Epactionotus bilineatus is named for the two parallel dark stripes running along its flanks, which distinguish it from most other small loricariids in its region. It belongs to the genus Epactionotus, a small group of otothyrine loricariids restricted to the Pelotas and Uruguay River drainages of southernmost Brazil — one of the most geographically isolated freshwater systems on the continent. The Pelotas River drains a temperate highland zone with some of the coldest freshwater fish habitats in South America.
The streams occupied by E. bilineatus are cool, clear, well-oxygenated mountain streams with rocky substrates. Temperatures in these highland drainages can approach 10–12°C in winter, making E. bilineatus one of the most cold-tolerant loricariids known. In summer, temperatures rise to comfortable subtropical levels. The species grazes periphyton, biofilm, and diatoms from smooth rock surfaces in moderate to fast current.
For aquarists, E. bilineatus is an extreme rarity that requires genuinely cool water — too cold for most tropical species and requiring specific care attention. It is best housed in an unheated or chilled aquarium with other cool-water specialists. Its twin-stripe markings are clean and attractive, and its scientific interest as a cold-adapted Pelotas endemic makes it a compelling subject for dedicated southern loricariid collectors.
