Description:
Knotfinger mud crabs reach approximately 2.5 - 3 inches in carapace length.  The chelae are unequal and have a granular appearance.  The dorsal surfaces of the chelae have a reticulate pattern that is heavily blotched or spotted with dark markings, especially along the frontal edges and upper surfaces.  The carapace has transverse, granulate lines on the ocular and hepatic regions.  On the carapace, the first two anterolateral teeth are coalesced, but separated by a deep, rounded notch.  The tips of the other carapace teeth are somewhat rectangular.  Color is typically grayish on the dorsal surface, often blotched or spotted with contrasting markings.  The ventral surface is typically lighter in color, ranging from cream to yellow-brown or light tan.   

Habitat:
Knotfinger mud crabs are common intertidal and shallow littoral species in the tropical western Atlantic.  They are associated with hard substrates such as worm reefs, oyster reefs, mangrove roots, and inlets.  They can also be common in seagrass meadows. 

Similar Species:
Panopeus herbstii, the common mud crab, is similar in size and general characteristics, but tends to be more olive-brown to gray or slate in color.  First and second teeth of the carapace are coalesced as in P. lacustris, but the notch separating them is rather shallow in comparison.  The other carapace teeth tend to curve toward the anterior.  The fifth tooth is hooked forward and significantly smaller than forth tooth.

Range:  
Knotfinger mud crabs range from Bermuda south through Florida, the West Indies, and the Caribbean, to Brazil.  They have been introduced into both California and Hawaii.