Atlantic Marsh Fiddler Crab (Uca pugnax) - likely a territorial dispute - there's no way to know if the small one is defending its burrow or if the larger one is evicting a squatter

2014-8-15 Bombay Hook NWR, DE
the larger blobs of mud in this photo aren't fecal pellets, that's mud excavated from the burrow
info
http://bugguide.net/node/view/361847
and
http://bugguide.net/node/view/1007880/bgimage