Possible Clues:

• Tuna scarce? Perhaps - and lobster, for example
• Such as a barnacle
• E.g. crab, lobster, woodlouse
• Woodlouse, say, can react badly if interrupted by us
• Such as a crab or shrimp
• Hard-shelled animal
• Arthropod with a carapace
• Ground nut carcase, which is shelled
• Eg, crab, lobster
• Tuna scarce? Switch to crab, perhaps