Morphology

This page describes the external morphology, locomotion, and sensory biology of Potamotrygon motoro.

Sensory

Potamotrygon motoro has a number of methods for detection of their surroundings. The dorsal position of the eyes offers a nearly 360° field of vision (Spiegel 2013). Due to the ray’s preference for murky waters and penchant for kicking up sediment, sight is not likely to be the dominant sense for detection of predators and prey. The mechanoreception system of lateral lines across the ray’s disc detect changes in water pressure while electroreceptors called ampullae of Lorenzini detect short-range electrical signals. These systems, as well as the strong olfaction system provided by cartilaginous bulbs placed laterally on the head, form the dominant senses used by the stingray to detect nearby prey (Spiegel 2013).

Locomotion and Feeding

The ocellate river stingray is dorsoventrally flattened. It lacks dorsal and anal fins, and its whiplike tail lacks a caudal fin. At the base of the tail is a spine with a potent venom used to ward off potential predators. The pectoral fins form a large disc with small pelvic fins near the tail. The disc acts as the main method of propulsion through undulating waves moving down the disc’s length (Gabler-Smith et al. 2023). This rajiform movement is pictured below. The most visually distinct feature of Potamotrygon motoro is the yellow and black spots covering the dorsal side of the ray for which the common name “ocellate river stingray” is derived.

To compliment its durophagous diet, the ray has crushing, molariform teeth used to break the shells of the molluscs and crustaceans that form the majority of its diet (Freshwater Stingray). The mouth is placed subterminally and features euhyostylic suspension like all batoids.

References

Freshwater Stingray (no date)https://nationalzoo.si.edu/animals/freshwater-stingray

Gabler-Smith MK, Coughlin DJ, Fish FE (2023) Morphological and histochemical characterization of the pectoral fin muscle of batoids. J Morph 284.

Spiegel J (2013) Potamotrygon motoro Black river stingray (Also: Ocellate river stingray; South American freshwater stin). https://animaldiversity.org/accounts/Potamotrygon_motoro/ (accessed April 25, 2024)