Otocinclus species: the littlest algae-eaters

Otocinclus species. Mixed-species stocks of Otos are usually imported, so all the Otos mentioned towards the end of this page are being given their conventional "trade" names here. There are seventeen Otocinclus species at FishBase, four of them new to science since 1997, but identifying Otos at the species level depends on professional techniques, like accurate counts of scales along the lateral line, or details of the dentition and the body armor, and symptomatic ratios of one body measurement to another. The details we amateurs might pick out, for instance minute differences in patterning, such as the blotch on their caudal peduncle, are less secure.
 
A specific name is rarely offered at the LFS. Often they are rather arbitrarily offered as O. affinis (now officially Macrotocinclus affinis), O. arnoldi, or O. flexilis:  O arnoldi and O. flexilis have backs and fins peppered with dark flecks of various sizes, with a broken sidestripe. The real Macrotocinclus affinis from the lower Parana basin of Paraguay and Uruguay and coastal streams in Brazil  has a light gray back and a very narrow black stripe, which ends before the tailfin and has no blotch on the caudal peduncle. My Otos— the commonest species on the market here in New York— seem to be either O. macrospilus or O. vittatus.
 
Some Otocinclus markings have specific protective value in their native waters. Corydoras catfish are armed with dorsal and pectoral fin spines that can deliver a painful sting, whereas the Otos are only lightly armed with minute versions of the prickles common to Loricariids. Batesian mimicry of the local species of stinging Corydoras has been noted for several sympatric Otocinclus species, that is, those that occupy the same waters:  Otocinclus flexilis  imitating Corydoras paleatus is a pairing that most aquarists could reproduce in their home tanks; the all-but-identical O. arnoldi, formerly considered a junior synonym of flexilis, also occurs together with Corydoras paleatus. Other pairings that have been noted in field studies are O. affinis with C. nattereri and O.  xakriata with C. garbei. In 2003 researchers identified two new species of miniature catfish in Paraguayan waters near Ciudad del Este, Corydoras diphyes together with its mimic, Otocinclus mimulus. A visually oriented predator,  perhaps a large wading bird, which has become wary of the Corydoras after a spiny encounter will also steer clear of its Otocinclus mimic.
 
Parotocinclus. One distinction is clear at the species level, anyway. Otocinclus don't have an adipose fin. If your Oto has an adipose fin, it's one of the fourteen species of Parotocinclus. A commonly-seen one is Parotocinclus maculicaudatus, a "Golden Oto" with brown blotches along the lateral line and a larger blotch on the caudal peduncle. The leading rays of dorsal, pectoral or caudal fins are red with brown banding, a feature you might not detect til you get a chance to photograph your Otos.
 
Otos are happier at room temperatures, yet Paul Kjaerland's Otos spawned at 25°C (77°F). Perhaps cooler "winter" months produce hardier Otocinclus. 
 
O. macrospilus (upper Amazon drainage in Colombia, Ecuador and Peru) is illustrated at Planetcatfish.  There is a shaped band on the tailfin, just inside the clear fin edge. A large blotch on the caudal peduncle. No adipose fin, of course.
 
Behavior. Otocinclus have a unique modification at the back of the esophagus, which is developed into a blind diverticulum which may function as supplementary aerial respiration and perhaps serves to maintain buoyancy for grazing in the uppermost levels of stream margins, Scott Schaefer of the American Museum of Natural History has suggested (Schaefer 1997). Every fishkeeper will have noticed that periodically Otos will dash to the surface for a quick gulp of air. 
 
You know that Otos spend most of their waking lives working their way across algal films. If they have the security of floating plants nearby, they'll even work the underside of the surface film that forms on still waters. It's a lipid layer to which algal cells and bacteria tend to stick, attracting plankton animals. So our little herbivores are getting a more balanced diet than you might think.
 
Otocinclus have three familiar behavior patterns: they feed, as I described; they do a skittering chaotic scramble that is hard to follow, like a butterfly's unpredictable flight, which could be a handy evasive activity with some practical survival value. And they "freeze." If you've ever seen a cryptically-patterned butterfly come to rest on rough gray bark, fold its wings and virtually disappear, you'll sense how useful it is for an Oto to "freeze" on a slender waterlogged branch about as big around as he is, and similarly disappear. LoMax13, posting from Gainesville FL at AquariaCentral a while ago, had this to say: "What's up with that mad dash thing? It's like they're having a panic attack. They move around like— 'oh crap I can't find enough algae. I'm going to die! Oh...OK here's some more right here. Whew.'" (I just had to pass this on.) Otos will also 'play dead:' Jinlong, Mission Viejo CA, noted: "When I initially added the otos to the 40g, one of the baby angels decided to suck an oto off the side of the tank. He pulled it off by the dorsal fin! As he did, another angel came up and grabbed the poor oto by the TAIL! The two angels then played tug of war with the poor oto until it flipped loose somehow and fell to the bottom of the tank, where it lay, belly-up. I was convinced it was dead, and apparently so were the angels, who nudged it with their noses for a minute or two before abandoning the toy as no longer interesting. Seconds later, the oto went from "freeze" to "dash" mode and hid behind the heater. Two months later, the same oto is still fat and happy."
 
"Solo Oto? Oh no! No solo Oto!" Don't keep them solo; they'll "freeze" unhappily under a leaf and pine for reassuring company. A "freezing" Oto may  just be resting, or merely too full to feel ambitious. But if there is no other Otocinclus in the vicinity, I think a sole Oto that does an unusual amount of "freezing," is expressing a symptom of unease and tension. It's social stress: "Where is everybody? Why is it so quiet in here? There must be some unseen danger lurking. Better just 'freeze'."

Why newly-arrived Otos can die like flies. Otocinclus are notorious for dying like—  well, like Otos— when you first get them home, though once they've acclimated to your planted tanks they live for years. Aquarists beat themselves up over this, but I think it's not our fault. Here's the thing: no vertebrate vegetarian can digest cellulose, not one! so each carries a species-specific community of anaerobic bacteria (and some protozoans) that do the work. Ruminants even have a special fore-stomach (the rumen) where grass is fermented in a rich bacterial soup, protected from stomach acids. Dairy cows are nourished, not so much by grass, but by bacterial by-products, which include some vitamins, and by digesting some bacteria: cow breath! Now, look at the size of the Oto. Scarcely room for a billion gut bacteria in there to do the work, eh? Starved Otos in transit can lose so much of their gut bacteria that the internal ecosystem doesn't revive— even with a glut of tasty algae in your tank! It just passes through their system, like when you were too hasty eating that corn-on-the-cob, remember? Not much nutrition when the kernels passed right through, because your system couldn't digest them open. Otos need a jungley tank with lots of leaf surfaces to run over. (If you can count your Otos, you haven't got enough plants.) But the vegetable supplement we give them (zucchini, spinach, etc.) has to be constant, or else they won't have the gut bacteria to process the green treat when it finally does arrive. Hopefully with your algae, and plenty of natural green cover, and your constant feedings of spirulina flakes or algae wafers plus veggies every few days,and perhaps sharing the gut bacteria of Otos already established, Otos that aren't too far gone should come round and thrive with you. 
 
Females are noticeably wider and plump, but though a healthy male is leaner, he shouldn't have a concave look, when seen from the side.
 
SegaDojo recently offered the suggestion that Otos might be unusually sensitive to nitrate. That might go far to explain Otos' sensitivity. "As a rule," G. Sterba wrote in 1967 (in Aquarium Care, p. 257) "newly imported wild-caught fish from tropical waters poor in nitrate and nitrite are particularly sensitive."
 
Breeding. Commercial Otos are still all wild-caught, I believe. Spawning Otos in aquaria is unusual enough to draw attention. Alec in Ontario had Otocinclus spawning for him in 1999:

"although they have not bred in a few months. For a while they were spawning every three weeks or so. My females are about 30-40% larger than the males, at least in the species that I have. There are also subtle structural differences, and the females usually have an extruding vent. I have two females and three males, but they do not pair off. It's basically the female and whatever male manages to fend off the others and get to her first. During the entire spawn, her eggs are usually fertilized at least once by each male. She will go along from leaf to leaf until she finds one she likes. Meanwhile, the males are all jostling along behind her. They will run their mouths along her entire body and position themselves as close as they can to her. When she finds a suitable leaf, she zips underneath it, and the lucky male wraps his body around her snout and holds her in place while she deposits single eggs on the underside of the leaf.
"They consistently produced fry, but I was never able to raise any, and by the time I set up a dedicated fry tank, they had stopped spawning. I am hoping they will resume in the summer again."

 
Chuck Huffine gave a detailed account of his spawning O. affinis to the Aquatic-Plants Digest. The spawning pair were in a densely-planted 20 gal. without the distractions of other kinds of fish (an important point), though Huffine senses that communal spawning might be more successful. "Oto's seem to be quite territorial despite their small size, and I suspect this fact may be one of the secrets in spawning them, along with water quality and diet. The Otos I keep in community tanks have not spawned, nor have they exhibited spawning behavior to my knowledge despite similar tank conditions. The pair that do spawn live alone with the exception of shrimp," Huffine noted. Weekly water changes with cooler water seemed to encourage spawning behavior. The eggs were carefully placed on a single plant of Bacopa caroliniana and were guarded, a characteristic Loricariid touch. Follow the hyperlink above, for more of the interesting details.
 
"Attack" Otos. I've never had a "problem" Otocinclus that developed a habit of rasping the body slime off other fishes, but I hear some individuals develop a taste for it. My hunch is that Otos are more inclined to snuffle at the flanks of other fishes in rather sterile environments, when the only available substiture for algae is a Hikari wafer. More natural grazing grounds are spread over all the surfaces of the planted aquarium. When Heinz Bremer and Ulrich Walter examined the nutrition of discus fry, which graze on their parents' slime and on specialized nutritious cells shed intact into the mucus, they found microorganisms, especially diatoms and bacteria, settled in the mucus surface. So the bizarre Oto habit isn't incomprehensible.  I think it's mildly aggressive behavior, too; I've seen an Oto that had been repeatedly pestered by another fish, turn on the harasser finally and do some defensive mouthing that was modestly aggressive.
 
Links. One of the best Internet articles on Otocinclus is Calilasseia's account, perilously archived in the "Bottom feeder Frenzy" forum at www.FishProfiles.com. Her Otos go under the name of O. arnoldi.
At WetWebMedia.com, Bob Fenner's article "The ideal algae-eater? the littlest South American suckermouth catfishes, genus Otocinclus" gives some of the Web's best pointers for buying Otos and keeping them hale and hearty, mostly from the perspective of the importer/wholesaler, with useful points for picking out healthy stock, handling and acclimating them. He ends with a long list of Oto articles in the print media. Two points that he doesn't mention, which I think are equally important: 1) Otos share gut bacteria: their need for the company of other Otos isn't merely social. And 2) partly because of their need for plentiful oxygen, Otos thrive at room temperature, in the 60s and low 70sF, rather than in the steamy conditions we are inclined to offer. 
Another excellent article at PlanetCatfish, "Otocinclus: 'Little Monkeys' in the Planted Aquarium," by Julian Dignall and Dinyar Lalkaka, outlines recent name changes among Otos, describes some of their natural ecotopes and techniques for acclimating them to the aquarium.  
Robyn Rhudy's Otocinclus page also has basic details — and some personal experience and further links, too.  There are excellent close-ups of Otos at the Aqualand (Des Moines IA) website.
 
...and Parotocinclus too. At Scotcat.com you'll find an interesting description of breeding the sister genus, Parotocinclus. For color photos to compare to your own dwarf Lory, search "Parotocinclus" at Planetcatfish. 
 
Otocinclus cf. affinis. Out of a couple of dozen published species, maybe half a dozen turn up in your LFS from time to time. Compare your mystery Otos first to O. affinis— "Golden Otos," Baensch calls them, and illustrates a very golden one indeed. Whether we have the right species name or not, these are the Otos you see everywhere, and the ones most commonly spawned. They come from southeast Brazil, the area round Rio de Janiero. The specific name is currently a little questionable. Not for us to fret about, though. That "cf." in the name above means "comparable to." It's a cop-out that makes me look less clueless than I am and more like I have some serious scientific reservations about the name I'm using.
 
O. cf. flexilis. From Rio Grande do Sul, in southernmost Brazil. In this Oto, the black lateral band breaks into blotches towards the tail.
 
O. cf. vittatus. This Oto, which grows to about 1.5 inches, is from the Mato Grosso of Brazil, and small tributaries in the Paraguay drainage. I have a trio, sold to me as "Giant" Otos! (snicker) Indeed they are slightly bigger than the most familiar Oto, O. affinis. Mine correspond very closely with the photos at Planetcatfish, where O. vittatus was Catfish of the Month for Dec 1996. O. vittatus features a speckled back on a gray ground, a broader lateral stripe ending in a peduncle blotch, and wavy black lines in the tailfin, with white spots in the base of the upper and lower fin lobes. Paul Kjaerland had some unexpected luck when a small group began laying eggs for him one summer, at a temperature of 25°C and a pH of 7.5, and he got some juveniles out of the resulting fry. Of course you wouldn't keep these or any Otos except in a well-planted tank with a tendancy to grow algae, in peace-loving company, and in water that was soft to moderately soft. Mine get a continual supply of blanched vegetables, a little at a time, and no more till they've finished what they've got. But I don't pull out the spinach leaf after 24 hours; they like it best when it's slimy olive-gray and coming apart. Then they have to forage for algae for 24 hours before they get veggies again. Even the male is mildly plump on this regimen, but I can recognize the female by her broader beam.