Alternative name: Laimosemion ubim.

Rivulus ubim new species, is described from a small stream tributary of Lago Amanã system, Central Amazon, northern Brazil, based on external and internal anatomical morphological characters. It is considered closely related to other species of miniature Rivulus, from the same region. It is distinguished from all other rivulids by having double-branched epipleural ribs, a condition seldom found among Cyprinodontiforms, and it differs from all its congeners by having hypertrophied teeth on the anterior portion of the outer row of the premaxilla and dentary in males. It reaches a maximum adult size of about 18 mm SL and exhibits several reductive characters, as expected for a miniature species, including a notable reductive character state – four branchiostegal rays.

Rivulus ubim is a minute species reaching about 18 mm SL as maximum adult size, distinguished from all other rivulids by having epipleural ribs branching at their sub-basal portion, and again at the sub-distal portion of the dorsal branch formed by the sub-basal branching in all other rivulids the epipleural ribs are unbranched, except for some species of Rivulus with a single basal branching, but the branches formed are united by an osseous flap (Costa, 1998). Rivulus ubim is also distinguished from all other congeners by having hypertrophied teeth on the anterior portion of the outer row of the premaxilla and dentary in males vs. teeth never hypertrophied. The type locality of R. ubim consists of a small terra-firme stream (i. e. not affected by large rivers flooding during the wet season) and adjacent pools draining into the Igarapé do Ubim, a tributary of the Lago Amanã.

The collection site was located under dense forest vegetation commonly found in central Amazon upland areas mixed with Mauritia flexuosa palmtrees. The water was black with high transparency. Maximum width was about 2 m and maximum depth at the stream bed was about 0.5 m, where the substrate was composed mainly by leaf litter and mud. Most specimens of Laimosemion ubim were actually collected in the adjacent pools and next to palm-tree roots, where depth was about 0.1 m or less. The following environmental data were obtained in January 2010, at the beginning of the rainy season in the Lago Amanã region: water temperature 24.9 °C, pH 5.9, conductivity 27.4 µS · cm-2, and dissolved oxygen 5.88 mg · l-1. Over 40 other species of fish were collected in the same habitat.

The name ubim is derived from the name of the type locality, Igarapé Ubim. Ubim is the indigenous name of small Amazonian palms of the genus Geonoma, usually reaching 2-4 m height. 

Source: Original description.

Max. size 18.1 mm,
Dorsal 5-6,
Anal 8-10,
Caudal fin rays 21-23
Pectoral fin rays 12,
Pelvic fin rays 5,
LL scale count (average) 29.5
Pre- dorsal length to % SL – 76.8 %
Depth to % SL – 18.7 %