This male fiddler crab (Uca vocans) was timidly walking past a burrow when a rival male's huge pincer popped out of it. What ensued was the usual fiddler crab arm-wrestling bout, except that the second crab never actually left its burrow. The battle ended with the 'trespasser' backing off; the rival left his angry pincer poking out of the burrow for a few minutes after the fight.
Chorao Island, Goa
Fiddler crabs are very territorial, and the males of all species have one of their claws greatly enlarged for use as a weapon to battle rival males and to signal in a manner similar to a flag. The name "fiddler" comes from the way these crabs wave the large claw, resembling a musician moving a bow across a fiddle.
Canon 500D, Canon 100mm Macro USM
The image lacks sharpness- this was clicked in low light early in the morning, and from a difficult position (my feet were stuck in the mud ).