Basic Ray Anatomy
Freshwater Stingray Morphology & Physiology
Stingrays belonging to the family Potamotrygonidae have done particularly well in their riverine environment for a variety of reasons. Below is a listing of physiological and morphological attributes that have made a significant contribution to their evolutionary success.
Method of Osmoregulation
Amazonian river stingrays differ from all other living sharks and rays in that they are the only ones known to permanently dwell in freshwater. Unlike the euryhaline whiptail stingrays (Family Dasyatidae), they have completely lost the ability to migrate between freshwater and marine environments. Such a major modification to a fundamental function like osmoregulation (the regulation of internal solute concentrations) is quite remarkable evolutionarily speaking, because even the smallest modification or mutation in the genes that regulate this important system could be disastrous for the organism. In a nutshell, there isn't much room for natural variation in this particular instance.
Sharks and rays are inherently marine animals simply based on their physiology. Seawater is usually saltier than the blood of most fishes, but instead of actively pumping ions and other solutes out of their bodies like marine bony fishes, elasmobranchs simply match their internal osmotic concentrations to that of their external environment. They accomplish this by maintaining concentrations of organic solutes (namely urea and an enzyme called trimethylamine oxide or TMAO) within their bodies. Although urea is toxic to fish, the TMAO counteracts the protein-de-stablizing effects of urea. Excess monovalent ions that they ingest, namely sodium and chloride, are eliminated from the body via specialized rectal glands.
Since Amazonian rays live in freshwater, they have exactly the opposite problem of their marine cousins: instead of loosing water to their external environment, they have to worry about gaining it, since their internal osmotic concentrations are higher than that of the water in which they live. One result of this situation is that freshwater stingrays no longer have any need for rectal glands, and these structures are now vestigial (greatly reduced in size and no longer capable of secreting salt). They have also lost the ability to retain urea, allowing them to sever their ties with the ocean and evolve into exclusively freshwater organisms.
To learn more about the osmoregulatory differences between the families Potamotrygonidae and Dasyatidae (marine and euryhaline), I recommend reading a short article entitled "The osmotic response of the Asian freshwater stingray to increased salinity: a comparison with marine and Amazonian freshwater stingrays" put out by the National University of Singapore in 2003.
A trimethylamine molecule, (CH3)3N. This substance helps to counteract the protein-damaging effects of urea in a shark's bloodstream.
The protruding "periscope" eyes of a freshwater stingray allow them to see what is going on in the water column above while they are buried in the riverbed. They also posess keen eyesight that allows them to navigate the murky environments in which they live. Most of the Amazon River is quite murky and often loaded with sediments, debris, and small floating organisms that either absorb or reflect most of the sunlight before it reaches the bottom. However, stingrays have adapted to low-light conditions in a special way. If you look at a ray's eyes in dim light, you will notice that they shine in the dark, similar to those of a cat. A layer located towards the back of the eye reflects light back into the retina, giving the ray night vision.
Ampullae of Lorenzini
These jelly-filled sensory pores, located on the skin around the nose and mouth on the underside of the disc, are present in most elasmobranchs and allow sharks and rays to detect minute electric fields generated by living things. This is especially useful for freshwater stingrays because it helps them to hunt down prey that might be buried in the riverbed or hiding in murky water.
Stingrays, both marine and freshwater, have two rows of five gill slits on the ventral side of their bodies and two modified sixth gill slits on the dorsal side called spiracles, located behind the eyes. This adaptation allows them to breath more easily while they are hiding in the substrate.
The stingray's infamous tail spines have two components: the sharp inner barb used for piercing, and a thin sheath surrounding it that contains the venom. When the spine is deployed, the barb pierces the venom sac along with the victim's skin, and the poisonous slime is introduced into the wound. The barb is extremely sharp (it has been known penetrate bone), and it operates under the same principle as an arrowhead-- it slides into flesh fairly easily, but the serrated edges make it very difficult and painful to extract. The tail is very flexible and can bend pretty much any direction within a split second, inflicting serious damage. In addition to causing great pain, the venom contains enzymes that cause tissue death.
Similarly to the teeth or dorsal spines of a shark, stingray spines are thought to have originated from placoid scales, the tiny tooth-like structures that protect the skin of elasmobranchs and give it a sandpaper-like texture (see drawing below). And, just like shark teeth, they are lost and re-grown on a regular basis.
Freshwater stingrays are matrotrophically viviparous, giving birth to one to seven live young at a time after a gestation period of several months (dependent on species).The uterus is formed from the expansion of the oviduct. The embryos obtain nourishment from their yolk sacs early in their development. During the later stages of pregnancy, small, filamentous appendages called trophonemata develop within the uterus and penetrate the spiracle of the embryo (Nikolskii, 1961), supplying it with a nutrient-rich fluid called histotrophe that feeds it until it is born (Charvet-Almeida et al., 2005).
Although freshwater stingrays produce fewer offspring per batch than many other fishes, they invest much more time and energy into producing well-developed children that can fend for themselves early on and have a better chance of surviving predation. At birth, the young rays look like miniature copies of their parents, and are able to search for their own food a short time after delivery.
A stingray's sex can be determined fairly easily based on the presence or absence of claspers, two "penises" connected to the inside of the pelvic fins of male rays. They are usually about the size of your pinky finger on mature specimens and are rolled up into hollow tubes. On juvenile rays, they look like tiny nubs and are a bit harder to identify, but it is still possible to tell males and females apart at this stage. You just need to look a bit closer.
Above: Immature, unrolled claspers.
The eggs are fertilized internally after the male inserts one of his claspers into the female's cloaca, which serves dual purposes in defecation and reproduction. In order to accomplish this, the male must grab hold of the female's disc with his mouth and roll underneath her, so that their bellies are facing one another. The actual sex act usually lasts no longer than a couple of seconds. Females can sometimes sustain slight injury during mating when the male bites her, which is why they have evolved thicker, more durable skin than males. If you are trying to breed rays, be sure to pick a male that is the same size or slightly smaller than the female, as a larger male can overwhelm and seriously injure the female in his overzealous mating attempts.