Unlike most crustaceans, the hermit crab is more active in the daytime than at night. Large specimens often use shells of the common whelk Buccinum undatum; smaller individuals are found in a wide variety of shells including moon snails (Polinices spp.) and periwinkles (Littorina spp.). Shells selected based on combination of weight, volume, and aperture size, and often covered with the hydroid Hydractinia echinata or the anemone Calliactis parasitica, whose stinging cells (nematocysts) help to deter predators. The crab actively transplants anemones onto its shell; it prods the anemone with its legs in a manner that elicits its release so that the anemone can reattach to the hermit's shell.
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