I remember seeing what looked like a giant underwater roach when I was younger, at what was then the Camden Aquarium in New Jersey. Other people remember this too, but none of us remember what it was really called. What the hell was this?
Was it one of these guys?
If so it was a horseshoe crab.
Isopods.
http://en.wikipedia.org/wiki/Isopod
http://en.wikipedia.org/wiki/Giant_isopod might be more what he was thinking of. Though they're not really that roachlike, more like a slater (woodlouse).
Interesting growth phenomena:
Never heard of 'manca' before - must check that.