Taxonomy & naming
Potamotrygon marquesi was formally described by Silva and Loboda in 2019 in the Journal of Fish Biology (volume 95, issue 2, pages 594–612). The holotype is a female 9 in disc width collected from the Tarauacá River, a tributary of the Juruá in Acre State, Brazil. Paratypes ranging from 8–9.5 in disc width came from the same drainage, with additional referred material from the Arari River on Marajó Island (Pará State) and coastal freshwaters of Amapá. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) recognises it as a valid species in the genus Potamotrygon, family Potamotrygonidae.
Potamotrygonidae are the neotropical freshwater stingrays — a natural group of South American elasmobranchs whose ancestors entered fresh water and became isolated in the continent's river systems. These are cartilaginous fish (Chondrichthyes), not bony fishes; their skeleton is cartilage, their skin is covered in tooth-like denticles rather than scales, and they detect prey with electro-sensory ampullae of Lorenzini. Placed in the order Myliobatiformes, they are closer relatives of the pelagic manta rays and eagle rays of the sea than of any cichlid, tetra, or catfish.
The species epithet honours someone named Marques; the original description formally dedicates the name. No widely used common name has been established in the hobby or the literature.
Morphology
P. marquesi has the characteristic rounded pectoral disc of its genus, with eyes and paired spiracles set on the dorsal surface and the mouth, nostrils, and five pairs of gill slits on the ventral face. The disc width ranges from approximately 4.5 in in the smallest examined specimen to around 12 in (roughly 12 in) in the largest, making this a medium-sized Potamotrygon. The holotype female measured 9 in disc width; total length runs 9–19.5 in once the tail is included.
Dorsally the ground colour is beige to grey, marked with open and closed bicolor rings that distinguish this species from its congeners. The skin bears asymmetrical star-shaped denticles (stellate denticles) — no scales — consistent with its nature as a cartilaginous fish. The tail carries a single regular row of spines on the dorsum and a lateral row near the sting insertion; the venomous serrated tail spine is a modified denticle sheathed in venom-producing tissue, shed and regrown periodically.
The skull includes three angular cartilages, a character noted in the original description. Sexual dimorphism exists in tooth morphology: males have cusped teeth, females do not — a pattern seen in sharks and rays generally, linked to the male's need to grip the female during mating. Males are also identified externally by their paired claspers, rod-like copulatory organs formed from the inner margins of the pelvic fins.
Habitat
Potamotrygon marquesi is known from three geographic areas: the Tarauacá River sub-basin of the upper Juruá in Acre State, the Arari River drainage on Marajó Island at the Amazon mouth in Pará State, and coastal fresh waters of Amapá State in northeastern Amazonia. All three are Amazon Basin drainages within Brazil. The species is described as sedentary and territorial, with a strong preference for muddy substrates — it lies buried with only its eyes and spiracles exposed, breathing by drawing water through the spiracles rather than the mouth.
The Amazon mainstem and its white-water tributaries carry moderately mineralised, near-neutral water; the coastal drainages of Amapá and the seasonally flooded landscapes of Marajó present broadly similar Amazon-influenced chemistry. Like all Potamotrygonidae, P. marquesi is a true freshwater obligate — it cannot tolerate salt — and the species is confined to the lowland tropical river environment it was described from. No temperature or pH data specific to the collection sites were documented in the available literature, but the Amazon Basin environment is characteristically warm (typically 75–82 °F) and soft to moderately mineralised.
Feeding
P. marquesi is a benthic carnivore, as are all members of its family. The disc-shaped body plan is adapted for life on the river bottom: the ray lies on or buried in sediment and uses the electro-sensory pores (ampullae of Lorenzini) distributed across its ventral surface to detect the faint bioelectric fields of buried invertebrates and small fishes. Prey is pinned under the disc and worked toward the crushing dental plates in the mouth.
The diet of P. marquesi has not been specifically investigated, but close relatives and sympatric Potamotrygon species feed on aquatic insect larvae, crustaceans, molluscs, annelid worms, and small fishes — a generalist suite of benthic prey typical for the genus. In captivity, related rays readily accept meaty foods offered on the bottom: earthworms, prawn, pieces of mussel, and whole small fish. As with all river stingrays, the metabolic rate is high relative to body size, making these heavy feeders that generate a correspondingly large waste load.
Mating
Like all elasmobranchs, Potamotrygon marquesi reproduces by internal fertilisation. The male locates a receptive female and grips the margin of her disc with his mouth — bite marks on the disc edge are commonly recorded in captive rays and are part of normal courtship. He then inserts one of his paired claspers into the female's cloaca to transfer sperm. The claspers are rod-like organs formed from the inner edges of the pelvic fins and are the diagnostic external character distinguishing males from females.
Sexual dimorphism in tooth morphology — males with cusped, more pointed teeth, females with broader, non-cusped crowns — is likely related to this gripping behaviour during courtship. There is no prolonged pair bond and no territory shared between the sexes outside the brief mating event. Fertilisation is entirely internal; there are no external eggs, no nest, and no spawning site.
Breeding
Potamotrygon marquesi is viviparous — it gives birth to live young — and employs the same mode of embryonic nutrition used by all members of the family: matrotrophic histotrophy. After the small initial egg yolk is exhausted, the developing pups are sustained inside the uterus by a protein- and fat-rich secretion, a "uterine milk" (histotroph), produced by the uterine wall and taken up directly by the embryos through specialised structures. This allows the pups to grow to a relatively advanced state before birth.
Fecundity in P. marquesi is low: the available data record an average of two to three pups per litter, consistent with the small-litter pattern seen across the genus. Gestation lasts several months. Females give birth to fully formed, independent miniature rays; there is no parental care after birth — the pups are immediately self-sufficient. This reproductive strategy — internal fertilisation, small litters, long gestation, and probable late sexual maturity — means the species replaces itself slowly and has very limited capacity to absorb harvest pressure.
In the aquarium
Potamotrygon marquesi appears occasionally in the ornamental trade — FishBase notes it has been illegally exported since the 1990s — but it is a specialist fish for which most domestic aquaria are wholly unsuitable. An adult disc of up to 12 in requires a tank with a very large floor area: floor footprint, not water volume, is the governing dimension, since the ray needs room to move and turn across the bottom. A deep bed of fine, soft sand is not optional — the species buries habitually, and coarse gravel abrades the delicate denticle-covered skin and disc margin.
Water quality must be pristine. River stingrays are scaleless, with a high metabolic waste load and extreme sensitivity to ammonia and nitrate; powerful biological filtration and large, frequent water changes are required rather than recommended. No salt should be added to the water. Medications containing copper are toxic to elasmobranchs and must never be used. Tankmates must be peaceful and non-nippy; even minor harassment causes chronic stress in a species adapted to a sedentary, buried existence. The venomous tail spine is a serious hazard — a strike is intensely painful and may require medical attention — making this an unsuitable fish in households with children or where accidental contact is possible. Many jurisdictions require permits to keep freshwater stingrays.
The ornamental trade in wild-caught P. marquesi is now regulated under CITES Appendix II (effective 2023). Prospective keepers should seek captive-bred specimens from licensed specialist breeders, which reduces pressure on wild populations and generally produces animals better conditioned to aquarium life.
Conservation
The IUCN Red List assessed Potamotrygon marquesi as Near Threatened in 2024. A population decline of 20–25 % over three generations (estimated at 21 years) is suspected, driven primarily by the ornamental trade and bycatch from demersal longline and shrimp-trawl fisheries. The species is described as rare in some parts of its range and more abundant along the Amapá coast, pointing to uneven and poorly understood population structure across three geographically separated areas.
The slow reproductive rate of P. marquesi — small litters of two to three pups, long gestation, probable late maturity — means even moderate and sustained removal quickly outpaces recruitment. Beyond direct take, mercury contamination from artisanal gold mining in Amazonia accumulates in benthic food chains and affects ray tissues; habitat degradation and seasonal flooding alterations affect the muddy-substrate benthic environments the species depends on.
P. marquesi is listed on CITES Appendix II (effective 23 February 2023) alongside several other commercially traded Potamotrygon species (P. albimaculata, P. henlei, P. jabuti, P. leopoldi, P. signata, P. wallacei), requiring that any international trade be accompanied by a non-detriment finding and export permit. Brazil had earlier listed selected Potamotrygon on CITES Appendix III and restricted exports under domestic regulation. Given the combination of slow life history, trade pressure, and bycatch, the Near Threatened assessment is likely conservative if harvest rates are not reduced.