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Hermit Crab Houses and Their Stinging Guests
By Randy Brooks

How would you like your house to be the shell of a dead snail? Ugh. But to a hermit crab, itās a good home. Hermits have a soft, curled belly that squeezes right into the twists and turns of a snail shell. Inside the shell, the hermit is protected. A hermit crab can carry its shell house wherever it goes.

While studying hermit crabs, I learned that snail shells make good homes for crabs, but not perfect ones. For one thing, snail shells do not completely protect hermit crabs from their enemy, the octopus. An octopus can simply stick some of its eight arms into the shell opening (the hermitās front door) and pull the hermit out.

An Astonishing Defense
Whatās a hermit to do? Home improvement! Some kinds of hermits arm their shell homes with creatures called sea anemones (pronounced uh-NE-muh-nee). Sea anemones fasten onto the hermitās shell and live there as in the picture above.

The anemones help protect the hermit crabs. Anemones have stinging cells in their tentacles that can hurt an attacking octopus. I found some hermit crabs with lots of sea anemones on their shells and other hermit crabs with only one or two. Also, some hermits had sea anemones stuck close to the opening of the shell, and some had sea anemones stuck farther back on the shell. So, I had two questions:

Question 1. Is the number of sea anemones on the hermitās shell important?

 To find out, I placed three hermit crabs in an aquarium with an octopus: One hermit had three sea anemones on its shell; one hermit had one anemone; and one hermit had no anemones on its shell.


 Every morning, I checked the aquarium to see if the octopus had eaten a crab during the night. I did this experiment 10 different times. I found that the number of sea anemones on the hermitās shell is very important! In eight out of 10 tries, the crab with three anemones on its shell lived longer than crabs with one or no anemones. Most hermits with no anemones were eaten within a few days, but several hermits with three anemones lived in the aquarium with the octopus for more than 40 days! (See graph.)
 
 


 



Question 2. Is the location of sea anemones on the hermitās shell important?

 To find out, I put two hermit crabs in an aquarium with an octopus. One crab had a sea anemone just 5 millimeters from the shell opening. The other crab had a sea anemone at least 20 millimeters from the shell opening. I soon discovered that the location of sea anemones on the hermitās shell is also very important. In all seven experiments, the crab with the sea anemone close to the shell opening did not get eaten. The octopus only ate the crabs with the anemones at least 20 millimeters away from the shell opening.

For Best Results
 To avoid getting eaten by an octopus, hermit crabs should arm their homes with lots of sea anemones close to their homeās front door. That way, an attacking octopus is more likely to brush against the stinging tentacles. The amazing partnership between hermit crabs and sea anemones is just one of the wonders youāll see when you study ocean life. Our great seas are full of mysteries. We need future investigators like you with a passion and respect for nature to help learn about and preserve these special places.
 


Dr. Randy Brooks

I first became interested in marine science by watching Jacques Cousteau nature shows on TV. (Cousteau was a world-famous undersea explorer and environmentalist.) "As you grow up, follow your heart toward what truly inspires you, and try to pursue that area as a profession. Youāll love going to work then!"÷Randy Brooks

Randy Brooks calls himself lucky to be a marine biologist studying life in the oceans. He is a professor of biology at Florida Atlantic University in Boca Raton, Florida.
 
 
 

Hermit Crab
Bird Nest
Roosting Bats
Dragonfly

 
 
 
 

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