That poor lobster

That's not invulnerable. They can still suffer from diseases. So immortal

Cniderians are also immortal (I think I spelled that right) such as jellyfish. Bacteria are also theoretically immortal because they're all the same cell that has been split. Which is the original cell after the split? None. Both are the same cell. Therefore, if they keep splitting, they keep making more copies of themselves. Therefore, they're immortal because the last ones are identical to the first one.

Their shells are made of carbohydrates, which have chitin in them, a very strong substance.

Lobsters (Homarus) are NOT immortal, they can live up to 100 years, but they eventually die as every other (higher) animate being. The span of life is limited for genetical reasons: every time a non-embryotic stem cell performs karyokinesis (that can be once a few hours, once a week, once in several months or never) the ends of its DNA (called telomeres) shorten a little bit. Are they too short (have reached the so-called Hayflick limit) the cell will die. This process is called aging and will eventually lead to the death of the whole individuum. However, some species are indeed potentially immortal: Bacteria, for example, simply don't have telomeres that can be shortened and their DNA can technically replicate indefinitely. Other creatures, such as the hydra (Hydra) use certain enzymes to restore their telomeres (the same way embryotic stem cells and cancer cells do) and by this extend their lifetime to immortality. Some sorts of jellyfish are able to live indefinitely, too, but...

...that's a rather complicated mechanism to explain here. However, every lobster will eventually die either from age or from being served with melted butter.

Almost forgot: chitin IS a carbohydrate.

Thus wiseguy increases my wiseness.