I don't think there are that many "species" of Crappie, but there certainly are several variations other than the Black & White Crappie.
I would assume these are the "7" he was referring to :
Black Crappie
White Crappie
Hybrid Crappie (natural hybrid)
Blacknose Black Crappie (Black Crappie w/marking gene)
Magnolia Crappie (lab produced hybrid)
Blacknose White Crappie (natural hybrid)
Golden Crappie
I've only seen/caught the first 4 ... but I've seen pics of all 7 versions.