Motoro stingray or ocellate river stingray (Lat. Potamotrygon motoro) is one of the most famous and popular freshwater rays.
We should mention that despite the motoro stingray popularity in aquarium husbandry, there is still no quite precise classification of this family representatives.
From time to time, some new kinds are discovered, which were not described before.
Habitat in the wild
This kind is quite widespread in South America. The fish is also encountered in Columbia, Peru, Bolivia, Brazil, Paraguay, and Argentina. Motoro stingray inhabits both Amazon River and its tributaries: Orinoco, Rio-Branko, Parana, Paraguay rivers.
Like other representatives of the kind, the fish is encountered in different biotopes. Mainly these are sandbanks of large rivers and their tributaries where the substrate consists of mud and sand.
During the rainy season, this stingray moves to flooded forests and during the dry season – to the lakes that were formed earlier.
Rays are relatives of sharks and sawfishes, which have a skeleton that differs from the one that ordinary fishes have, since it doesn’t have bones, and it completely consists of cartilageous tissue.
The scientific name of this kind is an ocellate river stingray, which means that the ray can sting. Really, the ray has a poisonous spine on its tail (actually, it used to be a scale once). The ray uses this spine to protect itself. The poison is produced by the glands located near the bottom of the spine.
Contrary to common belief, rays don’t attack humans with their spines. You should step on one of them or disturb somehow to make the fish sting you.
Motoro stingray loses its spine occasionally (every 6-12 months), and you can find it on the bottom of the tank. This is a normal situation, and it shouldn’t bother you.
Another feature of freshwater rays is the ampullae of Lorenzini. This is a special tube or channel located on the fish head (around its eyes and nostrils).
Using these, cartilaginous fishes detect electric fields, and they help them to navigate according to the terrestrial magnetic field.
In the wild, Motoro stingray is up to 50 cm large in diameter and up to 1 m long. It weighs up to 35 kilos. When keeping it in the tank, the fish, of course, doesn’t grow that large.
The ray is disk-shaped, and the disk is almost round. The fish eyes are updrawn above its back. The latter is usually beige or brown colored with numerous marigold yellow spots and dark circles.
The ray abdomen is white. The color and location of the spots may significantly vary from one fish to another. In the Amazon river basin, there were defined three basic coloring types of stingrays, but each of them includes several subtypes.
Difficulties in keeping
P. motoro is one of the most popular representatives of the kind among the aquarists. Many of them get very surprised when they find out that some rays dwell in freshwaters.
Freshwater rays are rather clever, and they can interact with humans. You can even train them to hand-feed. Nevertheless, this fish is not for everyone. It requires large tanks, perfect conditions, and a special diet.
But for those who are ready to put efforts to keep a ray in a tank, the fish will become a unique and the most loved pet. In the past, most rays on sale were caught in the wild. It means that they were quite stressed as well as brought some parasites and illnesses with them. Most rays sold nowadays were bred in captivity.
This fish is dangerous. Most locals in the countries where rays dwell are afraid of this fish more than of other life-threatening species such as piranha. For instance, in Columbia, annually, more than 2000 cases of injuries and even accidental deaths as a result of ray attacks are registered.
The poisonous spine is located on top of the ray’s caudal fin, where you can clearly see it. It is covered with a thin outer cover that serves to protect the ray itself from its poisonous glands.
The internal spine surface has a row of reverted spines. They help to tear the cover when the ray uses its spine and to widen any wound it makes. These spines direction also allows using them as a fly-hook, which makes it harder to remove them from the wound.
Although toxic properties of poisons of various ray kinds may differ, their composition, in general, is alike. The poison is based on proteins and has a cocktail of chemicals aimed to cause both severe pain and fast tissue degeneration (necrosis).
If a ray has stung you, be prepared for severe pain on the bite site and headache, nausea, and diarrhea. You should visit a doctor regardless of how strong the symptoms are.
It goes without saying that when keeping rays in a tank, you should be extremely careful. However, the chances of being stung are very low if you behave respectfully. As a rule, these are not aggressive fishes, and they use their spine only as a protection measure.
Actually, rays become very domestic; they learn to recognize their owner and come to the water surface to ask for food. Most injuries happen when reckless owners try to ‘stroke’ the fish or catch it with a scoop-net.
You should never use it for this purpose. It’s better to take some solid container instead.
Keeping in a tank
Freshwater rays are very sensitive to ammonia, nitrites, and nitrates content in water, that’s why it is important to have an understanding of a nitrogen cycle and keep the tank water crystal clear.
The latter is quite a challenging task since rays produce a large amount of ammonia. Large-sized tanks, efficient biological filtration, and regular water renews – is the only way to keep the proper tank and water conditions.
Most freshwater rays can be kept in a tank at рН from 6,8 to 7,6, alcalinity from 1° to 4° (from 18 to 70 ppm) and water temperature from 24 to 26°C. Ammonia and nitrites level should be zero, and nitrates level should be lower than 10 10 ppm.
In terms of the tank size for rays, the following approach works best – the larger, the better. The walls height is not a crucial parameter, but the tank should be from 180 to 220 cm long and from 60 to 90 cm wide to keep a ray in it for a long time.
A tank from 350 to 500 liters capacity will do to keep young rays in it, but for a long-term keeping of adult species, you will need a tank not less than 1000 liters large. Small-grained sand can be used as the tank substrate.
What to use as a tank substrated to a large extent, depends on your own preferences. Some aquarists use river sand that is a rather good option in this case, especially for young rays.
Others use standard aquarium gravel of various brands. Another option is not to use the substrate at all. This simplifies the tank maintenance but makes it look a bit severe and unnatural. Besides, rays like digging into the sand when they are stressed out, and usually in the wild, they dwell in waters with sandy or muddy bottom.
Thus, it seems quite cruel to let them live without having a chance to find shelter in the tank.
As for the tank decorations, they should be smooth without any sharp edges. Actually, tank decorations are not really necessary in a tank with rays. However, you can put some large snags or smooth stones, if you like.
Leave as much as possible free space for the fish to swim, to let them move and dig into the sand. Heaters should have protection around them or put them out of the tank to ensure that the rays won’t get burnt.
There should be some dim light with a 12-hours cycle (day/night). The fish will eat tank plants that require rooting in the substrate, but you can try some plants that can be fixed to the decorations such as java moss or Anubias spp.
However, even these may not survive the fish’s attention.
Freshwater rays are flesh-eaters that feed mainly with fish and crustaceans in the wild. These are active fishes with high metabolism level, and that’s why you should feed them at least twice a day.
They are also known as gluttonous ones, thus the food for them will be quite costly for you. In general, the fish diest should be natural, though some may eat artificial food.
Young species eat live or frozen blood worm, tubifex, brine shrimps, prawns, etc., while adult ones should be fed with the food of larger size. It can be whole mussels, shellfishes, prawns, calamaries or fish juveniles (or some other fresh fish), and earthworms. Diverse diest is a must to keep the fish in its best condition.
After you buy a Motoro stingray, it often eats quite unwillingly, and it’s not in the best shape as well. It must start eating as fast as possible because of its fast metabolism. Blood and earthworms (the latter can be cut in small pieces) are considered the best food to help the ray adapt to new tank conditions.
They shouldn’t be fed with flesh from mammals such as ox heart or chicken. The fish can’t properly digest some of the lipids that this meat contains, and this may cause excessive lipopexia and even some organs necrosis.
Also, there is no use it feeding it with some edible fish, such as livebearers or small goldfish. Such a diet may be the reason for the possible spread of illnesses and parasites.
Rays spend most of their time on the bottom. Their eyes and branchial apertures are on top of the body, which allows the fish to spend time under the sand waiting for food. They have perfect eyesight, and they jump out of the sand to catch their prey.
Rays are one of the main predators in ecosystems where they dwell in the wild, and it’s not safe to keep them together with most of the other fish kinds in a tank. Their tank mates should be large enough not to become food for them, but peaceful enough as well not to bite rays or steal their food.
Thus, fish that swims in upper and medium water layers will be the best choice in this case. I’d advise avoiding armored catfishes as rays tank mates, since there are a lot of documented reports saying that they get stuck to rays and damage their skin.
Female species are larger than males, and they have two uteruses, which means that they can have offspring from two different males simultaneously.
Males have modified fins that they use to fertilize the females.
Many aquarists succeeded in freshwater rays breeding, but this process takes time, you will need a large tank and dedication. Ocellate river stingray reproduces by ovoviviparity.
The female carries from 3 to 21 species, and they born completely ready to live on their own. The gestation period lasts from 9 to 12 weeks. An interesting fact is that this period is sufficiently shorter for rays that breed in tanks.
Possibly, due to the plentiful supply of food they have if compared to the fishes in the wild.
Rays may be quite picky when selecting their mate. So, just by buying a fish couple and putting them together in one tank, you won’t have any guarantee of successful breeding.
A perfect way of creating a fish couple is to get a group of juveniles, put them in a huge tank, and let them select their mates. However, this is quite a costly process, and most aquarists can’t afford it. Besides, it may take several years for the rays to become reproductive.
We should also mention that males of this kind are considered as the most furibund during the spawning period, and the females may not be ready for this.
If you keep a couple or a group of rays, be very attentive to their behavior during this period and be ready to separate them if necessary.