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How Hermit Crabs Trade Shells in a Line

7th grade nonfiction passage about hermit crab shell swaps: 246 words, six questions, an answer key and vocabulary.

7th grade nonfiction hermit crab shell swaps 246 words

How Hermit Crabs Trade Shells in a Line

A hermit crab is born with a problem. Unlike most crabs, it has a soft, curved body with no hard covering at the back. To stay safe, it backs into an empty snail shell and carries that shell everywhere, pulling its body inside when danger comes near.

The trouble is that a crab keeps growing and a shell does not. Sooner or later the crab is too big for its home and must find a larger one. Since hermit crabs cannot make shells of their own, they depend on the empty shells that snails leave behind. Good shells are not always easy to find.

Scientists who study some kinds of hermit crabs have noticed a remarkable pattern called a vacancy chain. When one crab discovers a large, empty shell, it does not simply slide in. Other crabs often gather around, and the group lines up by size. The biggest crab moves into the new shell first. Its old shell is now free, and the next biggest crab takes it. Then the third crab moves into the shell that crab left behind, and so on down the line, until every crab has a new and slightly roomier home.

A single shell can improve the lives of many crabs. The chain works because each shell that is left behind is the right size for the animal waiting just behind it. Even a creature that looks lonely and solitary can end up depending on the habits of its neighbors.

Questions

  1. Why does a hermit crab need a snail shell? in the text

  2. Why must a hermit crab change shells as it grows? in the text

  3. What is the name for the pattern of crabs swapping shells in order? in the text

  4. Which crab moves into a large new shell first? in the text

  5. Why does one new shell help many crabs and not just one? think about it

  6. What does the last sentence suggest about hermit crabs? think about it

Words to know

vacancy
an empty space that someone could move into
remarkable
unusual and worth noticing
roomier
having more space inside
solitary
living alone and not with others
pattern
something that happens in the same way again and again
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One passage, three levels

The same topic and facts written three ways, so a mixed class can read and talk about one text. Each group’s sheet shows only a shape; the key says which is which.

● Easier · 220 words · Flesch-Kincaid grade 2.2

A hermit crab starts life with a problem. Most crabs have a hard covering on their bodies. A hermit crab has a soft, curved back end. To stay safe, it backs into an empty snail shell. It carries the shell everywhere. When danger comes near, it pulls its body inside.

But a crab keeps growing, and a shell does not. Soon the crab is too big for its home. It must find a larger one. Hermit crabs cannot make shells. They need the empty shells that snails leave behind. Good shells are not always easy to find.

Scientists who study some hermit crabs have seen a pattern called a vacancy chain. One crab finds a large, empty shell. It does not just slide in. Other crabs often gather around, and they line up by size. The biggest crab moves into the new shell first. Its old shell is now free. The next biggest crab takes it. Then the third crab moves into the shell that crab left. This goes on down the line. In the end every crab has a new, roomier home.

One shell can help many crabs. The chain works because each shell that is left is the right size for the crab behind it. A crab may look lonely, but it can still depend on its neighbors.

▲ This passage · 246 words · Flesch-Kincaid grade 5.9

A hermit crab is born with a problem. Unlike most crabs, it has a soft, curved body with no hard covering at the back. To stay safe, it backs into an empty snail shell and carries that shell everywhere, pulling its body inside when danger comes near.

The trouble is that a crab keeps growing and a shell does not. Sooner or later the crab is too big for its home and must find a larger one. Since hermit crabs cannot make shells of their own, they depend on the empty shells that snails leave behind. Good shells are not always easy to find.

Scientists who study some kinds of hermit crabs have noticed a remarkable pattern called a vacancy chain. When one crab discovers a large, empty shell, it does not simply slide in. Other crabs often gather around, and the group lines up by size. The biggest crab moves into the new shell first. Its old shell is now free, and the next biggest crab takes it. Then the third crab moves into the shell that crab left behind, and so on down the line, until every crab has a new and slightly roomier home.

A single shell can improve the lives of many crabs. The chain works because each shell that is left behind is the right size for the animal waiting just behind it. Even a creature that looks lonely and solitary can end up depending on the habits of its neighbors.

■ Harder · 258 words · Flesch-Kincaid grade 8.7

A hermit crab is born with an inconvenient problem. Unlike most crabs, it has a soft, curved body with no hard covering at the rear. To remain safe, it backs into an empty snail shell and carries that shell wherever it goes, withdrawing its body inside whenever danger approaches.

The difficulty is that a crab continues to grow while a shell does not. Sooner or later the crab becomes too large for its home and must locate a bigger one. Since hermit crabs are incapable of manufacturing shells of their own, they depend entirely on the empty shells that snails leave behind, and suitable shells are not always easy to come by.

Scientists who study certain kinds of hermit crabs have observed a remarkable pattern known as a vacancy chain. When one crab discovers a large, empty shell, it does not simply slide inside. Other crabs often gather around the find, and the group arranges itself in a line according to size. The largest crab moves into the new shell first, which frees its old shell for the next largest crab. The third crab then occupies the shell that crab has just vacated, and so on down the line, until every crab has acquired a new and slightly roomier home.

A single shell can therefore improve the lives of many crabs. The chain works because each abandoned shell happens to be the right size for the animal waiting just behind. Even a creature that appears lonely and solitary can end up depending on the habits of its neighbors.

Download as one PDF

How to use it

  1. Press Read it big (or B) to put the text on the projector, or Full screen (F) to fill the whole screen.
  2. Ask the questions one at a time and use Show answer when the class has decided. A shows or hides every answer.
  3. Print student copy for the class, or Print with answers for yourself.

Ideas for class

  • Before reading, put vacancy, remarkable, roomier, solitary and pattern on the board with their meanings from the Words to know section and have students predict what the text will be about.
  • Read it aloud once while students follow along, then have them reread it alone or with a partner. At a calm 100 words a minute it takes about 2 minutes to read aloud.
  • Use the Read it big button (or B) to put the text on the projector, and Show answer on one question at a time so the class commits to an answer first.
  • Question 5 asks the reader to infer. Ask students to point to the words in the passage that led them to their answer.
  • Ask students to write one question the passage did not answer, then look for the answer in a book or on a classroom screen.
  • Print the student copy for independent work and keep the answer-key copy for yourself.

Related

Questions

What reading level is "How Hermit Crabs Trade Shells in a Line"?

It was written for 7th grade. It has 246 words and works out to a Flesch-Kincaid grade level of about 5.8. That is a formula's estimate from sentence length and syllables, and it is not a Lexile or a guided reading level, so check it against your students.

What do the questions ask?

There are six. 4 ask for something the text says directly and 2 ask the reader to work something out from clues. Each has an answer in the key, and the answers are written as a teacher would accept them, not as the only possible wording.

How long does it take to read?

About 2 minutes read aloud at 100 words a minute. For a one-minute fluency check, paste it into the fluency timer and use the word count.

Can I print it?

Yes. Print student copy gives the passage, the questions with lines to write on, and the word meanings. Print with answers gives the same page with the answers filled in. Both are made in your browser.

Is this passage from a published book?

No. It was written for Peabody Teach, so there is no book to buy and no login to read it.

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