That's the rule crabs follow when they walk that walk. Most crabs have eight legs in four pairs, but crabs in the antlered crab family have only six legs, for their rearmost pair of limbs have specialized into hook-shaped appendages curled up over their backs, which they can use like hands to grip sea anemones and shellfish for camouflage, and also as shields to ward off attacks from other creatures.
螃蟹一，爪八個（四對步足），屬於人面蟹科的螃蟹卻結結實實都只有六隻腳，最後一對腳朝背上特化成勾爪狀，像手一樣可以抱住海葵、貝殼，既可偽裝，又能充當盾牌抵擋外力襲擊。