P. motoro is one of the most popular members of the genus with aquarists. Whilst it is quite easily obtainable and hardy once settled in, it does grow very large so make sure you can house it adequately long term. It should also be noted that the males of this species are among the most violent when in the mood for spawning and females can end up on the wrong end of a serious beating. If you're keeping a pair or group of these keep a close eye on proceedings and be prepared to separate them if necessary.
The patterning of P. motoro is highly variable. Some of these variants have been given their own P numbers (listed above), whilst others are sold under trade names such as P. motoro "blue", "orange", "black" or "marbled". The latter in particular is a stunning fish and tends to cost a bit more than the standard form. P05, or P. sp. "B" has also been tentatively named P. sp. aff motoro. Worryingly there are a number of rays appearing on the market that seem to be hybrids of this species with others in the genus. We find this process abhorrent, and urge you not to buy these genetic misfits with the hope of stifling their popularity.
There are currently three genera in the family Potamotrygonidae, all of which hail from South America. Paratrygon and Plesiotrygon are both monotypic genera (contain only a single described species) and are rarely seen in the hobby. The vast majority of captive rays belong to the biggest genus, Potamotrygon. This contains 17 species at present, and its members are sometimes referred to as "big-eyed" stingrays for fairly obvious reasons. There are also a number of undescribed species that appear in the trade quite regularly and are believed to belong to Potamotrygon. For ease of identification all Potamotrygonids have therefore been assigned a "P-number". This system works in exactly the same way as the L-numbering system for identifying Loricariids, and was similarly used in Germany first before being adopted globally. We've included P numbers in the "common name" field in our stingray profiles. Even with this classification confusion sometimes occurs, resulting in some species being assigned several P numbers. More recently an alternative "R number" method of numbering has come into usage, but we've omitted these for now.
Along with sawfish and sharks all rays are included in the class Elasmobranchii. They have no bones in their body, with the skeletal structures being composed primarily of cartilage. The Potamotrygonids are part of the only group of Elasmobranchs that have evolved to occupy purely freshwater habitats. Like other rays they've also developed specialised breathing apparatus to allow them to breathe whilst lying in or on the substrate (the mouth and gills are on the underside of the fish, so can't be used when the fish is at rest like this). Behind each eye is an opening known as a spiracle, through which water can be drawn and passed to the gills, where oxygen is extracted.
These fish are dangerous. Most natives of the countries in which they're found are far more fearful of rays than other supposedly life-threatening species, such as piranha. In Colombia for example there are over 2000 annual cases of injury and even the odd death caused by ray stings. The sting is located on the top of the caudal fin, where it's clearly visible as a rather fleshy looking appendage. This has a thin outer layer known as the integumentary sheath, which serves to protect the spine of the sting and its venom glands. On the dorsal surface of this are a number of backwards pointing barbs. These help to tear open the sheath when the ray attempts to use its sting, as well as widening any wound inflicted. The orientation of the barbs also allows them to work like a fishing hook, making it difficult to remove the sting once it's embedded in the flesh of a target.
The actual stinging apparatus or spine is formed from dentin (the same substance that composes much of a human tooth) and possesses associated venom glands. A ray can't move its sting independently, rather it uses its tail in a flailing motion when trying to sting something. A sting occurs if the tip of the spine pierces the integumentary sheath and also makes contact with the skin of the intended target.
Although the venom of the various species may vary in toxicity (unfortunately there's little published work regarding the subject), all are broadly similar in composition. The venom is protein-based and contains a cocktail of chemicals designed to cause both intense pain and rapid tissue degeneration (necrosis). If stung, at the very least expect excruciating localised discomfort, headaches, nausea and diarrohea. More severe reactions than this are not uncommon, and medical advice should be sought no matter how serious the sting. An immediate form of treatment is to submerge the wound in water as hot as one can stand, as this helps to denature the toxic proteins.
It goes without saying that the highest form of caution should be exercised when keeping rays. However the danger involved is minimal if a respectful attitude is adopted. They aren't usually aggressive fish, using their sting only as a means of defence. In fact they often become quite tame, learning to recognise their owner and rising to the surface to beg for food. Many hobbyists like to hand feed their rays and this is fine as long as you're careful. For the sake of safety it may be wise to invest in a pair of thick gloves for feeding and tank maintenance though. Most injuries in the hobby occur when naive and/or foolhardy ray keepers attempt to "pet" their fish or catch them using a net. A net should never be used to catch a ray, instead use some kind of solid container or large fish bag.
Rays periodically shed their stings, usually around every six months or so. Often the new sting begins to appear before the original one is shed. Once shed the sting continues to be venomous for a period, so due care must also be exercised when siphoning the substrate or performing other tank maintenance.
The conservation status of Potamotrygonids is unclear. Some populations that have a restricted natural range are thought to be under threat from various anthropogenic impacts, including habitat destruction, ecotourism and to a lesser extent, collection for the aquarium trade. Whilst conservation projects have been suggested, a general lack of biological information regarding the various species, as well as frequent misidentification, has so far made the development of species specific management plans difficult. The Brazilian government has recognised that a problem exists and stingray exports for the trade are now controlled by the Brazilian Environmental Agency (IBAMA). Currently no ray species can be legally exported from the country.
Before buying a ray please read as much as you can on the subject. These are specialist animals that can live for over 20 years, so the purchase of one amounts to a considerable commitment. In the shop try to see the fish feeding before purchase if possible. A healthy specimen will rarely refuse food. If you can see a bony-looking growth protruding on each side of the caudal peduncle, this is a clear indication of malnutrition. The protruberances are the pelvic "bones" and should not be visible in a well-fed ray. An indentation between the eyes is another classic sign of undernourishment in rays. If the fish is still actively seeking food either situation is usually reversible. However if the edges of the disc appear to be curled upwards don't waste your money. This condition is commonly referred to as the "death curl" by ray keepers and the vast majority of affected specimens fail to recover from it. It's caused by extended periods of starvation and/or stress and is thought to come about as a result of the fishes' nervous system and muscle tissue breaking down.
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