The impermeability (or not) of a lobster's exoskeleton

I had lobster for dinner last night, steamed at home. When I broke open the claw a lot of water spilled out. Do live lobsters fill up with water inside their exoskeleton?

Sounds like you got a new shell lobster. If you’re paying by weight you end up paying for water. It’s happened to me on occasion. The best way to avoid this is to go to a place that’ll let you select your own and give the claw and body a squeeze, the claw should be hard as a rock.

The shells were definitely hard. I am just wondering if a live lobster’s shell seals out the water from its ambient environment. I suppose that when you cook them, the change to the outer tissue from cooking could allow water to get in through the joints or something. I bought them steamed at the grocery store then re-heated them by steaming in an inch of water at home. I was wondering if my re-steaming introduced the water.

[insert comment about OP name and topic correlation here]

One other possibility is that the shell is impermeable and juices released from cooking had no way to escape until you broke it open. Also, steam can get through openings water can’t (due to surface tension).

“Soft shell” is a bit of a misnomer for lobsters. Unlike soft shell crab, the shell of any newly molted lobster you’ll find in the market or on your plate at a restaurant is still pretty hard and would probably not be noticeable as “soft” by anyone other than a lobsterman or fish cook, unless you had a “hard” shell lobster to do a side-by-side comparison with.

Don’t lobsters live in saltwater? I thought they’d need to keep saltwater and their body separate.

I’ve seen this before - juices coming out of enclosed shell sections in cooked crustaceans - it’s really hard to tell whether it originates from the animal’s own fluids, or if it’s from cooking water ingress - because even in the latter case, it is still infused with the flavour of the meat.

I guess the question is whether this occurs when they are cooked by steaming or grilling.

Like humans or any animal, lobsters have an inside that they defend from contamination from the outside. We have our skin, with only selected openings and internal cavities that should, to lesser and greater extent, be considered to be part of the outside. Lobsters have their exoskeleton, with only selected openings …

Now I’m not a biologist, but to me that leads to the conclusion: Live lobsters do not fill up with water inside their exoskeletons.

Concur. A lobster us not an animal wearing a suit of armour as if it were a garment. The only question is whether the fluid the OP describes was there all along, or entered during cooking.

Also, I suspect a lot of the space you see inside the shell isn’t there in the live animal - the meat starts to shrink once the lobster is caught, and shrinks more on cooking.