Crabs also walk sideways, never moving forward. Prufrock sees himself as similar to the lateral-moving scavenger on the ocean floor, moving back and forth between meaningless parties, finding scraps, but never really going anywhere.
He is actually comparing himself to a crab in this line, not a lobster. Not only is this comparison a metaphor, but it is also an example of what Eliot called the "objective correlative." That is, in Eliot's words, "a set of objects, a situation, a chain of events which shall be the formula for that particular emotion; such that when the external facts, which must terminate in a sensory experience, are given, the emotion is immediately evoked" (see the link to The Literary Encyclopedia below).
The objective correlative evokes in the reader an emotional response without directly stating what the speaker is feeling. As one critic has said: "It is a means of communicating feeling, giving the 'internal world' a correlative relation in the 'external world,' and doing so in a way that was definit, impersonal, and concrete" (see the link below).
Eliot uses the image of a crab scuttling across the ocean floor to describe Prufrock's loneliness and desolation without coming out and saying, "I'm lonely."
There are other examples of the objective correlative in this poem. Visit the links below for more information.
Interesting question LOL. T.S. Eliot is using a metaphor here. Perhaps the narrator, Prufrock, who is very unhappy with his inability to approach women, references a crustacean due to the fact that it is a bottom-dwelling creature, meaning these are the "lowest of the low" sea creatures who eat what's at the bottom of the ocean. Lobsters and crabs also feed on dead carcasses.