For oddball fishes, especially for a designated "species" tank, it's hard to beat the freshwater stingrays. If they're allowed in your state (not in all), and you can meet their requirements (a BIG well-filtered system), and keep your hands out (yes, they're venomous), these cartilaginous fishes may be for you.
Let's place the group systematically, do a once over on how to pick out a good one, and tell you how to keep it alive.
Classification: Taxonomy, Relation With Other Groups
Harking back to the higher taxonomy of fishes, you'll recall that that they make up three of the seven living Classes of vertebrates (the others being the Amphibians, Reptiles, Birds and Mammals): For a more complete scheme see Nelson, 1994.
Class Agnatha; the Jawless Fishes (Lampreys and Hagfishes).
Class Chondrichthyes; the Cartilaginous Fishes (Sharks, Rays, Skates, Chimaeras); so named for their flexible skeletons.
Class Osteichthyes; the Bony Fishes (Lungfishes, Gars, Cichlids, Eels, Catfishes... and all the rest of "true" bony fishes).
Within the Cartilaginous Fishes are the weird-looking Chimaeras, aka Elephant or Ghostfishes. These odd, obscure marines are separable from the rest of the cartilaginous fishes on the basis of having their upper jaw attached to their cranium. That's right, sharks' jaws are not attached to their skulls via bone (and you wondered how they could open their mouths so wide!). So the Class Chondrichthyes is subdivided into the:
Can you stand a little more of this "splitting" up? If you've ever wondered how to distinguish the "flattened-sharks" (like Angelsharks, Hornsharks), from "real" ray Elasmobranchs, look to the placement of their numerous gill openings relative to their pectoral fin origins. For Sharks, they finish above, for Rays and Skates they're below.
For those keeping score, the provisional classification of Nelson (1994) further cuts the Batoids into four suborders, 12 families, 62 genera and about 456 species.
But, let's focus on the group at hand (sheesh!). The river or freshwater stingrays belong to the family Potamotrygonidae (or subfamily Potamotrygoninae of the family Dasyatidae in some folks minds). There are three genera (Paratrygon, Plesiotrygon <both monotypic> and Potamotrygon) of about 18 species.
All Potamotrygonids (recognize the "potamo" part like hippopotamus, "river horse"?) are freshwater, of South American rivers dumping into the Atlantic.
Small species to about eight inches; modest size species to about 16 inches in disk diameter, large ones to thrice that, plus their different tail lengths.
Species of Interest to Aquarists:
Selection: General to Specific
Time: That is, how long has the ray been at where you're seeing it? Don't purchase new imports; most that are going to die do so within a week or two. Wait a while.
Markings: Check especially around the mouth and light colored underside for bloody spots; signs of damage and infection. Don't buy a bruised, bloodied specimen.
Feeding: Is the animal? Have your dealer show you. Don't buy a skinny, non-eating specimen.
Coloration: Just a note to not rely too much on this characteristic; Potamotrygonids are aquatic chameleons, and will change markings and intensity arbitrarily.
Freshwater rays are collected for the trade via hook and line and seine-netting. They are also shot with bows and arrows, and gigged with hand spears to gather their young (released on the death of gestating females) and to get rid of them.
In the wild, Potamotrygonids are found in open streams and meandering rivers of a wide range of water types with sandy or muddy bottom and submerged vegetation. In captivity clear water of appropriate make-up and little decor suits them well. Most species need hundreds of gallons of uncrowded space and all require a soft substrate.
Softer (less than 10 DH), more acidic (pH 7.0 or lower) is encouraged (IF you are confident in going about such modification). Starting with reverse osmosis water is the simplest means of achieving these ends; otherwise you may want to look into water preparations offered commercially.
Temperature should be high (78 F. plus) and consistent. You are advised to place resistant heaters outside the main display tank (maybe in a filter/sump) to prevent breakage and damage, burning and electrical, to your aquatic charge.
Minimal lighting suits rays; I would only have it bright enough to suit you for viewing unless live plants are involved.
For large carnivorous fishes, the freshwater stingrays have stringent filtration needs; they do not appreciate any measurable level of ammonia, nitrite, and little nitrate (less than 10ppm). Frequent, massive water changes (e.g. 25% a week) are strongly suggested; a good time to vacuum the gravel.
Due to digging and burrowing habits undergravel filters should not be relied upon to provide regular, complete biological conversion. Instead an oversized external unit (wet-dry, canister, hang-on-back) with good, thorough flow-rate (two or more turns per hour) can be employed. Redundancy (using more than one filter/circulation system) is encouraged.
Potamotrygonids are definitely mal-affected by the presence of metabolites. You want to NEVER have detectable ammonia or nitrite and as small as practical nitrate (under 10 ppm). See elsewhere on WWM re how to achieve this.
These rays, like all cartilaginous fishes need SPACE; to maneuver, swim and grow. They lack gas bladders and so once in motion, stay that way. The practical consequences are obvious; either you must provide a rounded system or one that is very large. I would not keep the smallest specimen in anything less than one hundred gallons.
To preclude injury, hard decor should be kept to a minimum. If you must use rock, make sure and place/bind it together to prevent collapse from undermining. Finer, rounder sand (not silica) is called for to prevent abrasion and secondary infection.
Other Biology of Interest
For the sake of completeness we'll state that some of the family of whiptail stingrays, Dasyatidae are also known to be obligate and facultative freshwater inhabitants in Africa and Asia. None are currently utilized in the aquarium interest as far as I'm aware.
How do they manage to breath while sitting on or in the bottom? The openings behind the eyes, called spiracles are the intake ports, with excurrent water pumped out the gills or mouth.
One semi-last remark regarding all these rays stinging capacity; all are venomous and unless physically altered, capable of rendering a painful wound and toxic venom. Do not place your hands in harms way. If you get "stuck" seek medical attention, immediately.
Of the species I've seen, none have been overtly agonistic to their own kind during holding periods (they're generally kept together in large, flat tanks). Though I've seen smaller individuals kept as "pairs", I've read accounts of chewing and biting, and due to size and need for space I encourage home hobbyists to keep theirs one to a tank.
If I haven't been emphatic enough regarding the dangers of handling these fishes, allow me to add more "fat to the fire"; they are easily damaged. Though their skin is tough (before WWII shark skin was used for shoe "leather" and sandpaper!), it is covered with a slimy layer that functions the same as for "advanced" bony fishes (osmotic balance, barrier against infection...). Further, cartilaginous fishes lack much in the way of ribs et al. to support their internal organs (remember Flipper hemorrhaging those nasty sharks by ramming them?). The upshot of all this is that rays should not be netted at all; instead they can and should be directed and scooped into multi-layered (or extra-thick) bags. On arrival, much of the shipping water may be replaced with suitable (temp. and pH) system water and the animal poured into its new home.
Does the thought of swimming with piranhas give you the "willies"? Natives of the areas where freshwater stingrays are found fear them much more than "toothy tetras" (but less than Candiru catfishes).
For aquariums; make no mistake about it, Potamotrygonids will capture and swallow any fish small/slow enough to ingest. I've witnessed them in public and private aquariums around the world in the company of discus, other South American cichlids, and other eclectic mixes; most without mishap unless mis-sized. The rays are not above sticking tankmates (or aquarists) with their venomous tail barbs, but most seem to be sentient of this.
Reproduction, Sexual Differentiation/Growing Your Own:
Like sharks, freshwater rays have internal fertilization and the specialized organs (claspers; modified pelvic fins in the males) for genetic transmission. The pointy claspers make sexing rays a snap; males have them, females don't.
A few species of these ovovivaparous rays have "bred" in captivity, releasing live miniatures of their parentage.
Feeding/Foods/Nutrition: Types, Frequency, Amount, Wastes
Almost all specimens accept meaty foods readily. On being first imported and tanked, and occasionally for water quality or no discernible reason whatsoever, a ray will go on a feeding strike. Don't be unduly worried by this. If the animal is well fleshed it can, and will stay off-food for several days without harm. When/where in doubt, check water chemistry, effect a large water change, and/or shift to another meaty foodstuff.
Worms (blood, Tubificid, earth), shrimp (krill, "cocktail" w/o the sauce) clams, cut beef heart, fish fillet, live fish of many kinds have all been used intentionally and otherwise. These are intelligent animals that train (you) easily to take foods in a regular manner; however, no matter how "friendly", do not hand feed stingrays!
River stingrays are susceptible to ich and other tropical freshwater maladies; what's more they frequently harbor monogenic fluke and internal worm problems on arrival (all specimens are wild collected). As with all aquarium set-ups, these difficulties are best dealt with through quarantine, prevention and providing proper habitat.
Treating for infectious and parasitic disease is problematical with freshwater rays. Though many other authors encourage the use of copper-based, organophosphate (e.g. Dylox), and other commercial nostrums, I warrant against this. These fishes are quite sensitive to poisoning by such treatments, and frequently are killed by the "cure".
Some writers state good success using salt(s) (sodium chloride, artificial sea salt) prophylactically and for medicating; others, myself included, do not specifically advise salt use.
Ancient? Yes. Primitively backward? No. Rays and related cartilaginous fishes are well wired for their environments. Making the commitment to house one or more freshwater stingrays is serious. They need real room, good, consistent water quality, and no food competitors.
Be aware of all this should you take the plunge with the Potamotrygonids; and be wary of where you stick your hands.
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Fenner, Bob 1992. The Candiru- Vampire fish. FAMA 12/92.
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aquarium, pts. 1 & 2. FAMA 2,3/91.