Renee Grugett, age 7, of Lansing, Michigan, for her question:
How do iellyfish swim?
Fishes have fins for swimming and seals have flippers. A jellyfish looks like just a bag of soft jelly with no fins and no flippers. Yet he manages to swim far and wide through the ocean. When you watch him, you see a lot of long threads dangling down under his bag of jelly. No, these trailing tentacles do not help him along. The secret of his swimming is jet propulsion the same sort of push that sends our astronauts to the moon.
Many big and little jellyfishes live in the salty seas and a few smallish ones live in fresh water lakes and rivers. Each one has a body shaped like a thick umbrella made of clear or tinted jelly. The top of the umbrella is smooth and shiny. But on the underside there are a lot of dangling tenticles, trailing down through the water like stringy threads. The jelly umbrella opens and shuts ¬spreading out wide and then squeezing together like a fist. Each time it opens, the jellyfish grabs a bagful of water. Each time it closes, the water squirts out in a jet. The force of the jet pushes the floating jellyfish ahead. He swims by jet propulsion. His ancestors invented this way of traveling millions of years before we discovered it to push our jets and spacecraft.
A grown up jellyfish did not always travel by jet. During his younger life he went through several surprising changes. Scientists have different names for the different stages of his life. The grown up jellyfish, who travels by jet, is called a medusa. At the proper time, the medusa forms eggs under the jelly umbrella and frees them into the water. Each little egg is called a planula. It is covered with short, fine hairs and waves them to swim through the water.
After a while, the planula give up swimming altogether. It sinks down to the bottom and stands up straight like a stubby little plant with a bulging top. At this stage of life, the young jellyfish is called a polyp. Some jellyfish polyps grow larger trunks, circled with ridges. This stage is called a strobilia. Soon the round ridges grow delicate fringes. The strobilia looks like a ferny underwater plant, rooted to the spot or a stack of delicate daisies.
Soon the one at the top of the pile breaks free. The daisy type petals become tentacles and trail down through the water. The center becomes a jelly umbrella that opens and closes. The little creature travels by jet but it is too small to be rated as a medusa. At this stage, it is called an ephyra. The second ephyra breaks free, then the others one by one. Off they swim, jetting their small umbrellas open and shut. Soon they will grow as big as their parents big enough to be rated as grown up medusa jellyfishes.
Most Jellyfishes of the ocean are just a few inches wide, about as large as a man's hand. But a few members of the family are whoppers. One of them lives in the chilly Atlantic near Greenland and even farther north. This giant jellyfish opens his umbrella seven feet wide. And some of his trailing tentacles are 120 feet long. Altogether, this whopping jet traveler may weigh a ton.