This species is distinct from common otos in its possession of an omega iris (/iris operculum). O. flexilis and O. affinis are the two species that were suggested to form Macrotocinclus–however O. flexilis lacks the raised/enlarged odontodes found in other species with an omega iris: O. affinis, O. xakriba, and O. mimulus. Again, Macrotocinclus would unnecessarily make a mess of Otocinclus, so it generally is not used (and especially not for O. flexilis for whatever reason).
O. flexilis is a Batesian mimic of Corydoras paleatus–peppered cories! Sadly, this species is nowhere near as common in the ornamental fish trade as is the peppered cory. They’re probably most likely to show up as contaminants alongside wild-caught peppered cories. See PlanetCatfish for more.
Otocinclus arnoldi
This species was once considered a junior synonym of O. flexilis. As in, they were considered to be the same species, just described twice at different times under different names. However, O. arnoldi was resurrected as a distinct species in 2010 by Lehmann, Mayer, & Reis. They found that O. arnoldi consistently had only five branched rays in the pectoral fin vs. six for O. flexilis.
