Taxonomy & naming
Paratrygon aiereba was described by Müller and Henle in 1841 and is the type species of the genus Paratrygon, family Potamotrygonidae, order Myliobatiformes. Paratrygon is distinguished from the more familiar Potamotrygon by its very large, near-circular disc, its long, slender tail and its plain coloration, and the genus contains only a handful of species — alongside P. aiereba, the Catalog of Fishes recognises P. orinocensis of the Orinoco and the recently-described P. parvaspina.
P. aiereba as currently understood is widespread and variable, and genetic work suggests it may be a complex of more than one species spread across the major basins rather than a single uniform one — a reminder that even a large, conspicuous animal can be taxonomically unsettled. The epithet "aiereba" and the common name "ceja" are vernacular South American names; "discus ray" refers to the disc-flat, rounded shape.
Morphology
The ceja is built on a grand scale. Its pectoral disc is large and almost circular — a metre across in big animals, with reports of considerably larger — and noticeably thin and flexible at the margin, often appearing semi-translucent at the very edge. The coloration is plain by the standards of the family: grey, brown or olive above, sometimes finely mottled, without the bold ocelli or pearl spots of the showier Potamotrygon, and pale below.
The defining feature is the tail: long, slender and whip-like, far thinner and more fragile than in the round-tailed Potamotrygon, and bearing a relatively small venomous spine. The eyes and spiracles sit on top of the disc, the mouth and gills beneath. As in all river stingrays, males carry paired claspers and females, which reach the largest sizes, do not.
Habitat
Paratrygon aiereba ranges widely through the Amazon and Orinoco basins of South America, where it is a fish of the larger, deeper river channels and lakes rather than the shallow margins and beaches favoured by smaller rays. It lives on soft sediment in moving water, often in the deeper parts of the main rivers, and is encountered less often than the shore-dwelling Potamotrygon simply because of where it lives.
It occupies warm, soft, near-neutral fresh water and, like all its family, breathes through the spiracles on top of its head so it can respire while resting on or buried in the bottom. Its preference for deep channel habitat, its very large size and its sensitivity make it a fish that is genuinely difficult to maintain outside the largest installations — its space requirements alone put it beyond the reach of ordinary fishkeeping.
Feeding
The ceja is a benthic carnivore that takes bottom-living invertebrates and fishes — crustaceans, molluscs, insect larvae and small fish — detected in part through the electroreceptive ampullae on its underside and pinned against the substrate before being crushed by the dental plates. Its large size lets it handle correspondingly larger prey than the smaller rays.
In the few settings where it is kept — chiefly public aquaria — it is fed a meaty diet of whole fish, prawn, mussel and similar foods on the bottom. As with all rays it is a heavy feeder producing a large waste load, which compounds the water-quality challenge posed by its great size.
Mating
Reproduction is by internal fertilisation. As in the rest of the family the male grips the female and transfers sperm with a clasper; there is no nest, no egg-laying and no external development. Detailed observations of courtship in this species are few, reflecting how rarely it is kept and studied compared with the small Potamotrygon.
There is no pair bond and no parental care. What is known of its reproduction comes largely from examined wild specimens and a small number of captive events, and many specifics — frequency, seasonality, behaviour — remain incompletely documented for this large, deep-water ray.
Breeding
Paratrygon aiereba is viviparous, like all its family, nourishing its developing young inside the uterus by matrotrophic histotrophy — the secreted "uterine milk" that sustains the pups after the egg yolk is used up. Consistent with its large body size, litters are reported to be small, often just one or a few large pups, carried through a gestation of several months and born as sizeable, fully-formed miniatures.
Very small litters combined with large body size, late maturity and a long gestation make this one of the slowest-reproducing freshwater fishes — a life history closer to that of a large shark than of a typical aquarium fish. That slow turnover is the central fact for its conservation: populations depleted by fishing or habitat change recover only very gradually, and the species is poorly suited to absorb sustained harvest.
In the aquarium
Paratrygon aiereba is not a general-hobby aquarium fish. Its enormous adult size — a disc of a metre or more — and its preference for deep, open water mean that only the very largest private systems and public aquaria can house it properly; the floor area required is measured in many square metres. A deep bed of soft, fine sand is essential, and its long, fragile tail needs unobstructed space.
The family's exacting requirements apply with extra force at this scale: utterly pristine water, because a fish this large and this sensitive imposes a huge filtration and water-change burden; no salt; great caution with medication; and respect for the venomous sting. It is a delicate species that acclimates poorly and is intolerant of handling and of less-than-perfect conditions. For all but a handful of keepers it is a fish to admire in a large public display, not to attempt at home — and one for which wild collection is hard to justify given its slow reproduction.
Conservation
Paratrygon aiereba carries the conservation burden of its life history. It is among the slowest-reproducing of freshwater fishes — very small litters, large size, late maturity — and such a strategy leaves little capacity to rebound from exploitation. It is taken in fisheries for food and for its skin, killed because of its sting, and affected by habitat change, damming and pollution including mercury from gold mining across its range, while the aquarium demand for a metre-wide ray is small but the impact of removing slow-breeding adults is not.
The IUCN Red List now assesses Paratrygon aiereba as Critically Endangered. It is heavily targeted by unregulated fisheries for food and taken illegally for the ornamental trade, and — because the 2021 split of the genus narrowed the true range of aiereba in the strict sense (the Orinoco animals are now Paratrygon orinocensis) — it is more restricted, and more vulnerable, than the once-widespread name suggested. The priorities are better basin-level data, resolution of the species complex, and management of the fisheries and habitats on which this giant, slow-living ray depends.