The generic limits of Eruca are very uncertain, as is the number of species. As usually understood it is a Mediterranean genus with one variable species, E. vesicaria. Alternatively, some of the infraspecific taxa are sometimes accepted as distinct species. Schulz (1919) distinguished five species, whereas Euro+Med Plantbase now accepts two species (E. vesicaria and E. loncholoma (Pomel) O. E. Schulz, a North African endemic). One taxon (“subsp. sativa”) is often cultivated as a minor vegetable, rucola.
Molecular data shed new light on the genus. According to some authors (Al-Shehbaz & al. 2006) it is best considered part of a broadly circumscribed genus Brassica, whereas others propose to redefine Eruca as to include some representatives of Diplotaxis (Arias & Pires 2012). Pending further studies it is here treated in a conservative way, with only one variable species accepted.
Al-Shehbaz I.A., Beilstein M.A. & Kellogg E.A. (2006) Systematics and phylogeny of the Brassicaceae (Cruciferae): An overview. Pl. Syst. Evol. 259: 89-120. [available online at: http://www.telomereevolution.org/uploads/8/9/7/6/8976955/fulltext.pdf]
Arias T. & Pires J.C. (2012) A fully resolved chloroplast phylogeny of the brassica crops and wild relatives (Brassicaceae: Brassiceae): Novel clades and potential taxonomic implications. Taxon 61(5): 980-988. [available online at: https://www.researchgate.net/publication/259812488_A_fully_resolved_chlo...
Schulz O.E. (1919) Eruca. In: Engler A. (ed.), Das Pflanzenreich 70(IV.105). Engelmann, Leipzig: 180-190. [available online at: https://www.biodiversitylibrary.org/item/56527#page/190/mode/1up]