Megalopal and first crab stage morphology of Cyrtograpsus angulatus Dana was studied in specimens from Mar Chiquita Lagoon, Argentina and an identification key for the megalopae of the four southwestern Atlantic varunid crabs is proposed. Some anomalous first crabs were observed that differed in the carapace shape and in the orientation of the ocular peduncle. These crabs survived in laboratory cultures, molting to the following stage with normal aspect. An anomalous Chasmagnathus granulatus first crab was also found in the field. We speculate that these odd crabs appeared after a complete, but defective, ecdysis. Since the ultimate body structures to be separated from the exuvia are probably the cephalic appendages and the eyes, their survival in the field would be difficult since vision may help them to detect predators.