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Freshwater stingray history

freshwater stingray history

Freshwater stingray history

All stingrays both fresh and saltwater belong to the class Chondrichthyes. This includes all 850 species of sharks, rays, skates and chimaeras. Based on fossil remains it is known that this class of fish have been around since the Palaeozoic period (250 million years ago approx.), however it is believed that the adaption of the flattered (ray like) body only occurred in the Jurassic period some 100 million years ago. Freshwater stingray history is certainly an interesting topic for the fish keeper of today.

The development of the body shape and the loss of a swim bladder enabled these early rays to hide and lay practically invisible to a predator and to its pray. Rays have developed powerful jaws designed for crushing shells and molluscs which is a primary food source for these animals. Fossil remains prove that little has changed in the anatomy of stingrays over the last 50 million years.


Chondrichthyes (Cartilaginous Fish)


Elasmobranchii (Sharks, rays and skates)


Rajiformes (rays and skates)


Dasyatidae (stingrays)


Potamotrygonidae (river rays)



Freshwater stingray history – How did Stingrays adapt to Freshwater?

There are many theories on how the south american stingray adapted to freshwater to create a new family Potamotrygonidae (river rays).  A longstanding hypothesis suggests that before the Andes mountains were created about 80 million years ago, marine rays from the Pacific lived in and around the coastal areas near the Amazon and became tolerant to brackish water. Over time the Amazon reversed its flow as the Andes were formed.  The reversal of the river meant that stingrays became trapped in freshwater.

Another proposal put forwarded based upon DNA sequencing suggests that stingrays evolved far more recently. The theory suggests that marine rays from the Caribbean infiltrated into rivers draining into the Atlantic Ocean. If this theory is correct, then it suggests that the adaptation of the freshwater stingray could have been as recent as 20 million years ago.

It is not possible to know which theory of freshwater evolution is correct.  However it is extremely likely that the did develop from marine rays. Ray fossils prove that freshwater and marine rays have changed little in the last 50 million years.

Freshwater stingray history – The adaptation to freshwater

Sharks and rays are inherently marine animals simply based on their physiology. Seawater is usually saltier than the blood of most fishes, but instead of actively pumping ions and other solutes out of their bodies like marine bony fishes, elasmobranchs simply match their internal osmotic concentrations to that of their external environment. They do this by maintaining concentrations of organic solutes (namely urea and an enzyme called trimethylamine oxide or TMAO) within their bodies. Although urea is toxic to fish, the TMAO counteracts the protein-de-stabilizing effects of urea. Excess monovalent ions that they ingest, namely sodium and chloride, are eliminated from the body via specialized rectal glands.

The Amazon River

Since Amazonian rays live in freshwater, they have exactly the opposite problem of their marine cousins: instead of losing water to their external environment, they have to worry about gaining it, since their internal osmotic concentrations are higher than that of the water in which they live. One result of this situation is that freshwater stingrays no longer have any need for rectal glands, and these structures are now vestigial (greatly reduced in size and no longer capable of secreting salt).

They have also lost the ability to retain urea, allowing them to sever their ties with the ocean and evolve into exclusively freshwater organisms.

Freshwater stingray history – Taxonomy

The family of Potamotrygonidae belonging to the genus Potamtrygon consist of about 20 species of rays. In 1986 Ricardo Rosa re-assessed the taxonomy of freshwater stingrays.  This can be tricky. Most rays are highly polymorphic. This means that variations is patterns, physical structure and tail length are highly common.

The word Morph describes different looking specimens in the same species. In only a few cases the ray has a constant shape and pattern. A good example of a non-polymorphic species is the Tiger Ray which has a consistent colour, pattern and shape between specimens. On the other hand a great example of a highly polymorphic species is the Motoro stingray. This ray has many pattern variants and shapes within its species.

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9 Fascinating Facts about the Freshwater Stingray

freshwater stingrays facts

Freshwater stingrays facts

So to kick off – first the biology lesson.  Did you know that stingrays are members of the Elasmobranch class?  Well of course you did right? Just in case you didn’t..its is a group the includes both freshwater and marine stingrays, sharks and sawfish.  So now you know that, here are my 9 fascinating freshwater stingrays facts to help you understand your fishy friend a little more and marvel at how unique they are

Dermal denticles as viewed through an electron microscope

Freshwater Stingrays Facts # 1

Stingrays and their cousins the shark are covered with placoid scales Also called dermal denticles.  

Its the denticles that give the texture of smooth sandpaper.  

Placoid scales can’t actually grow in size, instead more scales are added as the fish increases in size.


Freshwater Stingrays Facts # 2

The barb is actually a modified denticle. The stingray has the ability to grow and shed its barb usually about twice a year. And its not uncommon for a ray to be seen with two barbs whilst the new one is growing. Usually the larger barb that is closer to the tail tip is the one that will fall off (Don’t forget  barbs can be found in your aquarium so please take care when removing them as they can still sting you even when they are no longer attached)

Freshwater Stingray Top Tip: If the barb is still black you can be sure that there is still a poisonous coating that you really don’t want to touch.

Freshwater Stingrays Facts #3

All Elasmobranchs shed their teeth continually thoroughout their life span.  Just take a look at the bottom of your tank if you don’t believe me. No, its not sand! Its teeth!

Freshwater Stingrays Facts # 4

Freshwater stingrays don’t have calcified bones instead their body is composed of cartilage – this enables the ray to maximise its movement. So it’s important to remember that great care should be taken when removing a ray from the water environment.

Freshwater Stingray Top Tip:  Moving rays should be done quickly with minimum stress to the fish.  The weight of the body can cause the cartilage to compress putting strain on the vital organs.

Freshwater Stingrays Facts # 5

Freshwater stingrays have adapted to living partially buried in the substrate. They rays have an accessory respiratory opening, the Spiracle. This is believed to be an adapted gill slit which has over time migrated to the dorsal side of the stingray. When the stingray is resting on the bottom and the under belly gills cannot allow water to pass through them then these spiricals open which allows the ray to breath whilst resting on the bottom.

Freshwater Stingrays Facts # 6

Stingrays have good eyesight which is a bonus for any ambush predator. Inside the eye there is a structure called the Operculum pupillare which controls the amount of light entering the eye. In dim light this will retract allowing greater light in and better vision at night.  So this is my favourite freshwater stingray fact.  Yes, they are watching you

Freshwater Stingrays Facts # 7

The digestive system of stingrays have evolved an unusual structure in the intestine. There are many layers or folds that have been added to the lining of the intestine. As food passes through the digestive tract it passes through this spiral valve. This explains why stingray waste has a ribbon like twist to it. This adaptation is also found in sharks and other primitive fish.

Freshwater Stingrays Facts # 8

The barb of the freshwater stingray is located midway along the tail of the ray. It is a defence weapon. The barb is only used a defence strategy and never used to hunt with. The ray is able to swipe its tail and reach its furthest point of its body in a very quick response. Want to know more about the venom and the spine read my other blog on the topic

Freshwater Stingrays Facts # 9

Potamotrygon is a genus of freshwater stingrays native to the rivers of South America. There are currently 27 recognized species in this genus.  So plenty of stingrays for us to put on our wish list

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How dangerous are stingrays?

How dangerous are stingrays?

How dangerous are stingrays?

It is a common opinion that Stingrays use their barb to kill their prey or to harm humans. You can hardly mention stingrays without the mention of Steve Irwin. But just how dangerous are stingrays? Mr Irwin’s death was only the third known stingray death in Australian waters, and wildlife experts agree “the normally passive creatures only sting in defence, striking with a bayonet-like barb when they feel threatened”

So stingrays really are peaceful animals and will actually try to avoid human interaction at all costs.  Stingray barbs are a defence. A protective weapon against other predators. Stingrays are the ‘pussycats of the seas’.  A name probably given due to their docile behaviour. That said, it is also reported that the natives of South America fear the stingray more than they do the piranha.

How dangerous are stingrays – captive vs wild

The majority of people stung by a stingray are unfortunate enough to have waded into the water and stepped on a ray.

Captive rays are generally docile and will tolerate the keepers hand being in the aquarium (such as cleaning).  Take extra care to make sure that the ray does not feel trapped or scared.

In either situation, if the stingray feels threatened it can lead to defensive behaviour which may result in a tail swipe.

How dangerous are stingrays – The Spine and Venom

The Stingray Spine is its main defensive weapon and it is therefore extremely rare for a fish to use it unprovoked. The barb is a modified denticle, (same as the rays skin). The location of the barb or spine is located about 2/3 along the tail or caudal appendage of the animal. The barb usually lies flush along the topside of the tail and points in the direction of the tail tip. In rare cases the barb has changed angle and points towards the head. This adaptation has no scientific explanation.

There is a wedge-shaped area of tissue located in close contact with the barb where the spine lays flat against the dorsal surface of the ray’s tail. It is covered in a cocktail of venom and mucus.

There is a lot of confusion referring to the correct terms fish keepers use when speaking about the spine/sting and barb. Here are the correct terms

The Sting:   Refers to the entire structure (the spine its sheath and venom glands)

The Spine:   Refers to the ridged surface of the sting made from dentin

The Barb:   Refers to the backward facing serrations along the lateral part of the spine

Stingrays are unable to raise and lower their sting however in response to a threat, but they can swipe their powerful tail to strike a predator.

How dangerous are stingrays – The Venom

All species of stingray have very similar venoms, yet some species are rather more potent than others. They all contain the same enzymes:  serotonin, 5-nucleotidase and phosphodiesterase. The last two enzymes are responsible for the necrosis and tissue breakdown seen in a stingray sting.

The serotonin is the enzyme which causes the excruciating pain in the area of the wound. Therefore, if left untreated the actions of these enzymes will continue to cause pain and tissue breakdown.

The stingray’s infamous tail spines have two components: the sharp inner barb used for piercing, and a thin sheath surrounding it that contains the venom. The barb pierces the venom sac along with the victim’s skin as the spine is deployed. As a result, the poisonous slime is introduced into the wound.

The barb is extremely sharp.  It can penetrate bone. Operating under the same principle as an arrowhead it slides into flesh fairly easily, but the serrated edges make it very difficult and painful to extract. The tail is very flexible and can bend pretty much any direction within a split second, inflicting serious damage.

How dangerous are stingrays – First Aid for Stings

The toxins or venom from the stingray’s spines are protein molecules.  You can “denature” proteins. To do this, the most effective treatment is to immerse the wound in as hot as water as the patient can stand.  Expect this to take a couple of hours.  You’ll know when the treatment has worked because it wont hurt when you take the wound out of the water.

How hot should the water be?  Don’t use boiling water.  That will just take your skin off. Take advice from the patient or trust your own instinct if you are the patient.  In our experience, the hotter the water, the less the sting hurts.  Continuously top up the hot water to ease the pain.  You/they will feel so much relief from keeping the wound in hot water.  Now, it’s just a matter of sitting with their hand in a bowl for a couple of hours.  Every so often, they might say – “its fine now”.  You’ll quickly figure out that the pain will return until all the proteins are denatured.

Please seek medical attention immediately if the wound itself is severe. Especially if you consider the barb’s serrated edges could cause more damage with its removal without medical advice. Most UK doctors might not have had to treat a stingray injury before, so please let them know that untreated necrosis can result in a build-up of decomposing dead tissue and cell debris at or near the site of the cell death. A classic example is gangrene.