Friday, April 5, 2013

Tadpoles in the Pond

We have several tadpoles in our classroom's pond.  To see them, you have to be a little bit lucky.  Now that it's warm, the fish are easy to see - they are out in the open water, swimming smoothly and darting skillfully.  But tadpoles are a different story.  They lurk near the bottom in the leaves and muck.  Every once in a while, they clumsily wriggle from one spot to another.  The best time to see them is when you first walk up to the pond - they will wriggle to a hiding spot in response to the shadow you cast over the pond (they respond as if you were a predator!).  If you don't see one right away, wait around and watch - it might be your lucky day.  You could also take matters into your own hands and scoop through the bottom of the pond with a net.  If you do, please keep the tadpole in the water - they are very fragile and can't survive being dry or being squished.
A tadpole in our pond at the outdoor classroom.
Tadpoles are truly strange creatures.  They are the larval (young) form of frogs.  Frogs lay eggs in water, each of which will hatch into a tiny tadpole.  The tadpoles use gills to breathe water - just like fish do.  Tadpoles swim and eat and grow larger and larger, all underwater.  Eventually when conditions are right and they have had enough food, the tadpoles' bodies change form completely.  Their bodies digest and absorb their tail, and they grow tiny forelimbs (arms) and hind legs.  As they change external forms from tadpole to frog, their internal structures change too.  They grow lungs for breathing air!  Adult frogs hop out of the water and live their adult lives mostly on land but near water.  A change in body form like tadpoles have is called a metamorphosis.  If your body changed and suddenly grew wings for flying, that would be a type of metamorphosis.  Other animals that do metamorphosis are insects (maggots become flies, caterpillars to butterflies, etc.).

Tadpoles and frogs are vertebrates.  Vertebrates are any animals that have an internal skeleton with a backbone.  That means mammals, birds, reptiles and fish are also vertebrates.  Can you think of animals that don't have backbones? (Answer below*.)  Any vertebrate that starts its life in the water and undergoes metamorphosis is called an amphibian.  The word amphibian makes sense if you know what the parts of it mean: amphi- means both, and -bian means life form.  Amphibians include frogs, toads, newts and salamanders. 

Why do you think fish are such better swimmers than tadpoles?  Compare the body shape of a tadpole to a fish, then try an experiment.  First, find a pool and a life guard.  Then jump into the swimming pool and swim like normal using your arms and legs to help you.  Then hold your arms and legs into your body and try to swim - it's not so easy without appendages, is it?  Tadpoles do not have fins like fish do, and fins are great for steering while you're swimming.  They have only a tail to help push them along.  That's why tadpoles wriggle around so strangely and fish swim with ease.

How do you think the tadpoles got to our pond?  Frogs had to have laid the eggs that grew into these tadpoles, but how did the frogs get to our pond?  They have to stay near water, and there aren't other ponds nearby.  Frogs could not have gotten to our pond to lay eggs!  One possible answer could be related to the fact that frog eggs are somewhat sticky.  If a bird stood in a different pond and frog eggs stuck to its feet, then the bird came to our pond, it could have brought the eggs that hatched to our tadpoles.  That makes frog eggs disperse just like the kinds of seeds that stick to animal fur - which would be animals imitating plants!

*Invertebrates include insects, spiders, clams, snails, sponges, jellyfish, sea stars, and thousands more types of organisms.

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