A review of the species complex published in 1998 was able to differentiate between specimens from the Mediterranean Sea and those from the Atlantic Ocean, and concluded that the Atlantic specimens were a separate species, accepted as M. brachydactyla Balss, 1922.
The 1998 study reviewed the species complex and found that specimens from the Atlantic Ocean differed enough to be considered a separate species, which was later named *M. brachydactyla* Balss, 1922.