# Oxydoras niger

Oxydoras niger, the ripsaw catfish or cuiu cuiu, is a species of thorny catfish native to the Amazon, Essequibo and São Francisco basins in Bolivia, Brazil, Colombia, Ecuador, Guyana, Peru and Venezuela. It is the largest member of the Dorididae family. This species grows to a length of 100 centimetres (39 in) SL and weights up to 13 kilograms (29 lb). This species is a minor component of local commercial fisheries. It has lateral thorns that can damage any potential predator or handler. It feeds by shifting through sand and detecting eatable parts with the taste receptors in the roof and floor of its mouth.

## Ecology

Oxydoras niger occurs over mud in streams and lakes. It is known from temperatures ranging from 24 to 29.8 °C (75.2 to 85.6 °F), pH range of 5–9, and an alkalinity range of 42–142. It is known to form schools. This species feeds on detritus, chironomid and ephemeropteran larvae, and crustaceans. They are often found in large freshwater lakes and basins from flooding during the rainy season, returning to their main river as these flooded areas recede. They are native to the Amazon River and have been found on the East Coast of South America and all along the Amazon. Due to the Amazon River being so large and their habitat increasing during rainy seasons, they have been able to spread out over large distances. This has allowed them to survive successfully despite struggling with reproduction.

## In the aquarium

Oxydoras niger is a popular aquarium fish species. In the hobby, it goes by many names, including black talking catfish, razorback catfish, mother of snails catfish, ripsaw catfish and black doradid. This species grows to a large size and are often bought by unsuspecting aquarists when small. They will rapidly outgrow smaller tanks, so the aquarium should be as large as possible. O. niger is especially light shy and should be provided with sheltered areas to hide. Although these fish are peaceful, very small tankmates are still at risk of being eaten. This species readily accepts prepared foods. O. niger has not been bred under aquarium conditions.

## Behavior

There have not been many studies on this species as to how they behave in the wild. Except it is very obvious that they require cover to feel comfortable. They have rarely been spotted freely swimming, often enjoying hiding for most of their daily life. Studies have observed that around ⅔ of the fish in the wild were found in the day. Which contradicts studies done on them in captivity. It was also found that they would not venture out from their cover, despite how dark their environments were. Behavior may vary with age as older individuals in captivity have been seen roaming their cages regardless of light or cover. Whether this is due to them realizing there are no predators in captivity or just unusual behavior from isolation. It is difficult to pinpoint the exact reasons behind their behavior because of the lack of evidence, more studies would need to be conducted to be conclusive.

## Reproduction

Ripsaw Catfish are known to participate in an annual run, known as the piracema. It is estimated that around 40 species perform this run and it lasts 4 months. Even though this run is extremely long and dangerous the fishes do it to ensure their offspring survive. As seen in the diagram they can range from the east coast of South America and all along the Amazon River, with many found in the Jurua River. There have even been some found in the Andes Mountains at elevations of 3000 feet. At the beginning of the dry season, they begin a massive migration upstream, covering distances from 100 to 400 km. This beginning of the dry season when water levels begin to drop is around June, with the start and end of their run in completely different seasons due to just how long it takes for them to complete it. This migration is thought to allow their species to disperse over large areas and increase survival. By taking over flooded areas and traveling to other rivers, their species is safer from natural disasters if a river is ruined. Although natural disasters are not the only thing they need to worry about, this run is in danger from the increasing number of dams in the Amazon. These fish are accustomed to the same route and with a dam in the middle it could completely prevent their spawning. Their spawning season starts in the rainy season when levels are at their highest, allowing them to have the most safety. When water levels are highest there is more space for them to spread out and hide. This decreases the chances of any predators finding them and also decreases the chances of their offspring being found. The females are known to lay up to 250,000 eggs with their sexual maturity being met at about 54 centimeters. Despite it being a large number, very few of the eggs survive to adulthood. Overall, Ripsaw Catfish have an inefficient and slow reproductive cycle that has hindered their species' success in increasing their population.

## Body

The ripsaw catfish is very unique in that it has the whiskers of a catfish and bony-like scales similar to a sturgeon. These scales have thornlike projections, running down both sides and can be used for defensive purposes. There is folklore surrounding these fish as they can be known as the "Talking Catfish." This name is due to people hearing them make noise as if they were talking to each other. They can do this by moving their pectoral spines in a certain way called stridulation. This process is not specific to catfish and is often displayed in insects, it is where animals will grind two bony or hard structures together to produce noise. Their pectoral spine has ridges on it and will rub against the cleithrum, in their pectoral girdle, almost like a bow on a violin. They can make different noises depending if their body is doing adduction or abduction movements. Where adduction refers to the body moving inwards and abduction has the body moving outwards. Having varying noises is thought to have purposes against predators and communicating with each other depending on the frequency. Their mouths are inferior, meaning they are downward facing, allowing them to suck food off the bottom of rivers and eat various organisms. Due to the Amazon being so nutrient-rich the sediment on the bottom has plenty of organisms for them to grow. Their backs are dark colored, allowing them to blend in when predators are looking from above. Which works well with their lighter-colored bellies to blend in with the light if a predator is below them. There are key differences among other fish within their genus, which include having a body width at the clavicles that is smaller than their head length. Their eyes are found on the back half of their head and on the sides, not on the top. They also have up to 40 scutes on each side of their body, as opposed to around 15–25. It has also been researched how their large swim bladders can help improve their hearing. In previous studies, larger swim bladders have been associated with a higher sensitivity to high frequencies, yet within thorny catfish, this was the opposite. It was found that smaller swim bladders have better hearing than large swim bladders, contradicting previous patterns observed in fish.

## Conservation

These fish possess very durable bodies, except they are sadly coded red on the endangered species list and this can be attributed to several factors. Locals love eating them due to their large bodies and delicious meat. They are not very dangerous to humans and have lots of meat so they are desirable targets for fishermen. They have a very appealing color, allowing them to be a great product to sell at markets. Constant developments are happening on the Amazon River and human interference has been increasing. The rising number of dams has made reproduction more difficult for them and could be a reason for fewer of them being born. It also takes them a long time to reach sexual maturity and this in turn means they reproduce slowly. Not being able to increase their population quickly from slow reproduction is detrimental for them as it is crucial to reproduce for species to survive.

## Parasites

Due to the Amazon being such a diverse ecosystem, parasites are very prevalent there within fish communities and often thrive. Despite its protective bony features the ripsaw catfish is susceptible to parasites just like any other fish. They are very important for parasite’s survival as they have been found to host a significant amount within their body. A study was conducted in the basin of Solimões River in Brazil to find out how many ripsaw catfish were infected with parasites. Twenty-seven fish were chosen for this study with 70.3% being infected by at least one parasite species: These parasites include Cosmetocleithrum gussevi, Ichthyophthirius multifiliis , Chilodonella, Cosmetocleithrum confusus, Cosmetocleithrum parvum, and Cosmetocleithrum sp. Of the fish chosen for this study, 18.5% possessed intestinal infections composed of Paracavisona impudica, 3.7% by Cucullanus grandistomis, 14.8% by Dadaytrema, and 3% by Proteocephalus kuyukuyu . There was a very diverse range of parasites among the Oxydoras niger, with monogeneans being the most prevalent, with acanthocephalans and digeneans after.

## Human Interaction

Due to their intimidating features, there was a River Monsters episode dedicated to them and a very large one was caught in that episode. They were featured not for having a large number of human attacks, but just for having an intimidating appearance. Yet their physical features are purely for defense and do not possess anything capable of harming humans. They are omnivores who feed mostly on food within the sediment of the water’s floor. They are sold for their meat quite often as they are described to look like salmon and are very delicious among the locals.