River Stingrays

River Stingrays: Two color morphs of Potamotrygon castexi

River Stingrays

The family Potamotrygonidae which is commonly referred to as the River Stingrays is in dire need of revision. There are presently 3 valid genera a one that has not been scientifically described but contains a ray that is visibly different enough to warrant a separate classification. There are at least 22 species although identification is extremely difficult in this unusual family group.

Below is a key to the genera of River Stingrays:

1. Large prominent eyes and a dorsal finfold (a band of skin on the upper side of the tail) = Potamotrygon. If the eyes are relatively small and less prominent and dorsal finfold absent, go to 2-4.

2. Anterior margin of disc concave. Spiracle contains a small protrusion = Paratrygon.

3. Anterior margin of disc broadly pointed. Spiracle simple (without protrusion) = Plesiotrygon.

4. Anterior margin of disc rounded (without any defined point). Thin poorly developed tail = undescribed Potamotrygonidae gen sp.

Identifying Individual Species

If you though identifying marine stingrays was difficult, try determining the exact species of a River Stingray. Body patterns and colors are extremely variable among the freshwater stingrays of South America making positive identification using pattern and coloration alone very difficult. This is known as polychromatism. Some morphologically different species can display almost identical patterns and to make identification even harder there is a degree of hybridization among sympatric species. Important characteristics to help the observer overcome these problems include:

• The lateral markings on the tail.

• The presence or lack of raised denticles along the tail.

• The ratio of tail length to disc width.

• the size and position of the eyes.

Although the problem of polychromatism implies that positive i.d. just from coloration alone is almost impossible, there are species that display distinct color morphs not shared with any other species.  Also, some characteristics among similar patterned species are more telling than it would at first appear. For example, Potamotrygon castexi can exhibit a large variety of patterns but they tend to fit within a certain range that blend into each other. When these patterns are considered along with other characteristics it often becomes possible to narrow down the species with reasonable certainty.

To help classify River Stingrays that are traded among aquarists, a more detailed (but also more subjective) classification system has been developed that splits up the recognized species into their various color morphs. In this system, our example P.castexi is further split by appearance into Jaguar Ray, Estrella Ray, etc. but although this works for aquarists it is difficult to follow when translated between languages as it does not have a universal Latinized equivalent and it serves no real purpose to taxonomists unless two morphs are eventually identified as separate species.

Once the genera has been ascertained, the best key to species identification (apart from DNA testing) is the markings on and shape of the tail.

Habitat and Geographic Distribution

River Stingrays are distributed throughout most of the tropical river systems of South America. They comprise the only elasmobranch group that is completely adapted to living exclusively in fresh water.

Brazil contains the greatest number of species (around 18) but potamotrygonid rays can also be found in Argentina, Bolivia, Columbia, Ecuador, French Guyana, Guyana, Paraguay, Peru, Surinam, Uruguay, and Venezuela. P.motoro which is the most common River Stingray has been reported from all of these countries except Ecuador.

River Stingrays can be found in rivers with sand, mud, or stony bottoms. During the rainy season they also move into areas of flooded forest.

River Stingrays spend a percentage of their time buried under the sand or mud with just their eyes protruding.

Freshwater Modifications

River Stingrays have developed permanent modifications to adapt to the freshwater environment that  they are confined to. One significant modification is the degeneration of the rectal gland that serves in the excretion of excess salt in marine elasmobranches. Salt water rays also have the ability to retain high levels of urea in their blood which counters the osmotic flow of fluids through their skin into the salt rich water. River Stingrays have lost this characteristic resulting in an inability to tolerate environments with a salinity greater than 3 ppt.

The electroreceptive ampullae of Lorenzini among River Stingrays is also modified to operate in freshwater.

Reproduction

River Stingrays practice a style of reproduction known as matrotrophic viviparity. Villi (umbilical filaments) are formed that nourish the fetuses while in the uterus. The reproductive cycle has been shown to coincide with hydrological changes in the rays environment. Gestation can last between 3-12 months with 1-8 embryos in gestation at one time.

Diet

River Stingrays eat a wide variety of available foods from worms to insect larvae, shrimp and other tiny crustaceans, and small fishes. Most of their food is located in the sediment and literally sucked in. Their tooth structure like other stingrays consists of small rounded molars that form a flat upper and lower surface designed to grip but not cut.

Defensive Mechanisms and Treatment of Stingray Wounds

River Stingrays carry one or more stingers or tail spines on the top of their tails. This weapon is capable of puncturing the hide of a menacing predator or impaling the leg or abdomen of a wading fisherman. It is sheathed in a mildly venomous covering of skin which is pushed back as the point enters the victim allowing the venom to come in contact with the cut tissue. Wounds inflicted by these stingers are apparently very painful but the toxins can be broken down quickly with heat. Treatment of a stingray wound should involve immersing the affected area in water as hot as the victim can tolerate. The wound should also be irrigated to ensure that no part of the spine has broken off. Infections are common and where medical attention is not readily available River Stingray wounds have resulted in fatalities. In the Amazon the River Stingray is highly feared among the natives that work along the river.

The Aquarium Trade

For the most part River Stingrays are at the top of the food web especially far up river where Bull Sharks are less likely to penetrate. The greatest threat that they face is removal by collectors for export into the aquarium trade. This has become a major problem but recently exports from Brazil were halted while a quota system is implemented. Fortunately for the stingrays, their cryptic appearance has led to so much difficulty identifying specific species that none can be exported until a better i.d. system is figured out. As DNA testing for aquarium collectors is impractical it may be many years before this hurdle is overcome. Under the right conditions River Stingrays will mate in captivity and many animals for sale now come from captive breeding programs. Interestingly, because species from different geographic regions are often housed together this has led to even more hybrids joining the melting pot.

Locomotion

River Stingrays are able to swim forwards (and slowly backwards) by undulating their pectoral fins that form their body disc. See: Elasmobranch Locomotion. Their tails with their rudimentary caudal fins are used for steering and balance and also to support their defensive tail stings.

Breathing

River Stingrays spend a percentage of their time buried with just their eyes showing. Their mouths and gills (like almost all rays) are positioned under their bodies which makes breathing while on the sand rather challenging. To overcome this hurdle they have developed large spiracles (spiracles are the openings positioned just behind the eyes through which a shark or ray can suck in oxygen rich water to flush over the gills). Through this mechanism River Stingrays are able to remain motionless for hours at a time.

River Stingray Evolution

It is unclear which group of marine rays the Potamotrygonidae evolved from but there are two main theories:

It was initially thought that an invading species of himantura (one of the whiptail stingray genuses) colonized the Amazon Basin from the Atlantic (Garman, 1913, Bigalow and Schroeder, 1953). Geographically this makes sense because the sprawling mouth of the Amazon is unprotected by rapids or other features that would protect it from invasion. Whiptail stingrays are also commonly found in estuarine environments and are euryhaline - able to tolerate fresh water for long periods.

The second theory based on parasitological evidence has River Stingrays evolving from the Urolophidae (a family of round stingrays confined to the Pacific) that were trapped during the creation of the Isthmus of Panama (Brooks et al., 1981). Parasitological evidence (comparative analysis of the parasites living in the gut and other areas with the body of an animal) is considered to be quite significant in evolutionary terms but apart from this characteristic River Stingrays have little in common with Round Stingrays as the latter are confined to saltwater environments.

Instead, an Indo-west Pacific Whiptail Stingray (Taeniura lymma or one of its predecessors)  may be the most likely ancestor of the River Stingray.