Carcinus aestuarii 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.

[2] The two taxa can be visually distinguished by the front of the carapace, between the eyes, which is short and toothed in C. aestuarii but longer and smoother in C. maenas.

Also, the gonopods of C. aestuarii are straight and parallel, whereas those of C. maenas are curved.

[3] It is found in the Atlantic Ocean around the Canary Islands, all along the coasts of the Mediterranean and the Black and the Azov Seas including the Suez Canal.

Eating habits can change throughout its lifespan and seasonally in relation with available food in its habitat.