Jonah crabs, Cancer borealis, and Atlantic rock crabs, Cancer irroratus, were collected from Maryland and Delaware coastal waters to create a species-identification card for local fishermen. Recently, Jonah crabs became federally and state regulated while Atlantic rock crabs remain unmanaged. For males of both species, the morphometric attributes studied were the length of the right third pereiopod merus (R3 merus), right chela height (RChH), right cheliped weight, right chela width (RChW), right chela length (RChL), total body weight, and carapace width (CW). There were distinct markings and color patterns on the manus that proved useful for identification purposes. The variables that differed the most between the two species are the length of the R3 merus, the RChH, and the difference between the long carapace width (LCW) (maximum width perpendicular to the medial axis) and the short carapace width (SCW) (distance between indentations anterior to the outermost anterolateral spines). The R3 merus length of the Atlantic rock crab was significantly greater than that of the Jonah crab (P = 0.0002) for a given CW. Additionally, the merus length showed a significantly (P = 0.038) steeper slope in rock crabs than in Jonah crabs. Both RChH and RChW are positively correlated with CW for both species (Jonah: R2 = 0.6647, R2 = 0.7673, respectively; Rock: R2 = 0.8714, R2 = 0.8632, respectively). We found that chela height of Jonah crab was greater than rock crab for the measured range (60-140mm). The chela width did not differ significantly between the two species (P > 0.05). The difference between the carapace widths are significantly different (Student’s t-test, P < 2.2e-16). The ratio of cheliped weight to total body weight did not differ with carapace width, nor did it differ significantly between the species (ANCOVA, P = 0.127). RChL increased with CW, but there was no significant difference among RChL between the species for a given size (P = 0.528).
