It would take an AWFUL lot of crabs acting as logic gates to model a crab brain or an entire crab. A single logic gate by itself is clearly less complex than a soldier crab.
But if you really could model a crab brain perfectly in a turing machine (not clear you can), then let's make a crab brain out of crab logic gates, and then use those mega-crab-brains to make more mega logic gates, and put together a bunch of those to... you'd probably run out of crabs.
But if you really could model a crab brain perfectly in a turing machine (not clear you can), then let's make a crab brain out of crab logic gates, and then use those mega-crab-brains to make more mega logic gates, and put together a bunch of those to... you'd probably run out of crabs.