An Introduction to Freshwater Stingrays :
By David Webber
In the last few years freshwater rays have become increasingly available and popular and tanks have become larger and cheaper, making them a reasonable pet for the dedicated home aquarist. Although some other major rivers around the world have ray populations, most freshwater ray species are found in Amazonia, and as with its other flora and fauna, the Amazon system has an abundance of ray variants found all along the river and in many of its tributaries, from Peru and Colombia in the West to the mouth of the Amazon in North Eastern Brazil. Some ray species are also found in other tropical South American rivers with no direct connection to the Amazon. Stingrays are very ancient species, tracing their evolutionary history as far back as 300 million years.
Evolutionary History :
The most commonly found species in South American rivers are Potamotrygon. There are also rays known as China or Coly rays, about which little is known at this time.
It is thought that all of these Amazon rays are most closely related to Pacific Marine rays. Their isolation would have occurred when the Andes Mountains rapidly rose up about 15 million years ago, blocking the Westward flow of the river as it then was and forcing it to flow east all the way to the Atlantic, trapping many rays in the new system. This isolation and the Amazon’s tropical climate and seasonal massive changes in water levels created ideal circumstances and great pressure for evolutionary changes, as represented by the huge variety of stingrays found in just the one system. Even individual species that are found along the whole river, such as Motoros andHistrix, are polymorphic, each exhibiting their own wide range of colors and patterns as habitat and available diet change subtly between regions.
The author with a few of his stingrays..
Book for stingrays..
It is thought that all of these Amazon rays are most closely related to Pacific Marine rays. Their isolation would have occurred when the Andes Mountains rapidly rose up about 15 million years ago, blocking the Westward flow of the river as it then was and forcing it to flow east all the way to the Atlantic, trapping many rays in the new system. This isolation and the Amazon’s tropical climate and seasonal massive changes in water levels created ideal circumstances and great pressure for evolutionary changes, as represented by the huge variety of stingrays found in just the one system. Even individual species that are found along the whole river, such as Motoros andHistrix, are polymorphic, each exhibiting their own wide range of colors and patterns as habitat and available diet change subtly between regions.
P. motoro - Peru.
Large China Ray middle, P. leopoldi top, P. motoro bottom
Estrella Ray center, Peru
Scobina Ray
Flower Ray from Venezuela/Columbia
A word of warning about a stingray's stinger :
Stingrays are so-called for their serrated, dagger-like stinger located on the top of and lying flat towards the end of the tail. Usually sheathed in a layer of skin and not always easy to see, this effective defense weapon is made of a protein complex , and is accompanied by a nasty venom usually released when the skin sheath of the stinger is ruptured. If the stinger cuts you it can cause large local blisters and intense burning throbbing pain. The immediate treatment for this is to immerse the wound in water as hot as can be tolerated, which helps to neutralize the toxins. This is not always possible, especially on the river. The traditional first aid treatment there is to urinate on the wound which works because urine is both hot , sterile and slightly acidic. Amazon folklore says that it must be a virgin who pees on the wound, leading to the joke 'there is good news and bad news......there are no virgins in Brazil'. Although extremely painful the ‘sting’ is rarely fatal unless by some bizarre accident it is in the chest near the heart, a tragic event which has been recorded as a fisherman was pulling a caught ray onto his boat. Fortunately for the aquarist even the feistiest ray is not normally aggressive with its stinger, though rays can be very accurate with their aim, either slashing or stabbing with their powerful tails. In Amazonia most casualties happen when a ray is stepped on as it hides under the sand basking and snoozing during the day and so most stings are on the feet or lower leg. Rays would much prefer to move than be stepped on so locals know to shuffle their feet or poke the sand ahead of them with a stick when in the water, so that a ray can sense the motion and move out of the way without incident. For hobbyists it is the opposite, most people get hit on the hand or arm as they mess around in the aquarium while cleaning the tank or moving the ray. Sometimes it is a pure accident, others it is from carelessness handling the ray. Even on a dead ray the stinger is still toxic and very sharp.
Types of rays kept by hobbyists
The majority of rays kept by hobbyists these days , and the main focus here, arePotamotrygonidae, species of Elasmobranchs which are found exclusively in freshwater and includes more than 22 distinct varieties. At this time there is a lot to be learned about the relationship between these different varieties, whether they are sub-species or how closely they are related to each other. There is a dramatic and beautiful range of size , patterns and colors between them . Black rays with striking polka-dot patterns, such as the Leopoldi, Henlii and the less common small spot Itaituba Black rays are found in Brazilian Southern Amazonian clear water rivers. Two of the more dramatically patterned types are Tiger rays, named for their striking patterns and the markings on their tails, found in Peruvian Amazonia and the similarly patterned Flower rays, found in Northern Amazonia in some of the Colombia/Venezuela/Brazil border region rivers. There are many other local ray variants that are not as beautifully patterned but still are interesting. Castexi rays, one of the most polymorphic rays, with large array of distinctly different body patterns, are mostly found in the Western regions of Amazonia, with morphs such as Otorongo
(translated: Jaguar), Motello ( tortoise), Hawaiian, Tigrinus and others, all named for their beautiful patterns.
Itaituba Ray
Rare P 14 Variant
Pearl Rays ..
The debate on species differentiation :
There is some debate concerning the different variants within species like Castexi and also the Black rays about how closely related to each other they may be. Whether they are truly separate species or are simply color morphs from slightly different habitats that would be expected to produce slightly different characteristics. Leopoldi, Henlii and Itaituba Black rays are prime examples often invoked in this debate, and their ability to easily interbreed throws more confusion on the whole subject. They are normally found in, and separated by, different Amazon tributaries.Leopoldi are found in the clear waters of the Xingu and Henlii are found in the waters of the Tocantins, clearly separate habitats according to the maps, but when the high water season is there it is impossible to know how different species are moved around by the massive inundation of floodwaters or if & how they migrate into different systems when water levels permit, only to become isolated again when the seasonal floods recede to lower levels. So it may be possible to find a species that may normally be only found in Peru or Colombia all the way down river in Brazil. It may also be that many of the species are in the middle of an evolutionary spurt, developing into new species. ‘Hybridization’ ( if they are actually separate sub-species) also seems to be a regular occurrence when natural circumstances permit and viable offspring suggests that Potamotrygon are not separate species.
Estrella and leopldi
Flower Rays middle, leopoldi to right
Newly arrived young leopoldi - Brazil - Xingu
Stingrays - a pest to some, a beauty to others :
"In the Amazon all things are possible". During the low water season, when fish are concentrated in small areas and main river and Igarape channels, it is relatively safe to travel to remote areas to investigate ray population distributions; but this season is relatively short, often as little as 4 or 5 months only before the new rains start and the water levels rapidly and dramatically rise again. Much of Amazonia is still unexplored, and river courses change making it impossible to be absolute when it comes to discussing Amazonian fish populations and distributions. Some ray populations are booming to the point where they are considered a pest on the sandy beaches of the river. People are employed on popular river beaches in Brazil to keep the beaches clear of the rays, which tend to congregate in ‘dormitories’ to bask just under the sand in shallow water during the day, just the place where bathers walk . Basically those rays are killed as a pest and left to rot and feed the scavengers. There is a certain irony in this as those same rays could be sold alive to the aquarium trade instead and generate income. Some ray varieties are only found in limited areas and are not common, such as ‘Pearl’ rays. Rays are also an important local food fish . These are both contributing reasons for annual stingray quotas out of Brazil. However, these quotas often produce paradoxical facts. As a food fish many rays are worth much more alive to the aquarium trade than they are as food. One Henlii , Leopoldi, or Pearl ray, sold alive, is usually worth more on the aquarium market than a meat animal that might feed a whole village for a week or give milk for a few years. With a few exceptions, most ray species are plentiful in their native habitats and are considered pests, which contributes to an attitude that it is OK to smuggle rays out during the off-season or when all the quotas for aquarium export have already been filled. The laws of supply and demand fuel this market, with the final retail price for them adding incentive. Smuggled rays are often more expensive and usually less healthy than those that are exported through legal channels because of the extra ordeal those smuggled rays have to endure when exported this way. There are many tales of rays (and other smuggled flora and fauna) being hidden inside adapted gasoline cans or under floorboards aboard fishing boats. Luckily they don’t fit into people’s pockets.
Tiger Ray ..
Otorongo Ray ..
Tank size, tank mates, and feeding :
One of the attractions for keeping rays in the aquarium is their behavior and intelligence. A healthy ray can learn very quickly how to recognize and hand feed from its owner. Hand feeding a large ray is a rewarding interactive experience for all those who do it. Of course, their ‘other-worldly’ appearance and their obvious beauty and character add to the pleasure. Most rays grow fast and if properly cared for will quickly grow to a size that is not suitable for a small tank . They require a large tank even at the beginning of their residency. Ideally they are kept as a single species, in other words they should be kept alone. Mixing other species can be precarious. One of the more popular tankmates are Arowanas, but if they are not matched for size with the rays then there can be problems. As a general rule of thumb when it comes to companion species ,"if a ray can fit it in its’ mouth then sooner or later it will eat it". If the Arowana is too big then it might attack the ray and will certainly beat the ray to food. On the other hand if the Arowana is not big enough then there is a good chance that eventually the ray will eat it. A friend of mine found this out the hard way when his 22" disc Tiger ray ate his 10" long $2000 red Asian Arowana. Plecos can present a similar problem, either they will be eaten by the ray or they will take free meals from the rays upper body slime coat, causing stress and possible secondary infections in the wounds and eventual death for the ray. Rays when healthy have voracious appetites and enjoy a range of foods. The easiest are live foods. California blackworms and earthworms are greedily sucked down. Ghost shrimp are a favorite food that often sends my smaller rays literally spinning with joy. Once used to it, most rays will also devour fresh frozen shrimp and other similar foods. Smelt , Salmon pieces, various shellfish and other kinds of seafood are often accepted, but some can be very messy and should be tested first or only fed just before a large water change. Rays can also be adapted to homemade recipes and prepared sinking commercial foods. Hand feeding your ray is one of the most rewarding experiences guaranteed to get a "WOW" from even the most jaded person. A ray is not inclined to sting, but can thrash about with happiness when being fed so when hand feeding it is important to be aware of where the rays tail is at all times and that the ray also be aware of you so that it won’t be startled. After a while rays can become very tame, allowing petting and actively investigating your hand for possible food.
Large China Ray ..
Water parameters :
Amazonian rivers are all very soft water, with few minerals at all. Only the massive volumes of rainfall prevents the pH from falling sharply. Most of their habitat is around the pH 6.5 mark. When properly acclimated most rays can thrive in our tap water and do not require extensive chemistry to maintain them. However, they eat a lot and consequently excrete a lot so good husbandry is important in order to keep the water clean and the filters clear from sludge buildup. I recommend at least 2 water changes each week of at least 25%, using aged water. More if the tank is the least bit crowded. Rays are susceptible to organ damage from Ammonia and Nitrates poisoning so it is essential to monitor those levels and maintain them at constant zero through regular water changes and good biological filtration. Ammonia poisoning is one of the main causes of the ‘silent death’ as spoken of by Dr. Ross in his books. The damage is often done in original transit, which is often more than 36 hours in a box . A ray may take up to a couple of weeks to die from this, as it refuses food and either wastes away or dies from the internal toxins, and so it is sometimes hard to understand the linkage between shipping in bad water and the eventual death.
Acclimating your new rays, tank decoration :
Some rays are more sensitive than others, the Paratrygon and China (small eyed) species are notoriously more difficult to keep than the Potamotrygons. Common popular species that are endemic to the Amazon are Histrix and Motoros and Reticulated. Many arrive under the catch-all name of ‘Teacup’ but this really only refers to their juvenile size and has no significance for knowing their species. These are often a first introduction to keeping rays as they are inexpensive compared to the more exotic rays. Some of those more exotic looking species, such as theLeopoldi, are very hardy and very active and aggressive in their behavior, others , like theMenchacai (Tiger) rays are thought of as being more shy, but this can be deceptive as most species are very active once they are properly acclimated and in a comfortable environment. Most people will never have to worry acclimating rays direct from import. If you are happy that the ray you are acquiring is healthy then acclimation to your tank or pool is not difficult. A few feet of airline tubing to siphon water slowly from the tank into the box/bag with your new ray are all that is needed. As the bag fills remove water until the ray is in tank water and all the original bag water has been diluted out. This takes from 30 minutes to an hour. The greater the differences in parameters between the tank water and the bag water the slower you should make the change. Once this is done remove as much of the water in the bag as possible and gently put the bag and ray into the tank and allow the ray to swim out of the bag into its new home. I keep my pools bare-bottomed but most hobbyists prefer a biotopical look and want substrate. It is important to avoid using anything sharp, such as sand that contains silicates, as this will tend to shred the rays underbody, allowing an infection to creep in, often leading to death of the ray. Estes is a brand name sand that is coated to make it smooth and is available in a variety of colors and is very suitable. The few times I have used a substrate I have chosen something to compliment my ray. For Black rays I use a black sand. Rays will tend to adjust their body color tone to blend in with their background, tending to get either noticeably lighter or darker over a period of a few days to a few weeks. Rays have relatively large brains and are one of the most intelligent of aquatic animals. A ‘dressed’ aquarium can help keep it amused, Driftwood and rocks to search around for food provides plenty of entertainment.
Breeding :
Gender identification is simple. Just like their relatives Sharks and marine Rays, male freshwater stingrays have claspers, one on either side of the base of the tail and slightly underneath. In sexually mature males these are easily visible from above and with immature young they can be easily seen from their underside. Females do not have claspers. When mature enough to breed rays reproduce by internal fertilization and give birth to live young after a gestation period of approximately 3 months. ‘Pups’ are usually born in litters of from 2 to 8, depending upon the age and size of the rays involved. While still not a common event in the home aquarium several stingray enthusiasts and many large public aquariums around the world are now successfully breeding many varieties. As with all animals, breeding is a biological imperative for rays that only requires enough space and plenty of good food along with reasonable aquarium conditions and good husbandry to make your pets comfortable . This also makes the aquarium more enjoyable to watch and interact with for the already entranced aquarist.
Large male Motoro center, young leopoldi sides..
Aquarium Fish: An Overview of Stingrays of the genus Potamotrygon :
By Kenneth Wingerter
While they require a high level of rather specialized husbandry, the rewards for successfully maintaining these remarkable animals are great.
There probably are few aquarium fish that are as beautiful, interesting and distinctive as the freshwater stingrays. They are typically the center of attention in any public or private exhibitry that they are displayed in. Certain special considerations, however, must be made to properly house them, which preclude many types of aquarium systems, aquascapes and tankmates outright. Still, while they require a high level of rather specialized husbandry, the rewards for successfully maintaining these remarkable animals are great.
Several genera of freshwater stingrays appear in the ornamental fish trade. However, those of the genusPotamotrygon, the so-called river stingrays, are undoubtedly the most common. Reasons for this are many. They are strikingly handsome fishes. They reach relatively modest adult sizes. They generally accept a variety of readily available aquarium fish foods. Under the care of an experienced aquarist, long-term survivability is quite feasible. Most notably, they even can be bred and reared in captivity.
During the rainy season, the Orinoco River can expand to a width of 14 miles (22 kilometers). Photo by Pedro Gutiérrez.
A Venezuelan river ecosystem landscape. Photo by Anagoria.
This piece discusses the classification, distribution, ecology and conservation of river stingrays; a following piece will discuss river stingray morphology, reproduction and husbandry.
Classification:
The true rays and skates, Superorder Batoidea, are assigned (along with all other jawed cartilaginous fishes) to Class Chondrichthyes. They share Subclass Elasmobranchii with sharks and chimaeras. Rays account for about half of all elasmobranch species. Of the 500 or more described ray species, there are over 150 stingray species that are assigned to approximately 20 genera. Freshwater stingrays of the family Potamotrygonidae are assigned to the genera Paratrygon, Plesiotrygon, Heliotrygon and Potamotrygon. To date, as many as 20 described species are assigned to Potamotrygon. P. hystrix is recognized as the type species. Some aquarists use a P-number system (which is similar to the L-number system associated with loricariid catfishes) to classify these animals.
P. hystrix, type species of Genus Potamotrygon. Illustration by Paul Louis Oudart.
The beautiful P. tigrinawas just given a scientific name in 2011. Photo by Franklin Samir Dattein.
Like other members of the genus, P. castexi can be found locked in ponds formed by receding floodwaters. Photo by Franklin Samir Dattein.
Below is a complete list of the currently valid names of species included in Genus Potamotrygon.
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Potamotrygon boesemani Rosa, M. R. de Carvalho & Almeida Wanderley, 2008 (Emperor ray)
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Potamotrygon brachyura (Günther, 1880) (Short-tailed river stingray)
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Potamotrygon constellata (Vaillant, 1880) (Thorny river stingray)
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Potamotrygon falkneri Castex & Maciel, 1963 (Largespot river stingray)
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Potamotrygon henlei (Castelnau, 1855) (Bigtooth river stingray)
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Potamotrygon humerosa Garman, 1913
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Potamotrygon hystrix (J. P. Müller & Henle, 1834) (Porcupine river stingray)
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Potamotrygon leopoldi Castex & Castello, 1970 (White-blotched river stingray)
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Potamotrygon magdalenae (A. H. A. Duméril, 1865) (Magdalena river stingray)
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Potamotrygon marinae Deynat, 2006
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Potamotrygon motoro (J. P. Müller & Henle, 1841) (Ocellate river stingray)
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Potamotrygon ocellata (Engelhardt, 1912) (Red-blotched river stingray)
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Potamotrygon orbignyi (Castelnau, 1855) (Smooth back river stingray)
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Potamotrygon schroederi Fernández-Yépez, 1958 (Rosette river stingray)
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Potamotrygon schuhmacheri Castex, 1964
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Potamotrygon scobina Garman, 1913 (Raspy river stingray)
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Potamotrygon signata Garman, 1913 (Parnaiba river stingray)
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Potamotrygon tatianae J. P. C. B. da Silva & M. R. de Carvalho, 2011
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Potamotrygon tigrina M. R. de Carvalho, Sabaj Pérez & Lovejoy, 2011 (Tiger ray)
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Potamotrygon yepezi Castex & Castello, 1970 (Maracaibo river stingray)
Genetic analysis of wild specimens suggests that the origin of the group can be attributed to a single colonization event. The role of hybridization in the speciation of the group, however, remains unclear. There is a considerable number of shared characteristics between species, as well as considerable variation within some species. The presently undescribed Itaituba river stingray (or P14), which evidently differs from P. henlei and P. leopoldi only in the size/number of spots, could possibly be one such variant or hybrid form. Indeed, some recent studies put into question the validity of the present taxonomic organization of these animals altogether.
In the wild, P. henleifavors muddy bottoms where it preys heavily on gastropods. Photo by Christine Schmidt.
Distribution/ecology :
Of the many, diverse elasmobranchs, Potamotrygonidae is the sole extent family that is completely restricted to freshwater. While potamotrygonids are primarily river dwelling (or potamodromous), they are capable of exploiting a variety of freshwater habitats. Potamotrygon is native to the murky river systems of neotropical South America. This highly specialized, monophyletic group occurs mainly within a narrow geographical range spanning the Amazon River Basin. Curiously, members of this genus are found only in those rivers that drain into the Caribbean Sea or Atlantic Ocean. However, they are not found in the upper Paraná basin, the northeastern Brazilian São Francisco basin, Argentinean rivers south of the La Plata River, or northeastern and southeastern Brazilian rainforest rivers that drain into the Atlantic Ocean. Potamotrygon usually inhabits ranges that are restricted to a single river system or basin. Usually, no more than a few species (P. motoro and P. orbignyi, for example) occur in the same basin. In certain cases, a species (P. leopoldi, for example) may be restricted to a single river.
River stingrays dwell in a diverse range of freshwater environments, such as sandy lake beaches, flooded forests, and small, muddy creeks. Some species thrive under unusual environmental conditions such as very low pH or low dissolved oxygen concentrations (hence, one interesting adaptation to freshwater environments: the ability to float on the surface when bottom waters are oxygen poor).
The distribution of P. orbignyi within Amazonian estuaries is influenced by seasonal fluctuations of salinity. Photo by Claire Powers.
However, river stingrays are restricted to water where salt concentrations do not exceed 3.0 ppt. Interestingly, potamotrygonid blood chemistry differs appreciably from marine and euryhaline elasmobranchs. For instance, because the rectal gland excretes little or no salt, they are incapable of retaining urea.
River stingrays tend to be more active at night, particularly while feeding. They are best described as nonspecialized predators. Wherever they occur, they generally are at the top of the food web. Adults prey mainly on fish, worms and small crustaceans, whereas juveniles prey mainly on small crustaceans and aquatic insects.
Conservation:
As they typically inhabit relatively restricted ranges, potamotrygonid stingray populations are especially sensitive to harvest as well as environmental disturbances. Both indirect threats (e.g., habitat destruction due to development, mining, and damming) and direct threats (e.g., the indiscriminate killing of stingrays as pests, collection for the aquarium fish trade) have resulted in tight regulations for stingray "fisheries" as well as CITES II protection. To date, five river stingray species have been registered in the IUCN Red List as threatened.
In the wild, P. leopoldifavors rocky river bottoms where it preys heavily on freshwater crabs. Photo by Michael-David Bradford.
While river stingrays are seldom fished for food, they are often taken as trawl net bycatch. They are also under significant pressure from ornamental fisheries. Owing to a high incidence of hybridization (both intentional and accidental) within captive populations--and a growing demand for "pure" lines--trade in wild-caught specimens has become quite lucrative. This has not escaped the attention of individuals who now harvest river stingrays heavily in unprotected areas just outside the borders or poach where harvest is prohibited. 20,000 specimens are legally exported from Brazil annually, with some unknown number of individuals (especially P. henlei and P. leopoldi) exported illegally. Strangely enough, another estimated 20,000 individuals are destroyed each year during "cleanups" along stretches of river beaches frequented by tourists; the waste involved in this practice should be obvious to anyone.
Commercial river stingray breeding facilities are currently operating in the United States, Germany and Southeast Asia. Fortunately, the use of PIT tagging in the trade is slowly regaining the confidence of consumers who are again relying on breeders, rather than collectors, to supply "pure stock." In fact, as breeders continue to increase production, they could potentially flood the market with captive bred product and all but neutralize the export of river stingrays from their native lands altogether. At the very least, relieving pressure on wild populations in this way could help to ensure that the existing legal harvest quotas will not be reduced, thereby keeping supply lines for wild genetics open.
Sources
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Kuba, Michael J., Ruth A. Byrne and Gordon M. Burghardt. (2010). A new method for studying problem solving and tool use in stingrays (Potamotrygon castexi). Animal Cognition, 13(3), 507-513.
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Toffoli, Daniel, Tomas Hrbek, Maria Lúcia Góes de Araújo, Maurício Pinto de Almeida, Patricia Charvet-Almeida. (2008). A test of the utility of DNA barcoding in the radiation of the freshwater stingray genusPotamotrygon (Potamotrygonidae, Myliobatiformes). Genetics and Molecular Biology 31(1), 1-116.
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de Araújo, Maria, Lúcia Góes, Patricia Charvet-Almeida, Mauricio Pinto de Almeida and Henrique Pereira, Brazil. (2004). Conservation perspectives and management challenges for freshwater stingrays. Ichthyology at the Florida Museum of Natural History. 14, 10-12.
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Charvet-Almeida, Patricia, Maria Lúcia Góes de Araújo, Ricardo S. Rosa and Getúlio Rincón. (2002). Neotropical Freshwater Stingrays: diversity and conservation status. Ichthyology at the Florida Museum of Natural History. 14, 10-12.
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de Araújo, Maria, Lúcia Góes, Patricia Charvet-Almeida, Mauricio Pinto de Almeida and Henrique Pereira, Brazil. (2004). Conservation perspectives and management challenges for freshwater stingrays. Ichthyology at the Florida Museum of Natural History. 14, 10-12.
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Charvet-Almeida, Patricia, Maria Lúcia Góes de Araújo, Ricardo S. Rosa and Getúlio Rincón. (2002). Neotropical Freshwater Stingrays: diversity and conservation status. Ichthyology at the Florida Museum of Natural History. 14, 1-4.
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http://www.monsterfishkeepers.com/forums/showthread.php?t=172190
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http://www.raylady.com/Potamotrygon
Rays of the genus Potamotrygon, the river stingrays, are among the most extraordinary animals that are available to the freshwater aquarist. They are also among the most challenging to keep. While the likelihood of lasting success with these species is considerably greater than that of other rays (whether freshwater, marine or anything in between), a serious attempt to keep them should begin with a large measure of research, planning and patience. This involves:
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developing an understanding of how these highly specialized creatures have physically and physiologically adapted to their natural environment, and
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learning how to simulate this environment in a manner that meets the unique needs of each target species. With time and the necessary resources, river stingray keepers can construct a captive environment in which their animals can not only thrive, but in due course reproduce
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This piece discusses the morphology, reproduction and husbandry of the river stingrays. A previous piece discussed river stingray classification, distribution, ecology and conservation.
Morphology :
Species of the genusPotamotrygon are roughly average in size among the batoids (i.e., rays and skates), ranging from approximately 25 cm in disc width to 100 cm or more. The smallest species of the genus, P. scobina, reaches 20.5 to 27 cm in disc width; P. brachyura, the largest species of the genus, is known to reach a disc width of as much as 150 cm. Generally, the disc is slightly longer than it is wide. Note that disc width (or DW) and disc length (or DL) are standard measurements of stingrays. Total length is generally not used, given that a portion of the tail is often missing.
River stingrays (such as these P. motoro) require a deep bed of fine substrate. Photo by Karel Jakubec.
Small openings located on the top of the head (or spiracles) take the place of the mouth to draw water into the gill chambers while resting in the substrate. Photo by Guérin Nicolas.
The disc is formed mainly by greatly enlarged pectoral fins, which are fused to the head. Posteriorly it overlaps most of the pelvic fins. Although there are no dorsal or caudal fins, membranous skin folds (or finfolds) are present on both the upper and lower tail midlines. Compared to other members of their family, river stingrays have a tail (or caudal appendage) that is stout and short (typically shorter than disc length). The dorsal surfaces of the disc and tail are often quite spiny, being covered with denticles, thorns and tubercles.
Like other stingrays, fishes of this genus have venomous barbs (or caudal stings) located on the dorsal surface of their tails. Stings are hardened sections of dermal tissue with acute distal tips. They are continuously shed and replaced. An individual may bear up to four stings at a time. The sting is well developed in Potamotrygon. It is comprised of a spine, an integumentary sheath and venom glands. The spine, which gives the surface of the sting its stiffness, is composed of dentin. It contains several small, lateral serrations oriented toward the base. Special glands at the base produce venom that is carried along longitudinal grooves. When the spine is relaxed, it rests on a wedge-shaped piece of tissue that keeps it bathed in venom and mucus.
Most potamotrygonids bear highly distinctive markings. These include various spots, reticulations and ocelli under a grey, black or brown background coloration. Patterns of pigmentation are presumed to be species-specific.
Reproduction:
The Potamotrygonidae are similar to marine elasmobranchs in that they are characterized by late maturation, slow growth and low fecundity. The hydrologic cycle appears to exert an appreciable influence on the reproductive cycle of potamotrygonids. Studies suggest that the reproductive cycle includes a resting interval in at least some populations. As males reach spawning condition, they begin to seek out and chase females. Courtship can become violent, particularly if a chosen female is unreceptive to a male's advances; males will resort to biting and wrestling in order to assume a belly-to-belly position. Copulation transpires quickly, with the male inserting a clasper into the female's cloaca and releasing milt. If a successful fertilization occurs, the oviduct undergoes changes that allow it to function as a uterus.
Barbs on the caudle sting (as on this P. henlei) cause severe exit wounds that are highly susceptible to infection. Photo by Stan Shebs.
Just after birthing, females may be removed from a breeding group for a recovery period. Photo by Steven G. Johnson.
Sexing adult river stingrays is uncomplicated; as in other chondrichthyans, males (left) can be identified by the presence ofclaspers on their pelvic fins. Illustration by Rafael Ruivo.
All known freshwater stingrays employ a reproductive strategy called matrotrophic viviparity, wherewith uterine milk (or histotrophe) secreted by specialized uterine filaments (or trophonemata) nourishes the developing embryo during gestation. Gestation may take place either intermittently or throughout the year. The gestation period is variable among wild populations, lasting from 3-12 months; however, in captive populations, this stage generally lasts from 9-12 months. The birthing season can last from 3-4 months. Depending upon species, environmental conditions and the fitness of the mother, the number of offspring produced from each gestation is usually from 2-7, though litters as large as 15 have been reported. The pups are born live and are fully formed. In captivity, pups are best transferred immediately after birth to a dedicated system for solitary grow-out. Absorption of the yolk sac lasts up to 7 days. By this time, pups can be offered a variety of live and frozen foods. With proper nutrition, excellent water quality and ample living space, growth is rapid.
Husbandry:
One of the most important elements of a river stingray aquarium is the tank itself. Here, the best tank is a big tank. Some sources recommend minimum tank volumes of 90 gallons. Even so, one would do best to use a volume of 120 gallons or more. A "long" tank is preferable to a "tall" tank, as the inhabitants will make better use of horizontal (i.e., bottom) space than vertical space. Thus, even the smaller species need a minimum tank size of 48 in long x 30 in wide x 20 in tall per trio (i.e., one male and two females).
A river stingray aquarium should have a sandy substrate. The sand bed should be deep enough that the rays can completely bury themselves (i.e., such that only their eyes will be visible).
Sub-gravel heaters should never be used, as they may burn the animal. Conventional heaters (either submersible or non-submersible) should instead be used, albeit with a protective cover (such as a Hagen® heater guard).
As river stingrays are somewhat sensitive to suboptimal water conditions, they require highly effectual water treatment/filtration. Only an efficient biofilter (such as a trickle filter with high surface area media) should be relied upon to carry out biofiltration. Aggressive mechanical filtration (and frequent cleaning of sponges/pads) is advisable, as rays can be unusually messy eaters. Chemical filtration (particularly those types that remove metals) can be very useful in protecting the animals from contaminants and bioaccumulations.
Because river stingrays (such as this P. leopoldi) have relatively small mouths, some food items are best chopped down to a manageable size. Photo by Noel Weathers.
It is said that South American natives fear stingrays (such as this P. hystrix) more than they do the piranha. Photo by Jim Capaldi.
River stingrays are not especially territorial; provided that a large enough aquarium is used, they may be kept in groups or with certain other types of fish. They cohabitate especially well with surface dwelling fishes such as gars, which tend to stay out of their way. They should not be housed with aggressive or nippy fishes such as piranhas, puffers and certain cichlids. Caution should be exercised if they are to be housed with plecos (e.g.,Plecostomus sp.), which tend to irritate them by sucking at their disc. While fast and flighty little fishes such as tetras will generally be safe, river stingrays will eat any small fish that they can catch.
River stingrays benefit from a highly varied diet. They may be offered some combination of live items such as blackworms, earthworms (chopped), bloodworms, ghost shrimp and/or grass shrimp, with frozen items such as clam, mussel, silversides, krill and/or mysis shrimp.
Great care must be taken at all times when handling stingrays. It is far more preferable to capture them with a bucket or bowl than with a net. Never lose sight of these animals when handling or working around them. River stingray injuries are extremely painful and potentially life threatening. If a blood vessel is punctured, apply hard pressure directly to the wound to minimize any bleeding. The affected area should immediately be placed under water that is as hot as the victim can tolerate. After most of the pain has subsided, the wound can be cleaned by way of Betadine™ treatment followed by a rinse with disinfectant soap. Then--no matter how minor the injury appears to be--seek immediate medical attention. The examination should include radiology to locate any fragments of the sting that may be embedded in the wound. Return to the physician at the first sign of any infection.
Though river stingrays are not a major target of the ornamental fishery in the State of Amazonas, they are credited with helping to increase sales of the cardinal tetra (Paracheirodon axelrodi). Photo by Axel Rouvin.
A beautiful example of Potamotrygon sp. pearl. Photo by www.stingraywebshop.com.
Conclusion :
Stingrays of the genus Potamotrygon can be stunning aquarium animals. While they have a much better record of captive survivability than other batoids, their husbandry is hardly undemanding or uncomplicated. In actual fact, properly caring for these unusual creatures requires a considerable amount of preparation and resources. Moreover, one must take precautions to avoid serious injury while handling or working around them. Still, a healthy river stingray is an exceptionally fascinating, beautiful creature and is definitely worth the extra effort. The relative ease with which Potamotrygon spp. can be successfully bred and reared makes them even more appealing. Breeding river stingrays is not only an interesting (and potentially very lucrative) activity, but is also important for conservation efforts in that it reduces demand for wild-caught specimens. With growing commercial production, one could hope that more river stingray species and varieties may be available to home aquarists in the very near future.
Sources
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Kuba, Michael J., Ruth A. Byrne and Gordon M. Burghardt. (2010). A new method for studying problem solving and tool use in stingrays (Potamotrygon castexi). Animal Cognition, 13(3), 507-513.
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Toffoli, Daniel, Tomas Hrbek, Maria Lúcia Góes de Araújo, Maurício Pinto de Almeida, Patricia Charvet-Almeida. (2008). A test of the utility of DNA barcoding in the radiation of the freshwater stingray genusPotamotrygon (Potamotrygonidae, Myliobatiformes). Genetics and Molecular Biology 31(1), 1-116.
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de Araújo, Maria, Lúcia Góes, Patricia Charvet-Almeida, Mauricio Pinto de Almeida and Henrique Pereira, Brazil. (2004). Conservation perspectives and management challenges for freshwater stingrays. Ichthyology at the Florida Museum of Natural History. 14, 10-12.
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Charvet-Almeida, Patricia, Maria Lúcia Góes de Araújo, Ricardo S. Rosa and Getúlio Rincón. (2002). Neotropical Freshwater Stingrays: diversity and conservation status. Ichthyology at the Florida Museum of Natural History. 14, 10-12.
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de Araújo, Maria, Lúcia Góes, Patricia Charvet-Almeida, Mauricio Pinto de Almeida and Henrique Pereira, Brazil. (2004). Conservation perspectives and management challenges for freshwater stingrays. Ichthyology at the Florida Museum of Natural History. 14, 10-12.
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Charvet-Almeida, Patricia, Maria Lúcia Góes de Araújo, Ricardo S. Rosa and Getúlio Rincón. (2002). Neotropical Freshwater Stingrays: diversity and conservation status. Ichthyology at the Florida Museum of Natural History. 14, 1-4.
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