See Mahmood, S. et al, Validity, identification, and distribution of the roundscale spearfish, Tetrapturus georgii (Teleostei: Istiophoridae): morphological and molecular evidence, Bulletin of Marine Science, Volume 79, Number 3, November 2006, pp. 483-491(9), available at http://www.ingentaconnect.com/content/umrsmas/bullmar/2006/00000079/00000003/art00005