The nautilus is negatively buoyant; it is unable to regulate its buoyancy and just sinks to the bottom. This is not something that occurred after receiving it. The nautilus has been in the aquarium collection for about 2 years now. I do know that if you do experience buoyancy issues upon arrival, the best thing to do is to wait it out and the issues usually resolve themselves.
While some people do feed chicken to their nautilus, I will never feed that out to captive nautiluses. Not very natural at all and I would much rather feed out animals thought to be its prey such as crab and shrimp, like you suggest.
Thanks for the info.
Greg