Taxonomy & naming
Potamotrygon motoro was described by Müller and Henle in 1841 and is placed by the Catalog of Fishes in the genus Potamotrygon, family Potamotrygonidae, order Myliobatiformes — the stingrays. The family is a natural group of South American freshwater rays, and Potamotrygon is its large core genus. These are elasmobranchs: cartilaginous fish whose skeleton is not bone but cartilage, kin to the sharks and the marine rays rather than to any of the bony fishes that make up the rest of this atlas.
The motoro is a widespread and variable species, and the name has historically been applied to several distinct-looking forms across a huge range, so the literature carries a long list of synonyms and the boundaries with similar rays are still debated. Aquarists recognise many regional colour variants under trade names, but these are forms of one variable species rather than separate taxa. "Motoro" is a vernacular name carried over from South America; the species is also widely called the ocellate or peacock-eye river stingray for its eyespots.
Morphology
The motoro has the characteristic shape of its family: a rounded, almost circular pectoral disc, flattened for life on the bottom, with the eyes and the paired spiracles set on top of the head and the mouth and gill slits underneath. The disc reaches roughly 19.5 in across in well-grown animals, and large females can exceed that; the whole animal, tail included, is considerably longer. The ground colour is grey to brown, marked with the rosette-like orange or yellow ocelli ringed in black that give the species its name, on a pattern that varies widely between populations.
The tail is moderately long and bears, near its base, one or more serrated venomous spines — modified dermal denticles sheathed in venom-producing tissue, which are shed and regrown periodically. The skin is otherwise covered in tiny tooth-like denticles rather than scales. Males are readily told from females by the pair of claspers — rod-like copulatory organs formed from the inner edges of the pelvic fins — while females grow larger and lack them.
Habitat
Potamotrygon motoro is one of the most widely distributed of all the river stingrays, recorded across much of tropical South America from the Amazon and Orinoco basins south into the Paraná–Paraguay system. It is a fish of slow and moderate lowland waters — the margins of rivers, floodplain lakes, sandy beaches and flooded forest — where it lies partly buried in sand or silt with only its eyes and spiracles exposed.
It favours warm, soft to moderately mineralised water that is roughly neutral to slightly acidic, and it is found over sand and fine sediment rather than rock. Like all of its family it breathes by drawing water in through the spiracles on top of the head, rather than over the mouth, so that it can respire while buried and while feeding. Its wide range spans white-water, clear-water and some black-water rivers, which is part of why it is more adaptable in the aquarium than its narrow-range relatives — though "adaptable" for a river stingray still means demanding.
Feeding
The motoro is a carnivore and a benthic hunter. It feeds on a range of bottom-living animals — aquatic insect larvae, crustaceans, molluscs, worms and small fishes — which it detects partly by the electro-sensory pores (the ampullae of Lorenzini) scattered over its underside, used to sense the faint electrical fields of buried prey. It pins prey against the substrate with its disc and works it back to the crushing tooth-plates of the mouth.
In the aquarium it takes a meaty diet — earthworms, prawn, mussel, fish and other whole frozen and live foods — and feeds best on the bottom, often learning to take food from the keeper's hand (an interaction to manage with great care, given the sting). It is a heavy feeder that produces a large amount of waste, which is one reason its water-quality demands are so high; a ray's appetite and its sensitivity to its own pollution pull in opposite directions, and the keeper has to make up the difference with filtration and water changes.
Mating
River stingrays reproduce by internal fertilisation, like the sharks and the marine rays. A male grips the female's disc with his mouth and uses one of his paired claspers to transfer sperm; courtship can be vigorous and the male often bites at the female's disc margin, leaving marks that heal. There is no nest, no spawning site and no external eggs — fertilisation and the whole of early development happen inside the female.
The motoro is, by stingray standards, willing to breed in large aquaria, and captive reproduction is well documented. As in the rest of the family the relationship ends at mating; there is no pair bond and no parental care of the kind seen in many bony fishes. The interest of the system lies entirely in what happens next, inside the mother.
Breeding
Potamotrygon motoro is viviparous — it gives birth to live young — and its mode of nourishing them is one of the most remarkable in fishes. After the small yolk of the egg is used up, the developing pups are sustained inside the uterus by a protein- and fat-rich secretion, a "uterine milk" (histotroph) produced by finger-like extensions of the uterine wall, which the embryos take up directly. This matrotrophic histotrophy lets the pups grow large before birth.
A female carries a modest litter — commonly a handful of pups, up to roughly a dozen in large animals — through a gestation of several months, and gives birth to fully-formed miniature rays a few centimetres across that are independent and feeding within days. There is no parental care after birth. The combination of small litters, slow maturity and a long gestation means river stingrays reproduce slowly, which matters greatly for how heavily they can be harvested from the wild.
In the aquarium
The motoro is the most frequently kept freshwater stingray and the usual starting point, but it is an advanced fish that is very often bought far too small for what it becomes. An adult needs an enormous, wide-footprint aquarium — the floor area matters far more than the height, and a tank measured in square metres, with a length and width several times the disc width, is the realistic minimum. A deep bed of soft, fine sand is essential for a fish that buries; sharp gravel abrades the disc.
Above all, river stingrays demand pristine water. They are scaleless, with thin, delicate skin and a high metabolism, and are exquisitely sensitive to ammonia and to accumulated nitrate; powerful filtration and large, frequent water changes are not optional. They tolerate no salt, react badly to many medications (copper especially), and must never share a tank with fin-nippers or aggressive bottom-feeders. And the venomous sting is a genuine hazard — a stingray wound is intensely painful and can be medically serious — so they are emphatically not a fish to keep where children might reach them, and many places restrict or license their keeping. None of this makes them impossible; it makes them a long, expensive, serious commitment.
Conservation
Potamotrygon motoro is listed as Least Concern on the IUCN Red List, reflecting its very wide range and tolerance of varied habitats — among the broadest of any Potamotrygonidae. That should not be mistaken for abundance everywhere, however: local populations are still reduced by the same pressures that affect the whole family.
Those pressures are real: river stingrays are slow-breeding, are caught for the ornamental trade, for food, and are killed out of fear because of their sting, and they suffer from habitat change, dams and pollution including mercury from gold mining across Amazonia. Because of trade concerns several Potamotrygon species and exporting countries have sought controls under CITES, and some range states (Brazil among them) regulate or restrict the export of wild rays. For the motoro specifically, its wide range and Least Concern status are no substitute for well-managed, captive-bred supply rather than continued reliance on wild-caught animals for an aquarium fish that lives for many years and breeds slowly.