Hermit Crabs:

Are they picky when choosing their shells?

Why does a hermit Crab need a shell?

How can he carry that heavy shell?

A Hermit Crab uses his shell for a home. His shell serves as protection for his soft abdomen. His abdomen contains all of his major organs. The outside of his abdomen has little appendages so he can hold on to his shell. Since the Hermit Crab lives in the water the weight of his shell is supported by the water. This lets the crab carry his heavy shell.

Questions we had:

1. Do hermit crabs claim old shells as theirs?

2. How do they pick their shells?

3. How long can they live outside their shell?

4. Are they all red and brown?

5. Where do they fall in the food chain?

6. How often do they reproduce?

7. How long is their life span?

8. Do they pick specific types of shells?

9. Do they pick specific sizes of shells?

10. How big is too big?

We finally decided on a question that we knew we could answer with only the information available on the island. The question was: through the life span of a Hermit Crab what is the smallest and largest shell they would call home? Do they prefer a specific type of shell?


We hypothesized that Hermit Crabs had to be some what particular when choosing their homes. If they werenít they would not be able to carry them. What we had to do was figure out the size and type of shells the crabs preferred.


We began our experiment by first finding the Hermit Crabs. The result of our search concluded that Hermit Crabs could only be found in tidal polls. However, we did find very small crabs in the slough. These crabs were all found in half-moon snail shells. We did not find any crabs in the open ocean. Next, for every hermit crab we found we measured the length of the shell in millimeters and the length of the shell opening. We were not able to detach the crab without hurting him so we were not able to measure the actual crab.

After we had measured about sixteen shells, we began to graph our findings. We listed the shells in the order we found them. Each shell had a measurement for length and opening. This information led us to our conclusion.


Our data found that Hermit Crabs are picky when they choose their homes. Small shells ranging in size from 29-80 mm in length and an opening ranging from 3-25 mm are the most popular. After shells reached a length of 80 mm crabs found them very hard to carry. All of the shells we found in this range were empty.

Small Hermit Crabs prefer half-moon snail shells. We found these are the only shells they can wrap their abdomen in and still carry. We also found that the larger crabs all preferred conch shells to accommodate their anatomy.

What types of shells did we find?


Where were the hermit crabs found?


Range of shell lengths and openings

Shell LengthShell Opening

1. 70mm
2. 80mm

3. 55mm

4. 47mm

5. 35mm

6. 40mm

7. 29mm

8. 150mm

9. 100mm

10. 115mm

11. 90mm

12. 80mm

13. 50mm

14. 20mm

15. 20mm

16. 3mm

Shells contained

Live Hermit Crabs

(found only in

Tidal Pools)

Only empty shells found in this range.

Only Half-moon snail shells found.

20 mm

19 mm

20 mm

10 mm

5 mm

29 mm

150 mm

35 mm

38 mm

23 mm

30 mm

20 mm

3 mm

3 mm

1 mm