If you go to the beach to collect marine specimens, in a small bay, you can see a creature similar to shrimp but also crab-like, carrying a “house” on its back—a snail shell, lively moving in the water or on rocks, immediately retracting its body into the snail shell when alarmed. This creature is called a “hermit crab.”
The morphology of hermit crabs is more advanced than shrimp but still retains primitive characteristics of shrimp; it also resembles crabs to some extent but still maintains a certain distance. They fully exhibit a transitional form from shrimp to crab during evolution.
Hermit crabs lead a parasitic life, dwelling in sturdy and durable little houses, which they neither register nor pay rent for. Sometimes they even forcefully occupy other snail shells as their own.
Why do hermit crabs inhabit snail shells? This is because they have lost the agile swimming ability of shrimp, and they lack the hard shell of crabs and defensive weapons against enemies. Therefore, the snail shell becomes their natural shelter for protection.
Upon further examination of the body structure of hermit crabs, we can see that their unique form is best suited for living in snail shells:
The chelipeds on the cephalothorax of hermit crabs, one large and one small, serve as their primary tools. The first and second pairs of walking legs are long and strong; the third and fourth pairs of walking legs are smaller and can support the interior wall of the snail shell, keeping the body stable. The abdomen is soft, long, and curved, allowing it to spiral inside the snail shell, with the tail fan hooked onto the top of the shell to prevent the body from sliding out. The abdominal appendages have degenerated, remaining only on one side of the abdomen, which is more pronounced in female crabs. If disturbed, they quickly retract their bodies into the snail shell and block the entrance with their chelipeds, remaining unharmed.
As their bodies grow larger, their original homes become too small, so they must search for suitable homes in the ocean. When they encounter empty snail shells, they use their chelipeds to probe inside. If they deem the size suitable, they move in; if they cannot find a suitable home, they become restless. Eventually, when they spot a suitable snail shell, they suddenly attack it, devouring the living snail inside and safely hiding their bodies inside.
Interestingly, sea anemones like to “settle down” on hermit crab houses, allowing them to travel around the ocean, searching for food. However, hermit crabs never object to the sea anemones because they also benefit from them. When danger approaches, the sea anemone displays its power, using its stinging cells to fend off enemies and protect the hermit crab’s safety.