Abstrat
Rivulus illuminatus, new species, collected in the floodplains of córrego da Queixada, upper rio Paraná basin, central Brazil, is described. It differs from all other species of the subgenus Melanorivulus by possessing six pelvicfin rays. Rivulus illuminatus belongs to a group, herein referred as the Rivulus pictus clade, diagnosed by having ventral process of angulo-articular vestigial, first epibranchial curved, and flank intense greenish blue or greenish golden, to purplish blue above anal-fin base; R. illuminatus differs from other species of the R. pictus clade by possessing frontal squamation F-patterned. Rivulus illuminatus belongs to a species assemblage included in the R. pictus clade, here called the R. pinima clade, also including R. pinima and R. kayapo, which is diagnosed by color patterns of males (longitudinal rows of red dots on flank; reduction of black pigmentation on head) and females (white to light yellow horizontal mark above caudal spot). It differs from other species of the R. pinima clade in having the combination of 32-33 caudal-fin rays, 33-35 scales in longitudinal series, 8 scales in transverse series, dorsal-fin origin in vertical through base of 7th or 8th anal-fin ray, and tip of pelvic fin reaching between base of 1st and 3rd anal-fin rays in males.

http://www.pfeil-verlag.de/04biol/pdf/ief18_3_01.pdf