Taxonomy & naming
Potamotrygon marinae was described by Pierre Deynat in 2006 from specimens collected in the Lawa River on the Suriname–French Guiana boundary, with type material deposited at the Muséum National d'Histoire Naturelle in Paris. It is placed in the genus Potamotrygon, family Potamotrygonidae, order Myliobatiformes — the stingrays — by the Catalog of Fishes.
The family Potamotrygonidae is the only group of stingrays to have made a permanent transition into fresh water. These are elasmobranchs, cartilaginous fish whose skeleton is composed of cartilage rather than bone, and they are more closely related to sharks and to the rays of the sea than to the bony fishes that make up virtually every other family in this atlas. Like all elasmobranchs, they possess ampullae of Lorenzini for electroreception, spiracles for benthic respiration, and a skin of tooth-like denticles rather than true scales.
Within the large genus Potamotrygon, marinae belongs to the Guiana Shield radiation of relatively small-disc endemics. Its specific epithet, marinae, was assigned by Deynat and refers to the individual for whom the species was named; it should not be confused with anything to do with the sea.
Morphology
Potamotrygon marinae has the characteristic flattened, nearly circular pectoral disc of its genus, adapted for life on the river bed. The holotype measures 15.5 in disc width (15.5 in), representing the known upper size limit; sexually mature males average around 9.5 in DW and females around 12 in DW, making this a relatively small species within Potamotrygon. The total length including the tail is considerably greater than the disc width alone.
The eyes and paired spiracles are set on the dorsal surface of the disc — the spiracles being the paired openings just behind the eyes through which the ray draws in water for respiration while buried, bypassing the mouth. The mouth, nostrils and five pairs of gill slits are positioned on the flat ventral surface. The tail is moderately long and armed with one or more serrated venomous spines near its base, modified dermal denticles sheathed in venom-producing tissue that are shed and periodically regrown. The skin is covered in minute tooth-like denticles rather than scales.
Sexual dimorphism is straightforward: males possess a pair of claspers, rod-like copulatory organs formed from the inner margins of the pelvic fins, and are smaller than females at maturity.
Habitat
Potamotrygon marinae is a narrow-range endemic confined to a cluster of rivers in the northern Guiana Shield: the Lawa River (the type locality, forming the Suriname–French Guiana border), the Maroni system of which the Lawa is a tributary, and the Grand Inini and adjacent drainages in southern French Guiana and Suriname. It is a benthic freshwater species that lies partly buried in sand and fine sediment on the river bottom, with eyes and spiracles exposed.
The rivers of the Guiana Shield are characteristically soft and acidic — clear-water or blackwater systems draining ancient, nutrient-poor basement rock, with low dissolved mineral content, and naturally low pH. Water conditions are warm year-round, typical of equatorial lowland rivers. Potamotrygon marinae occupies the shallow sandy margins and slower reaches of these rivers, where it buries to ambush prey and to avoid detection. Its restriction to a narrow geographic zone of geologically similar rivers means it has little capacity to tolerate the kind of environmental variation that characterises the more widespread species in the genus.
Feeding
Potamotrygon marinae is a benthic carnivore, like all members of its family. It detects buried and bottom-dwelling prey using the ampullae of Lorenzini, a network of electrosensory pores distributed across the underside of the disc that pick up the faint bioelectric fields produced by live animals. Prey is pinned beneath the disc against the substrate and worked toward the crushing dentition of the jaws.
The diet likely consists of aquatic invertebrates — insect larvae, crustaceans, worms and small molluscs — typical of the benthic fauna of Guiana Shield rivers. Specific dietary data for this species are sparse in the published literature; the account above reflects the known foraging biology of small Potamotrygon generally rather than documented records for marinae in particular. In aquarium conditions, the species takes meaty foods offered on the substrate, including earthworms, fresh prawn and other invertebrate items.
Mating
River stingrays reproduce by internal fertilisation, as do all elasmobranchs. Courtship in Potamotrygon typically involves the male tracking the female and biting at the margins of her disc; he then positions himself alongside or beneath her and inserts one of his paired claspers into the female's cloaca to transfer sperm. The claspers are elongate, rod-like structures formed from the modified inner edges of the pelvic fins and are present only in males, making sex straightforward to determine visually.
No pair bond forms and no spawning site is involved — fertilisation and the entirety of subsequent development take place inside the female's body. The venomous spine is not used in mating interactions. Specific mating observations for P. marinae in either the wild or captivity are not well documented, and much of what is understood draws on the broader biology of the family Potamotrygonidae.
Breeding
Potamotrygon marinae is viviparous, giving birth to live young, as is the entire family Potamotrygonidae. After fertilisation, the embryos exhaust the small egg yolk quickly; thereafter they are sustained inside the uterus by a protein- and fat-rich secretion — a "uterine milk" or histotroph — produced by specialised extensions of the uterine wall called trophonemata. This mode of nutrition, matrotrophic histotrophy, allows the embryos to grow substantially before birth.
Litter size in Potamotrygon typically ranges from one to eight pups, varying with the size of the female. Gestation lasts several months. At birth the pups are fully formed miniature rays, independent immediately, and no parental care follows. The combination of small litters, multi-month gestation and late sexual maturity means that river stingrays have an inherently low reproductive rate. A population under sustained pressure from trade collection or habitat loss has limited ability to recover quickly, which is a central concern in the conservation of this species.
In the aquarium
Potamotrygon marinae appears in the ornamental trade, and the pressure of collection for aquaria is one of the two primary threats assessed in its IUCN listing. As a relatively small species — adults in the 12–15.5 in DW range — it requires somewhat less floor space than the largest Potamotrygon, but the fundamental demands of the family are unchanged and are non-negotiable. The tank footprint must be large enough for the ray to turn freely and to move without restriction; floor area is far more important than water depth.
The substrate must be a deep bed of soft, fine sand — the ray buries, and any sharp or coarse substrate will abrade the delicate disc skin. Water quality must be maintained at very high standards: the species is scaleless, with thin skin and a high metabolic waste output, and is exquisitely sensitive to ammonia and to accumulated nitrate. Heavy biological filtration and frequent, large water changes are essential. No salt should be added; the water should be soft and moderately acidic in line with the chemistry of Guiana Shield rivers. Medications containing copper are toxic to elasmobranchs and must never be used.
The venomous tail spine is a genuine medical hazard — stingray wounds are intensely painful and can be serious — making these fish unsuitable in households with young children and subject to legal restriction in a number of jurisdictions. Captive-bred specimens are strongly preferred over wild-caught, both for animal welfare reasons and to reduce pressure on the wild population. Acquisition from reputable breeders who can confirm captive origin is considered best practice.
Conservation
Potamotrygon marinae is assessed as Near Threatened on the IUCN Red List, with the assessment dated 2 March 2023 under criteria A2cd. The two main documented threats are collection for the ornamental aquarium trade and habitat degradation from river mining — specifically artisanal and small-scale gold mining (orpailleurs) active throughout the rivers of French Guiana and Suriname, which increases turbidity, deposits mercury, and destroys the sandy benthic habitats the species depends on.
The species' narrow range makes it particularly vulnerable. It is known only from a cluster of rivers in a relatively small area of the Guiana Shield, and there is no population elsewhere to buffer a local decline. The slow reproductive biology — small litters, long gestation, late sexual maturity — means that a depleted population cannot rebound quickly even if pressure is removed. Potamotrygon marinae is listed in CITES Appendix III (under Council Regulation 2017/160 Annex C), which requires export documentation from range states and provides a degree of international trade monitoring.
Improved population data are urgently needed. River survey effort in the remote rivers of southern French Guiana and Suriname is limited, and much of what is known about the species' abundance in the wild comes from trade records rather than systematic field surveys. Better understanding of population size, range limits and reproductive rates would allow a more precise threat assessment.