Carcinus aestuarii is a littoral crab, native to the Mediterranean Sea. It bears some similarities to Carcinus maenas and was sometimes considered to be a subspecies thereof, rather than a species in its own right, but a molecular biological study using the COI gene found the difference between the two taxa to be substantial, supporting their status as separate species.
The two taxa can be visually distinguished by the front of the carapace, between the eyes, which is short and toothed in C. maenas but longer and smoother in C. aestuarii. Also, the gonopods of C. aestuarii are straight and parallel, whereas those of C. maenas are curved.
Whereas C. maenas has invaded many shorelines throughout the world, C. aestuarii has only been implicated in one invasion; the coastline of Japan has been invaded by either C. aestuarii or a hybrid of C. aestuarii and C. maenas.