Grades 3–5 · Organisms
Why does a hermit crab carry a shell it did not grow?
A hermit crab carries a useful secondhand shelter. Its front has a hard outer skeleton, but much of its abdomen has a softer covering. An empty snail shell provides extra protection around this vulnerable rear part.
In the diagramA borrowed shell shelters a softer abdomenempty snail shellhard-covered frontsoft abdomen insideCutaway: the borrowed shell is separate from the crab.
The crab fits its abdomen inside the shell’s spiral. Small legs and structures near the end of its body press and hook against the inside. This grip helps keep the shelter attached as the crab walks.
The snail originally built the shell, so the shell does not grow with its new occupant. A growing hermit crab must find another empty shell with enough room. It examines the opening and inside before moving in.
Changing borrowed shells is different from molting its own outer skeleton. The crab does both as it grows. One process replaces body covering; the other changes the separate shelter carried around that body.
About this explanation
Starting knowledge: Recognize an animal’s protective covering and know that growing changes body size.
Sources
- Smithsonian National Zoo — Land hermit crab