I asked ‘Bob”

Yes, Doc—but the risk is usually low, and it depends heavily on what kind of clam/mollusk and where it came from.

If you’re thinking about putting a live clam or other mollusk into your pet lobster aquarium, the main concerns would be:

• Parasites: Some parasites use mollusks as intermediate hosts, though many are quite host-specific and won’t infect lobsters.
• Bacteria: Wild or poorly handled shellfish can carry bacteria, including Vibrio species. These can potentially contribute to disease in crustaceans, especially if the lobster is stressed.
• Fouling organisms and hitchhikers: The clam’s shell can carry protozoans, worms, algae, hydroids, or other organisms into the system.
• Water-source contamination: Honestly, this may be the bigger concern. A clam collected from the wild can bring in pathogens from the environment even if the clam itself isn’t a direct disease vector.
• Decay: If the clam dies unnoticed, it can deteriorate quickly and create a substantial ammonia/oxygen-demand problem.

For your lobster tank, I’d consider a clam from an established disease-free captive marine system substantially safer than introducing a wild-collected one. If your goal is simply to give the lobster a clam as food, freezing it first would eliminate many—but not all—biological risks.

If you tell me what mollusk you’re considering and whether it’s wild-caught, grocery-store, or aquarium-sourced, I can dig into the actual lobster pathogens and tell you how much of a cross-species risk there really is.