Potamotrygon motoro

Geographic Range

Ocellate river stingrays are endemic to, and widespread throughout, several South American river systems. Most studies on this species have been conducted in the Brazilian Amazon region, and while its presence has been confirmed in the rivers of other South American nations, such as Uruguay, the details of its distribution outside of the Brazilian Amazon are not fully understood. In addition to the Amazon River basin, this species is found in the Uruguay, Paraná-Paraguay, and Orinoco River basins, including the middle and lower portions of Rio Paraná in midwestern Brazil (where it is the most abundant species of stingray), middle portions of the Río Uruguay, and in the Río de la Plata, Río Pilcomayo, Río Bermejo, Río Guapore, Río Negro, Río Branco, Río de Janeiro and Río Paraguay. This species has recently gained access to many upper regions of the Amazon river basin and other non-native areas due to hydroelectric dams eliminating natural barriers.

• Biogeographic Regions
• neotropical neotropical
  • introduced introduced
  • native native

Habitat

Ocellate river stingrays are habitat generalists of tropical (24°C-26 C) freshwater rivers. As a bethopelagic animal, habitat depth varies with the depths of the rivers they inhabit; studies have found these stingrays at depths of 0.5-2.5 meters in the upper Paraná River, but at depths of 7-10 meters in the Uruguay River. Ocellate river stingrays prefer calm waters with sandy substrate, particularly the edges of brooks, streams and lagoons, where they are often found partially buried.

• Habitat Regions
• tropical tropical
• freshwater freshwater

• Aquatic Biomes
• pelagic pelagic
• benthic benthic
• rivers and streams

Physical Description

These stingrays can be distinguished from closely related species (such as large spot stingrays ( Potamotrygon falkneri )) by the presence of orange to yellow dorsal eyespots, each surrounded by a black ring, with diameters larger than the eyes. Body color is otherwise greyish-brown. They are oval in shape with a robust tail, bearing a venemous spine. Maximum total length has been reported at 100 centimeters and maximum weight at 15 kg, though individuals tend to be much smaller (50-60 cm and under 10 kg). Females tend to be slightly larger than males.

• Other Physical Features
• ectothermic ectothermic
• bilateral symmetry bilateral symmetry
• venomous venomous

• Sexual Dimorphism
• female larger

Development

Ocellate river stingrays are ovoviviparous, with eggs approximately 30 mm in diameter at ovulation. They engage in lipidic histrophy, a mode in which developing young are provided with nutrient-rich secretions within the mother's uterus, providing for far more extensive development than the yolk sac can alone. Most development proceeds isometrically, with the overall body proportions essentially established from birth, but there are a few areas of significant allometric development. Total length relative to disk length decreases continuously throughout development, as does tail length relative to total length. This animal's eyes become less anterior in location and smaller in diameter relative to body size as development proceeds. The most rapid growth occurs early in life, tapering off in later years. Gestation of ocellate river stingrays lasts approximately 6 months in the wild, but has been observed within 3 months in an aquarium environment. From 3-21 pups may be born in a litter and the number of young in a litter is always odd. If caught, pregnant wild females are likely to abort their young.

Reproduction

Mate location methods have not been studied in this species. Information on mating systems has been observed in a captive population, and may exhibit differences from wild populations. Copulation occurs mainly at night. A male attaches himself to a female by firmly clamping his jaws onto the posterior margin of her disk, sometimes leaving prominent bite marks. Around the time of mating, the lips of the female's cloaca become swollen and bloody, presumably from copulation (this may have given rise to the folk notion that female stingrays menstruate). This species exhibits polygyny, although it has been suggested that a male will only mate every few weeks; in one study, a wild-caught male was housed with two captive-born females and, while he bred with both, fresh bite marks were not seen on both females at once, instead being seen at intervals of at least several weeks.

• Mating System
• polygynous polygynous

The reproductive cycle of this species is triggered and directly influenced by the hydrologic cycle of its river environment, rather than seasonal cues. Gonadal maturation takes 3-4 months and ovarian asymmetry, in which only the left ovary is functional, is a common characteristic. Ocellate river stingrays mate during the dry season, corresponding roughly to June through November. Gestation lasts for 6 months, and birthing occurs during the rainy season, corresponding roughly to December through March. Reproduction appears to follow a cycle in which one litter is birthed each year for three years in a row, followed by a several-year period of reproductive inactivity. Observations suggest that the mechanism of this cycle involves three sets of ova being induced to develop during each reproductive pause, with one set of ova finishing reproduction at the start of each breeding season. These stingrays have 3-21 pups per litter; average litter size is 7 pups and smaller females tend to give birth to fewer young. Young are independent at birth.

It has been suggested that male offspring are slightly more likely in a given litter than females (55% male to 45% females). At birth, young average 96.8 millimeters in length, with female disk diameter averaging 110-135 millimeters and male disk diameter averaging 95-120 millimeters (no data for birth mass is currently available). Age at sexual maturity has not been definitively determined, with estimates varying from 20 months to 7.5 years. Most recent studies report that sexual maturity occurs at a disc width averaging 390 mm in males and 440 mm in females, although earlier research reported sexual maturity at smaller sizes (200-250 mm and 240-320 mm, respectively). The relative length of pelvic claspers is a key indicator of sexual maturity in males, increasing from approximately 5% of total body length as juveniles to approximately 20% as mature adults.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
  • internal internal
• ovoviviparous ovoviviparous

Females continuously invest in their young during development through the constant provision of nourishment, through lipidic histrophy. Pregnant females in the wild are highly likely to abort fetuses upon capture.

• Parental Investment
• female parental care female parental care
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • provisioning
    • female
  • protecting
    • female

Lifespan/Longevity

Information regarding lifespan in the wild is unavailable. These animals are known to survive up to 15 years in captivity.

Behavior

Ocellate river stingrays are potamodromous, migrating within freshwater rivers and streams. Although little is known of their specific migratory behavior (for example, more of these animals are captured in the Paraná Medio from late fall through early winter, with few to none captured during the spring and summer, but it is unknown if this is due to migration or other causes), but migration patterns are assumed to be cyclical and predictable and to cover at least 100 kilometers. They are assumed to be solitary outside of breeding. During the day, these stingrays can be found remaining still and mainly buried in sandy sediment.

• Key Behaviors
• natatorial natatorial
• nocturnal nocturnal
• motile motile
• migratory migratory
• solitary solitary

Home Range

These stingrays are not known to defend a home territory or occupy a specific home range.

Communication and Perception

Ocellate river stingrays have eyes positioned on the dorsal surface of the head and oriented in opposition to one another, giving a nearly 360° field of vision, with little binocular overlap and blind spots directly in front of and behind the head. They have crescent-shaped pupils with dynamic irises that can adjust the pupil's size in response to light conditions. These stingrays have lateral line systems, consisting of subepidermal fluid-filled canals distributed throughout the body, which perceive changes in the surrounding water pressure. They also possess an elaborate electroreceptor system that provides for extremely sensitive perception of low-frequency electrical stimuli and allows for detection of prey that cannot be visually perceived, comprised of subdermal groups of electroreceptive units known as the ampullae of Lorenzini. It is also used for detection of predators and recognition of conspecifics, and sometimes in navigation. Olfaction is a major and well-developed means of perception for these stingrays; their olfactory organs are situated in laterally placed cartilaginous capsules on the top of the head.

• Communication Channels
• tactile tactile
• chemical chemical
• electric electric

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical
• electric electric

Food Habits

Food types consumed depend on age and environment. Shortly after birth, young eat plankton and juveniles add small mollusks, crustaceans, and aquatic insect larvae to their diets. Adults are primarily consumers of fish, including loricariids, Astyanax species, and graceful pimelodellas ( Pimelodella gracilis ), as well as crustaceans (Palaemonidae sp.). They are also known to eat gastropods (Ampullariidae and Hydrobiidae sp.), aquatic insects (Baetidae, Chironomidae, Elmidae, and Naucoridae sp.), and flying insects (Pyralidae, Corduliidae, Gomphidae, Hydropsychidae, Leptoceridae, and Odontoceridae sp.).

• Primary Diet
• carnivore carnivore
  • piscivore piscivore
  • insectivore insectivore
  • eats non-insect arthropods
  • molluscivore molluscivore
• planktivore planktivore

• Animal Foods
• fish
• insects
• mollusks
• aquatic crustaceans
• zooplankton zooplankton

• Plant Foods
• phytoplankton phytoplankton

Predation

Humans regularly catch and eat ocellate river stingrays. They have few predators, although caiman are known to eat these stingrays and it is assumed that large fish may as well. However, its serrated, venomous tail spine serves as an obvious anti-predator adaptation.

Ecosystem Roles

Ocellate river stingrays play a role in controlling insect populations through their diet. They serve as hosts to a variety of ecto and endoparasites.

Economic Importance for Humans: Positive

Ocellate river stingrays are consumed regularly in the Parano-plata Basin. However, in many regions (such as midwestern Brazil) there is no significant commerce of stingrays for human consumption. Juveniles are considered desirable, and are regularly seen in, the ornamental fish trade.

• Positive Impacts
• pet trade pet trade
• food food

Economic Importance for Humans: Negative

These stingrays can deliver a painful, venomous sting to humans and other organisms. Reports of incidents have become more common recently, likely due to its recent introduction to regions such as the Paraná River.

• Negative Impacts
• injures humans
  • bites or stings
  • venomous venomous

Conservation Status

This species is categorized as "data deficient" by the IUCN Red List of Threatened Species. The status of its population is completely unknown. In many areas, no regulations exist regarding the traffic and/or exportation of freshwater stingrays; an ongoing project in Uruguay is encouraging sport fishermen to return caught stingrays to the water. It may be inferred, from the species' generalist diet and habitat, fairly widespread range, and relatively low demand as a food source, that ocellate river stingrays are most likely not currently a critical conservation concern.

Additional Links

Encyclopedia of Life

Contributors

John Speigel (author), The College of New Jersey, Matthew Wund (editor), The College of New Jersey, Jeremy Wright (editor), University of Michigan-Ann Arbor.

References

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