Okay, this game actually makes you a lobster. Close enough. It's a fighting game not dissimilar to Virtual On, except you are a crab or a lobster or a barnacle and duke it out with another sea creature like you. You can shed your shell, detach legs to attack separately and also shoot homing energy rays. The barnacle can't really move and gets around by cloning itself. The battle system is actually quite involved.
It also has online play, which I haven't tested.
, you can get a demo here (need to register), or buy it DRM-free for 9 bucks.