Taxonomy & naming
Potamotrygon signata was described by Samuel Garman in 1913, and the Catalog of Fishes (Eschmeyer, California Academy of Sciences) treats it as a valid species in the genus Potamotrygon, family Potamotrygonidae, order Myliobatiformes. Like all members of this family it is an elasmobranch — a cartilaginous fish whose skeleton is built from cartilage rather than bone, making it a fundamental outlier among the freshwater species covered in this atlas. Its closest relatives are the other South American river stingrays of the family Potamotrygonidae, a group that became isolated in fresh water millions of years ago and diversified into more than thirty species across the continent's major river systems.
P. signata is the sole stingray endemic to the Parnaíba River basin, a drainage that lies east of the Amazon basin in north-eastern Brazil and drains into the Atlantic separately. This isolation has made the species of interest to biogeographers as well as conservationists. The species epithet signata is a Latin past participle meaning 'marked' or 'distinguished', referring to the patterning on the disc.
Morphology
Potamotrygon signata has the flat, disc-shaped body characteristic of the family: the paired pectoral fins are fused to the head and trunk to form a nearly circular disc, and the animal lies flush against the substrate for much of its life. FishBase records a maximum disc width of 12 in, making it a small to moderately sized river stingray. The eyes and the paired spiracles — dorsal openings that draw water across the gills while the animal is buried — sit on top of the head; the mouth and five pairs of gill slits are on the underside.
The skin bears minute tooth-like denticles in place of scales, giving the disc a slightly rough texture. The tail is whip-like and carries, near its base, one or more serrated venomous spines — shed and regrown periodically — sheathed in venom-producing tissue. Males are distinguished from females by the paired claspers extending from the inner margins of the pelvic fins, which are the external copulatory organs. The underside of the disc is studded with the electro-sensory pores of the ampullae of Lorenzini, used to detect the electrical fields of buried prey.
Habitat
Potamotrygon signata is restricted to the Parnaíba River basin of north-eastern Brazil — a drainage that lies east of the Amazon and empties into the Atlantic independently. This makes it a genuine single-basin endemic, and its entire global population occupies that one river system. FishBase categorises it as a tropical freshwater benthopelagic species, meaning it lives in association with the bottom in freshwater environments.
Like all river stingrays it spends much of its time buried or resting on sandy or fine-sediment substrates in relatively shallow, warm river stretches. The Parnaíba is a large seasonal river subject to marked wet and dry seasons that strongly influence water temperature, depth, and chemistry — conditions that P. signata has evolved alongside but which also make it susceptible to hydrological alterations from dams and water extraction.
Feeding
Potamotrygon signata is a carnivore and a benthic hunter in the manner of all its family. It detects prey — aquatic invertebrates, crustaceans, worms, and small fishes living in or on the sediment — through the ampullae of Lorenzini, which sense the faint bioelectric fields produced by living animals. The ray pins prey against the substrate with its disc and draws it back to the crushing jaw plates.
No feeding studies specific to P. signata have been published, and precise prey lists are not available in the literature. Based on closely related Potamotrygon species of similar size and habitat, the diet is expected to encompass insect larvae, small crustaceans, molluscs, and worms, with opportunistic predation on small fish. In the aquarium, river stingrays of this genus are generally offered earthworms, prawn, mussel, and other meaty whole foods placed on the substrate or tank floor.
Mating
River stingrays reproduce by internal fertilisation, as sharks and marine rays do. A male locates and approaches a receptive female and grips the edge of her disc with his mouth; he then inserts one of his paired claspers — the rod-like copulatory organs formed from the inner edges of the pelvic fins — to transfer sperm directly. Courtship and mating can be physically demanding, and disc-margin bite marks on females are a common observation in the related species that have been studied.
No mating observations specific to P. signata have been published. The species has rarely been held in captivity, and detailed behavioural accounts are absent from the literature. What can be said with confidence, shared by every member of the family, is that fertilisation is entirely internal, there is no external spawning event, and the male takes no further role after mating.
Breeding
All members of the family Potamotrygonidae are viviparous, giving birth to live young rather than laying eggs. After fertilisation the developing embryos consume a small initial yolk supply, then are sustained through the rest of gestation by a protein- and fat-rich secretion — histotroph, commonly called 'uterine milk' — produced by specialised glands lining the uterine wall and taken up directly by the embryos. This mode of nourishment, matrotrophic histotrophy, allows the pups to grow substantially before birth.
Gestation across the family ranges from roughly three months to close to a year depending on species and conditions. Litter sizes across Potamotrygonidae vary from a single pup to around twenty in large females of the largest species; specific litter size data for P. signata are absent from the available literature. At birth the pups are fully formed, independent miniature rays that begin feeding within days. There is no parental care after birth. This slow reproductive schedule — small litters, long gestation, and late sexual maturity — means that P. signata can sustain very little additional mortality before population decline, which is a central concern for its conservation.
In the aquarium
Potamotrygon signata is rarely seen in the aquarium hobby. FishBase notes it is very infrequently imported, and its restricted wild range and Vulnerable conservation status make wild-caught animals both scarce and ethically problematic. For anyone keeping this species, the husbandry requirements are the same as for any river stingray of similar size: a wide-footprint tank with floor area measured in square metres (far more important than depth), a deep bed of soft fine sand — sharp gravel or bare glass abrades the disc and causes chronic stress injuries — and exceptionally clean, ammonia-free water maintained by heavy filtration and large, frequent water changes.
River stingrays are scaleless with thin, permeable skin and are exquisitely sensitive to ammonia, nitrite, and accumulated nitrate; they are also sensitive to many medications, particularly copper-based treatments. No salt should be added. Tankmates must be peaceful and cannot include fin-nippers or aggressive bottom-dwellers. The venomous tail spine demands constant awareness: a puncture wound from a freshwater stingray causes intense local pain and can escalate to tissue necrosis and secondary infection, constituting a genuine medical hazard. Many jurisdictions require permits for stingray keeping. Captive-bred stock should always be preferred where available.
Conservation
The IUCN Red List assesses Potamotrygon signata as Vulnerable (VU), assessed in 2024 under criteria A2cde — indicating a population reduction inferred from declining range, exploitation, and habitat quality. Its status is driven primarily by its extreme endemism: as the only stingray in the Parnaíba basin, the entire global population occupies one river system, and any basin-wide impact is a species-wide impact. FishBase notes very low resilience, with a minimum population doubling time estimated at more than fourteen years — a direct consequence of the family's slow viviparity.
Pressures on P. signata include collection for the ornamental trade (even rare imports represent pressure on a small population), artisanal fishing for food or bycatch, killing of rays encountered by fishers fearful of the sting, and the growing threat of water-infrastructure development in the Parnaíba drainage. CITES has listed freshwater stingrays under Appendix III, and proposals have been made to escalate to Appendix II for tighter international trade control. Brazil regulates the export of wild-caught stingrays, and the rarity of P. signata in trade partly reflects these restrictions. Given the species's narrow range and slow reproduction, any management plan must combine strict harvest limits, habitat protection in the Parnaíba basin, and investment in captive-breeding programmes.