Alternative name Rivulus (Oditichthys) tocantinensis.

Rivulus tocantinensis is found a dense forest black- water stream in the middle section of the Tocantins River drainage, at the floodplain's of the left bank 4 km from the city of Sampaio, Estado Tocantins, Brazil.

Following the description the new species is distinguished from related species as Rivulus urophthalmus, R. stagnatus, R. mazaruni, R. lungi and R. deltaphilus by the number of scales in the longitudinal series having 33-35 (vs. 36-43) and by the absence of well-defined yellow stripes on dorsal and anal margin of caudal fin in males. (vs. present) from R. urophthalmus by the absence of diffuse grey bars on the anterior portion of the flank in males (vs. presence); from R. xinguensis by having a well- developed dermosphenotic (vs. absent, sometimes rudimentary) horizontal red spots on the flank ( vs. rows of wide brownish orange stripes.) and transverse rows of of dark brown dots on the caudal fin in males (vs. large dark orangish brown blotches on the middle of the caudal fin)

The given name refers to the occurrence of this species in this river.

Material of the species was collected by G.C. Brasil on 21 february 2008. Coordinates are: 5º 23'00"S; 47º 51'41"W.

It would be interesting to run a DNA sequence on this new species and R. xinguensis as well and compare them to the R. urophthalmus clade- members mentionned in the description. Results could give a more detailed view on the position of these two species within this clade or may result in exclusion from it.

Their resemblence to the R. urophthalmus clade is not to obvious to me.

The given size is from the samples taken in the wild. It is not known what their maximum size can be after getting adult.

If the size given in the description is their actual adult size the species is more related to other small Rivulus in the Amazone area.

Not known yet, but should be not very different from other members of the genus.

Remarks :

Size largest sample measured 38.4 cm.
Dorsal 7-8,
Anal 12-14,
caudal 26-29
D/A 10.5,
LL scale count (average) 35-37
Pre- dorsal length to % SL – 00.0 %
Depth to % SL – 00.0 %

Costa W.J.E.M, 2010. Two new species of the Rivulus urophthalmus group from the Tocantins and Xingu river drainages, eastern Brazilian Amazon. (Cyprinodontiformes: Rivulidae) Ichthyol. Explor.Freshwaters, Vol. 21, No. 1, pp. 79-85, 2 figs., 1 tab. March 2010.