I found a nice comparison image of three...

I found a nice comparison image of three Otocinclus species that are horribly misidentified all over the web. The lower illustrations seem to originate from here.

To the far right is Otocinclus affinis. The O. affinis name is slapped on like, every oto ever, probably because it’s the first alphabetically in a list of Otocinclus species. You can look forward to almost NEVER finding this one sold in shops. It is very distinct from the commonly encountered species–and not just in coloration! O. affinis has an omega iris, where most otos have a completely circular pupil. Some sources might use the combination Macrotocinclus affinis, which is bunk since it renders the genus Otocinclus paraphyletic. There are in fact other otos which are most closely related to O. affinis–multiple species that also possess the omega iris as well as a tuft of raised odontodes on the head.

In the middle is Otocinclus vittatus. This is one of the species commonly found for sale. O. vestitus is difficult to distinguish from this species, but in Schaefer’s big 1997 Otocinclus paper, he had over 1,500 museum specimens of O. vittatus to examine vs. ~70 for the other. O. vittatus is likely far easier to collect, and it is widely distributed across South America. Note how the lateral stripe reaches all the way to the caudal fin; it widens a bit just before it reaches the fin.

On the left is Otocinclus macrospilus, another common species. With these fish, the lateral stripe tends to disappear or fade as it approaches the caudal fin. The stripe is punctuated with a tail blotch that seems to have two halves (or, in Schaefer’s words, “a double caudal pigment marking, comprised of a large, diffuse triangular anterior mark separated from the [lateral] stripe, followed posteriorly by a separate rectangular to sickle-shaped pigment blotch”). IME the disappearing stripe and halved blotch are most apparent in pale/stressed fish and tend to become more like the photo as the fish colors up.

(Source: aquavitro.org)

otocinclus fishblr aquablr loricariidae