The Marsh Squareback Crab. Similar in size and habitat as the Fiddler Crab, the Squareback lives in the upper reaches of the marsh, but also makes forays into areas inhabited by man. Those living near the marsh will often find these endearing little crabs scurrying about the property. The Marsh Crab is one of the few animals that feeds on the marsh grass itself, preferring the older, outer leaves of Spartina Grass.