 C. maenas has a carapace up to 60 millimetres ( 2 @.@ 4 in ) long and 90 mm ( 3 @.@ 5 in ) wide , but can be larger outside its native range , reaching 101 mm ( 4 @.@ 0 in ) wide in British Columbia . The carapace has five short teeth along the rim behind each eye , and three undulations between the eyes . The undulations , which protrude beyond the eyes , are the simplest means of distinguishing C. maenas from the closely related C. aestuarii , which can also be an invasive species . In C. aestuarii , the carapace lacks any bumps and extends forward beyond the eyes . Another characteristic for distinguishing the two species is the form of the first and second pleopods ( collectively the gonopods ) , which are straight and parallel in C. aestuarii , but curve outwards in C. maenas . 
