Potamotrygon motoro

Motoro Stingray, P01, P03, P044

Classification

Potamotrygonidae

Distribution

This species has a very wide natural range, taking in parts of Colombia, Peru, Bolivia, Brazil, Paraguay, Uruguay and Argentina. It’s been recorded in the main Amazon basin and numerous smaller rivers including the Rio Orinoco, Rio Uruguay, Rio Paraná and Rio Paraguay.

Habitat

Like other members of the genus it inhabits a variety of biotopes. These include sand banks, the shallows of major rivers and slow-moving tributaries with substrates of mud or sand. It also move into areas of flooded forest during the annual wet season and can later be found in terrestrial lakes and ponds formed by the receding flood waters.

Maximum Standard Length

Depending on locality, up to 36″/90cm across the disc. This would be exceptional in the aquarium, although many captive specimens top 24″/60cm.

Aquarium SizeTop ↑

An adult or pair will need a tank measuring at least 96″ x 36″ x 30″/240cm x 90cm x 75cm/1700 litres.

Maintenance

Strictly speaking, decor isn’t really necessary in a tank set up to house rays. You can however add some large chunks of bogwood, beech branches or smooth rocks if you wish. Ensure that any such furnishings are too heavy to be moved around or secured to the tank in some way, and that there is as much open swimming space as possible. Dim lighting is preferable, although once settled in most rays will also be active under brighter conditions. Plants that require rooting in the substrate will be eaten, but you could try species that can be attached to items of decor, such as Java fern or Anubias spp. Even these may not survive the rays’ attention though.

Choice of substrate is to a large extent down to personal preference. Some hobbyists use river sand which is an excellent choice, especially for juveniles. It’s arguably the closest representation of what the species encounters in nature. Others use standard aquarium gravel of varying grades. The third possiblity is simply to omit substrate completely. While this certainly allows for easier tank maintenance, it can make the set-up appear a little stark and unnatural. Rays also like to bury themselves in sand when stressed and are usually found in sandy or muddy habitats in nature so to deny them the option of cover seems rather cruel to us.

Filtration is one of the most important aspects of a ray set-up. A large and efficient biological filter is needed to cope with the amounts of biological waste produced by an active, predatory fish of this size. If possible choose a sump–type arrangement, as this allows most of the equipment to be located outside the tank. Not only will maintenance be far simpler but there is less chance of the rays destroying anything or worse, burning themselves by resting on a submerged heaterstat. Alternatively install one or more high qualty external canister filters. Aim to turn over the entire tank volume around four times per hour, and position the outflow from the filter so that there is a bit of a current running down the tank. If this is placed at the surface the resulting agitation will also provide the high oxygenation levels required by rays. If using a sump allowing the water to fall a couple of inches between sections will also oxygenate the water sufficiently.

Tank maintenance must be similarly stringent. Weekly water changes of around 50% should be considered standard for a ray tank. Although it’s been proven that they can withstand fairly high levels of nitrates, build-ups of other nitrogenous wastes are not tolerated.

You’ll also need a very tightly-fitting cover as rays will sometimes (particularly when feeding)come right up to the water surface and may breach as much as half their body. Attempting to retrieve a couple of feet of venomous fish from the floor would likely be a memorable experience for all the wrong reasons!

Water Conditions

Temperature: 75 – 80°F/24 – 26°C

pH: 6.0 – 7.5

Hardness: 1 – 12°H

Diet

Wild rays feed chiefly on other fish and aquatic invertebrates, including worms and crustaceans. They’re active fish with a high metabolic rate and as such will need feeding at least twice a day. They’re also notoriously big eaters and it’s going to cost you a considerable amount of money to keep even a single specimen in good health. In general an exclusively meaty diet is preferable, although some will also learn to accept dried foods.

Juveniles (often sold simply as “teacup” rays regardless of species) relish live or frozen bloodworm, Tubifex, Artemia, krill and suchlike. Adults should be fed correspondingly larger foods, such as whole mussels, cockles, prawns, squid, whitebait (or other fresh fish) and earthworms. A varied diet is needed to keep the fish in top condition.

They’re often a little reluctant to feed when initially imported, and usually arrive in quite a skinny state. It’s very important to get them feeding as quickly as possible due to their metabolic requirements. Frozen foods may be refused at first, so bulk them up on live foods until they have enough weight to be safely weaned onto dead alternatives. Live bloodworm or earthworms (the latter can be chopped for small specimens) are generally considered to be among the best foods for conditioning newly imported rays.

Rays should not be fed the meat of mammals such as beef heart or chicken. Some of the lipids contained in these meats cannot be properly metabolised by the fish, and can cause excess deposits of fat and even organ degeneration. Similarly there is no benefit in the use of ‘feeder’ fish such as livebearers or small goldfish. Risks involved with these include the possible introduction of disease or parasites.

Behaviour and CompatibilityTop ↑

Rays are among the top predators in the ecosystems they populate in nature and are unsafe to keep with most other species. Conversely they also seem to prefer a quiet life and will often fail to thrive when kept alongside very aggressive or territorial companions. The best tankmates are large enough not to be eaten, peaceful and ideally occupy the upper parts of the tank. Some cichlids, such as Heros or Geophagus species work well, as do many bigger characins and cyprinids. Plenty of enthusiasts keep Asian or South American arowana with their rays, and in a roomy tank this can be a very striking combination. Other suitable options include Cichla or Datnioides spp., and in a tank with a very large footprint other bottom dwellers such as bichirs or Pimelodid catfish (Brachyplatystoma tigrinum is a popular albeit expensive choice). Obviously all these species grow to an impressive size so tank volume should be the primary consideration before any choices are made.

Other commonly chosen ray companions are recommended only reservedly. For example some ray keepers suggest that discus make good tankmates, while others have reported that smaller fish such as these are picked off by their rays at night. Similarly while many of the larger Loricariids would certainly make attractive additions, there are many documented instances of these suckermouth catfish attaching themsleves to the rays’ delicate discs and causing injury. Many are also highly territorial. The common Hypostomus plecs are safe by all accounts, though.

One potential pitfall when keeping more than one species of Potamotrygon together is that most will hybridise freely. Given the potentially precarious conservation status of some species this really should be avoided wherever possible. More recently there has been a somewhat alarming rise in popularity for the so-called “bat” or “batman” rays. These are the result of a genetic defect and have certain parts of the disc missing, usually at the rear edge and the area around the head. When viewed from above these do indeed resemble the sign of the fictional superhero.

Sexual Dimorphism

Rays are easy to sex. Males have a pair sexual appendages known as “claspers”, one on each pelvic fin. These are used to inseminate the female when mating and are clearly visible, appearing as finger-like extensions extending backwards from the inside of the fin. In juvenile males they’re much smaller, but can still be seen if you look closely.

Reproduction

Potamotrygon species utilise a breeding strategy known as matrotrophic viviparity. The young fish (often referred to as “pups”) develop inside the mother and are born live and fully-formed. Inside the uterus of the female specialised filaments or villi develop. These secrete a milky substance known as histotrophe, from which the growing pups derive their nourishment once their yolk sacs have been used up. Litter size usually varies between 1-8 and gestation can take between months. Interestingly this period seems to be significantly shorter with rays breeding in aquaria, possibly due to the abundance of food they receive compared to wild fish. Successful captive breeding of several species has occured regularly in recent years, .

Rays can be picky when it comes to selecting a mate. Simply buying a pair of fish and putting them together will not guarantee a successful pairing. The ideal way to obtain a pair is to buy a group of juveniles, housing them in a huge tank and allowing them to select their own partners. However this is probably beyond the means of most hobbyists. It can also take several years for rays to become sexually mature, so a good degree of patience is required when starting with young fish.

If selecting a single pair try to choose similarly patterned specimens, and a bigger female than male. The comparative size of the pair is particularly important as courtship can be a somewhat violent affair, particularly if the female is unwilling to spawn. It’s therefore essential that the she is large enough to defend herself. When in spawning condition the male will chase her incessantly, often biting her on the body and around the edge of the disc. He does so as in order to mate the pair must position themselves so that their bellies are facing each other. The male uses his mouth to take hold of the female and slide underneath her. If this behaviour continues for too long with no successful mating event real physical damage can occur. Keep a close eye on developments if your rays begin to show signs of mating behaviour, and have the facilities on hand to separate them if need be. You can try reintroducing them a few days later if necessary.

The spawning act itself is quite brief, lasting only a few seconds. Fertilisation occurs internally, the male inserting one of his claspers into the cloaca of the female before releasing his milt. Following a successful mating event the male should stop harassing his partner.

Gestation in captive rays generally takes between 9-12 weeks. During the latter stages the developing young can sometimes be seen as a visible (sometimes moving!) lump rising from the posterior end of the female’s back, although in well-fed specimens this can be tricky to spot. It’s essential to feed the female in sufficient quantities during this period as she will expend a lot of energy providing for her pups, and her appetite will increase significantly. Pregnant females are generally safe to be left in situ, although once the pups are born they’re best removed to avoid predation by other tankmates. If the rays are being maintained alone the adults won’t usually harm them although the chance is always there. Most breeders prefer to remove the pups for the sake of safety. The water in the tank is usually cloudy following a birth (this is thought to be caused by fluids released as the pups leave the body of the female), and a large water change is recommended once they’ve have been removed.

The pups usually have a small yolk sac attached at birth, and they will feed from this for anything up to a week. After the sac has been absorbed they should be offered high quality live and frozen foods several times a day. Some may initially refuse dead foods, but they can usually be weaned quite easily by mixing in a little live food when feeding. Growth is quite rapid with a stringent regime of water changes and the correct amount of food.

If you’re lucky enough to own a pair of rays that are breeding regularly, give the female a break from the male after every 2-3 litters. Females use up a lot of energy in producing young and it may drastically shorten their lifespan if they’re forced to mate continuously.

NotesTop ↑

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.