Neatly done Abhishek. U caught it displaying a typical behavior. I think i know this kind with a sort of jerky movement of the limbs it disappears under the sand.
Also if Im not wrong some other crab varieties have a similar appendage (Oar shaped) Ive seen it on the mud crabs and also on a couple of varieties found in the sea, mostly caught for the pot.

TFS
Roopak