is a little acrobat. Check out the drawings on the next page to see it in action.
Besides being an acrobat, a hydra can shrink and stretch with amazing speed. When it senses danger, or when it's bothered by something, the hydra seems to disappear. It becomes as small as many of the creatures it eats! But as soon as the danger has passed, the hydra stretches itself out again.
HUNTING FOR DINNER
A hydra eats copepods, water fleas, and other speck-sized crustaceans (krus-TAY-shunz). It also slurps down tiny worms and insects.
How does it catch these little water creatures? On the hydra's tentacles are thousands of stinging capsules. The capsules work a lot like guns. Each capsule has a hair sticking up next to it that acts as a trigger. And each is loaded--not with a bullet, but with a coiled thread.
When a small creature bumps into a few triggers on a tentacle--zip! zing! zap!--the capsules explode and shoot out their threads. Some of the threads are covered with sharp barbs. They jab into the prey, injecting it with poison. Other threads hold on to the creature.
Then the hydra wraps several tentacles around its prey. It pulls the prey toward its wide-open mouth and swallows it. While digesting its food, the little monster waits patiently for another victim. If prey does not come by soon enough, the hydra may move on to new hunting grounds.
About six hours after eating, the hydra gets rid of the leftovers from its last meal. They come spewing out of its mouth, like lava shooting from a tiny volcano.
A hydra makes more animals like itself in an amazing way. Several tiny hydras will sprout right out from the main body! This is called budding. The young hydras stay attached to the parent for a while. Then they drop off and start lives of their own.
Hydras bud all the time. When they have enough food, they will double in number every two days. So 10 hydras will become 20 in two days, and in two more days there will be 40!
In fall and sometimes in spring, an egg may form like a lump on the side of a hydra. The egg is fertilized by sperm from the same hydra or from a different one.
A hydra egg is a real survivor. It can live through times that might kill a grown hydra, such as hot summers when the pond water dries up. If that happens, the egg stays in the mud until rain fills the pond again. Then out hatches the baby hydra. The egg can also survive freezing winters that would kill an adult.
A LONG-AGO MONSTER
The hydra got its name from a tale people told about a giant nine-headed monster named Hydra. It was the job of Hercules (HER- kyuh-leez), a Greek superhero of the ancient world, to kill this weird monster. But every time Hercules cut off one head, Hydra grew two more to take its place.
In some ways, a real hydra is like the storybook one. If a real hydra is cut in half, each half regenerates (re-JEN-uh-rates), or grows back, the missing parts. In fact, a hydra can be cut into many pieces, and each piece will soon become a whole new hydra. Imagine what old Hercules would have thought of a monster like that!
A clean pond (above) is a great place to find tiny animals. The photo at left is a close-up of some water from a pond. The water is full of sparkling, see-through creatures that are shown here 20 times their actual size. Look for three hydras (numbered 1 to 3), a water flea (4), and a copepod (KO-puh-pod) (5). The green "ribbons" are algae (AL-jee).
Watch the hydra below do a disappearing act--almost. At first it's all stretched out (1). Then it senses an enemy nearby. Poof! The hydra scrunches itself into a tiny lump (2). Now it'll be harder to find!
Uh-oh. A water flea is getting tangled in a hydra's stinging tentacles (left). Gulp! See where the flea ended up? (below left) Right inside the hungry hydra!
Baby hydras can hatch from eggs. But many start growing as buds on the outside of a parent (below). When a baby gets big enough, it breaks off and floats away (below right).
WATCH IT CARTWHEEL! To move fast, a standing hydra (1) arches over (2) and sticks its tentacles next to wherever its foot is attached (3). The hydra then flips its foot up and over its body (4). Then it plops the foot down (5) and stands with its tentacles on top again (6). Over and over the hydra flips, until it gets to a place it wants to be.
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|Title Annotation:||microscopic animals known as hydras|
|Date:||Jul 1, 1995|
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