What is Pirarucu?

Known as one of the biggest freshwater giants, Pirarucu is Amazon’s monster. The fish can go up to 3 meters in length with brute strength and hunting capabilities. It is also known as Arapaima and belongs to the family Arapaimidae. This fish used to rule the Amazon’s water but due to overpopulation and human intervention, the population has reduced considerably. This fish has lost most of its natural habitat but has been introduced to different other regions within South America. The name Pirarucu has been derived from ‘pira’ and urucum’. It means red fish in Portuguese. In this section, we will study more about this fish, its class, family, and morphological features. It is found mostly in the Essequibo and Amazon basins. The population is currently thriving away from human habitations.


Arapaima or Pirarucu: What is it?

Pirarucu or arapaima is the name given to the fish in the genus Arapaima. This species has bony-tongue features suggesting its order to be Osteoglossiformes. This fish has a huge bony body structure reaching a length of 3 m (almost 9.8 feet) in the adult stage. The bony structure of the body gives them brutal strength to hunt smaller fishes and rule the freshwater rivers in Essequibo and Amazon. In fact, its voracious eating style and monstrous size along with fast-growing features make it an invasive species. It generally invades a freshwater source and starts reproducing fast. The fauna of that location gets disrupted due to its eating style and behavior.


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The name comes from the combination of the Portuguese words ‘pira’ and ‘urucum’. The red-colored fish was previously thought of as a monotypic one. Eventually, many other species have been discovered and identified by zoologists in different parts of the world.


Taxonomy and Classification of Arapaima or Pirarucu

As per the latest zoological definitions, Pirarucu has 4 species identified in the freshwater sources. Among the 4, Arapaima arapaima is the most prevalent one. The discovery of this species dates back to 1847. The other species are A. agassizii, A. gigas, A. leptosoma, and A. mapae.

As per the latest information and discovery from the leading zoologists, this species is non-migratory. It means that there are more species to be discovered deep in the Amazonian freshwater sources. The diversity of this species is expected to be witnessed in the depth of the freshwater harbors of the Amazon.

Its Classification Features are:

  • Kingdom: Animalia

  • Phylum: Chordata

  • Class: Actinopterygii

  • Order: Osteoglossiformes

  • Family: Arapaimidae

  • Genus: Arapaima

The 4 species that have been discovered centuries ago are mentioned above. The most prevalent among them is the A. arapaima. The studies were generally done by observing the museum specimens preserved in different parts of the world. Zoologists suggest the discovery of more species when the depths of the Amazon will be scouted. They are expecting to find exceptional species in the deep water. Apart from these 4, all the other species are virtually unknown to mankind.


The Evolution of Pirarucu Fish

As per the evidence collected as fossils, a similar species of age 23 millions years has been discovered in Columbia. The morphological features of this fossil match to that of the arapaima fish of the contemporary era. Similar specimens have been found in Guyana, Brazil, Ecuador, the USA, France, Peru, and England. It shows that arapaima is one of the oldest known freshwater fish in the world.


Morphology of Pirarucu Fish

As mentioned earlier, this gigantic fish can easily reach up to 2 meters in length generally. As this fish is hunted by the native Amazonians, they suggest that the fish can reach up to 2.6 meters in length. The weight of this bony fish reaches more than 100 kg or 220 lbs easily. As per the records, the heaviest fish that was caught weighed more than 200 kg or 440 lbs. The longest fish caught in this species is 3.07 meters or 10’1”. However, the anecdotal reports from the old times suggest that this fish can reach a length up to 4.57 meters. It is as much as 15 feet in length.

The verification of the size of Pirarucu fish from the anecdotal reports cannot be verified as overfishing and reduced food source might have affected the population. Arapaima of length up to 7 feet can be easily found in the world. This is a wild fish with a torpedo-like body. Its shape has become slender due to its huge size and fast swimming requirements. The entire body apart from the head and tail are covered with greenish-black scales with iconic red markings. It is this marking that gave its name. The streamlined body can easily swim in the freshwater. Its anal and dorsal fins are located closer to the tail. It helps them to steer and balance in the water and to swim faster.

The scales are quite hard due to the mineralization of the outer surface. The outer surface is corrugated. The layer beneath the outer surface comprises collagen. Collagen is found in the form of several layers of fibers forming a typical Bouligand arrangement. In simpler words, the arrangement resembles that of plywood. The orientation of the successive layer forms larger angles with the previous layer. This arrangement gives the fish strength and flexibility. As mentioned earlier, this fish hunts smaller fish. It has to be very much agile and flexible. This type of scale and collagen arrangement gives them their skin the right armor required to survive in harsh conditions. The locals use harpoons to hunt this fish.


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The fish has unique morphological properties that cannot be found or observed in any other freshwater fish. The skin, for example, has exemplary strength to provide protection to the fish. Even if the fish weighs more than 200 lbs, it can still move very fast due to its sleek and streamlined torpedo body. The flexibility is also offered by the collagen fibers and the scale arrangement. Pirarucu fish also depends on atmospheric oxygen rather than dissolved aquatic oxygen. This is why it has an evolved unique swim bladder system. This balder is made of lung-like epithelial tissue that can exchange oxygen and carbon dioxide and function like lings. It often surfaces and engulfs air to float. This engulfed air supplies the extra oxygen required by its huge body. It has to depend on the surface air even though it has a strong pair of gills protected by the bony operculum.


Ecology of Pirarucu Fish

You will be astonished to know that this fish can eat anything ranging from small fruits to small animals strolling on the bank of the freshwater sources. This huge fish has a voracious diet and can eat much fish at a time. It generally eats crustaceans, small fish, and insects also. It is an air-breather and uses its super-developed swim bladder to extract oxygen and release carbon dioxide. It means that this fish will have to surface and breathe in the fresh air with its mouth. It also uses its labyrinth organ. This organ is rich in blood vessels. The thick epithelial tissue on the outer surface of this organ helps the fish to exchange gases and breathe. This organ opens in its mouth.

Amazon is known for its oxygen-deprived freshwater zones. The oxbow lakes present in the Amazon’s trail are sometimes deprived of oxygen due to a lack of water flowing in. Research suggests that the oxygen level can dip up to 0.5 PPM where bigger fish cannot survive. Pirarucu fish can easily survive in such conditions using its exemplary breathing system. Apart from using gills, it uses its labyrinth and the swim bladder for supplying oxygen to its monstrous body. Its energy level suggests that it gets ample oxygen to show agility and speed in its habitat.

Even in the low water season, the water depth reduces a lot. The fish population increases causing a depletion of oxygen level. To everyone’s surprise, this fish can easily survive in that condition. It is at the top level of the ecosystem as a predator. It uses the dipping oxygen level as its prey starts feeling lethargic. There is no dip in its energy level. Using its super agility and strength, it hunts down the smaller fish. It can also jump out of the water or leap when harassed or constrained. Its strong muscles help it to lift its huge body quickly above water to get free from constraints.


Reproductive Behavior of Pirarucu

Arapaima is a freshwater fish and it solely depends on the rainwater flowing in from different parts of the region. Its lifecycle is also influenced by the seasonal flooding of the Amazonian regions. As per the zoological reports, the color of the fertile pirarucu changes slightly from that of the normal adults during the reproductive season.

During the fall of the dry season, the water level will start to rise. It is then this fish starts laying eggs after fertilization. The eggs are generally laid in shallow depths. Once these depths are filled with rainwater, the eggs will start hatching. This fish chooses to build a nest in muddy areas. The nest is around 15 cm deep and 50 cm in diameter. It lays eggs here. The eggs wait for the right time. The water starts rising and the offspring start to come out from their eggs. This happens during the flooding season ranging between May and August. This is how the yearly spawning of this fish is regulated by seasonal flooding.


Breeding of Pirarucu

As described before, Pirarucu is a bony-tongue fish. It means this species is a mouth-brooder. The young offspring enter the mouth of their fathers when threatened. This continues until the offspring are capable enough to take care of themselves. The mother also provides food and encircles the father and offspring to protect them. She also hunts and provides food to the father and the offspring. She also becomes quite aggressive during this cycle and scares the other predators.


How is Arapaima Used By Humans?

The South American natives consider the bony tongue of this fish as medicine. The local population hunts this fish, dries the tongue, and mixes it with guarana bark. It is said that this is a potential medicine to kill intestinal worms. This natural alternative medicine has anti-parasitic properties.

Dried guarana is also used as an ingredient in local beverages. The bony tongue is also used to scrape the cylindrical barks of this plant. The dried scales have a corrugated surface. It is used as files by the local females to smoothen nails.

This fish is also used as a food source for the locals. The boneless steaks are considered a delicacy among the locals. The meat is cut, salted, and dried. It is then rolled like cigars and stored. It can be stored for years and consumed. There is no need for refrigeration. This technique is ideal for the locals who do not have such facilities. They can consume this fish all year round. Pirarucu is also considered as the ‘cod of Amazon’.

FAQs (Frequently Asked Questions)

1. What Do You Mean By Bony-Tongue Fish?

Ans: Osteoglossiformes or bony tongue refers to those fish species that carry their newborn offspring in their mouth to protect them from predators. One of the prime examples is arapaima or pirarucu. The male carries the fries in his mouth whereas the female hunts, protects, and feeds the family.

2. How Can Pirarucu Breathe In Shallow Water With Low Oxygen?

Ans: It has a swim bladder made of thin epithelial cells that can absorb oxygen. It also has a blood vessel-rich labyrinth organ that helps to absorb oxygen from the atmosphere. It takes its mouth out and breathes in the fresh air.

3. Why is Pirarucu Agile Even If It Weighs Almost 100 Kg?

Ans: The muscular, skeletal features, scale arrangements, and the multiple layers of collagen underneath the scales give it exceptional strength and agility to swim and even leap when threatened. It can easily catch small fish and even small animals strolling on the river banks.