Thanks both.

Roopak: You are right, several marine crabs have these oar-shaped appendages. I believe it is an adaptation for their more aquatic habits as compared to their terrestrial/freshwater counterparts. Matuta lunaris has all 4 pairs of legs modified into the oar-shape, in others such as Scylla serata and genus Portunus crabs (many species of which are fished commercially), only the last pair shows this modification. I have never seen it on mud crabs though...