This is what a lobster looks like:
This is what a crab looks like:
The observant observer will notice that a lobster has a long body, a tail, and antennae, while a crab has a round body, no tail, and no antennae.
Now, bearing those distinctions in mind, this is what Sebastian from The Little Mermaid looks like:
Note that Sebstian has a round body, no tail, and no antennae. He does have pincers with which to hold maracas, but since pincers are common to crabs and lobsters, that is irrelevant. The point is, Sebastian is clearly, unequivocally, undeniably, inarguably, a crab. He's a crab. Sebastian is a crab. Not a lobster. A crab.
Nonetheless, at a party this evening I found it necessary to defend, albeit unsuccessfully, the crabhood of Sebastian in the face of completely unsupportable allegations of lobsterhood. The primary evidence I offered in support of Sebastian's crab status was:
(1) No tail.
(2) Round body.
(3) He runs sideways at at least one point in the movie.
(4) He gets stuffed by the chef (a weak point since, as one of my adversaries pointed out, you can stuff lobster).
(5) "Come on! He's a crab! Are you crazy?" (perhaps a poor choice of words since I was one of the only people at this party who wasn't a psychologist).
I couldn't convince the lobsterites that Sebastian was a crab. Fortunately there are so many people who don't know the difference between a lobster and a crab that the Internet has answered the question for us. From Disney Online Guest Services:
Q: Is Sebastian from "The Little Mermaid" a crab or a lobster?
A: Sebastian is a crab.
That settles it.