Simana (Aka-Karashina) is a leaf mustard of the Brassicaceae family. It looks similar to the commonly known Karashina (blue), but its green leaves have reddish-purple veins.
In Okinawa, the blue Karashina is currently known as "shimana," while the term originally referred to the native Ryukyuan Aka (red) Karashina.
It is in season from December to May.

Shimana is not eaten raw. It is often consumed as salted pickles (Chikina) or stir-fried (Chikina Irichi).

A local family has been passing down native Shimana seeds from their ancestors for more than a century in the Futami area on the east coast of Nago City, Okinawa. Even during the time of war, the family safeguarded these precious seeds without loss. However, after Blue Karashina, whose seeds are easily obtained, was introduced, the Simanas began to hybridize, with the leaves merging red and blue. In response to this, a backcrossing attempt was started to restore the leaves' natural red coloring. The "Futami Red Karashina Production Cooperative" was founded in 2017 to help with this initiative.