Arapaima – Freshwater Giants of South America

Ancient armored freshwater fish crushes prey with a toothed, bony tongue

Guest curator: Daniel Gurdak (SUNY-ESF)

Arapaima sp. from Guyana. Image: D.J. Stewart

Arapaima are the largest scaled freshwater fishes in the world. Known by several names, including pirarucu (Portuguese) and paiche (Spanish), they can grow to an amazing 3 m in length and weigh up to 200 kg! These tropical giants are naturally found in the rivers and floodplain lakes of Brazil, Columbia, Ecuador, Guyana, and Peru, but have been introduced to other parts of South America and around the world.

A little bit about a big fish

The groups of species found in the genus Arapaima are part of the family Osteoglossidae, an ancient group of fishes known as the “bony-tongues”.  These freshwater monsters have not changed much in the last 13 million years!  Large, powerful and covered with an armor of hard, overlapping scales, arapaima are well equipped to survive attacks from piranhas, crocodilians and even people. Arapaima are fearsome predators -prey are sucked in and crushed between their bony, toothed tongue and a bony plate on the roof of their mouth! (See the feeding video below).

Unlike most fish, arapaima need to come to the surface every 15-20 minutes to breath air. They have a weird swim bladder lined with blood vessels which works as a primitive lung. Indeed, if an adult arapaima can’t surface to breathe it will drown! Baby arapaima hatch with working gills but can only breathe under water for just over a week. In the tropics the ability to breathe air is an advantage. This is because the combination of slow moving water, high temperatures and decomposing plant material often deprive the water of dissolved oxygen.

About their ecology and behavior

Arapaima live mostly in lakes, quiet backwaters of large rivers and adjacent floodplains. The tropical floodplain is a unique ecosystem with high and low water seasons, it is neither “terrestrial” nor “aquatic”, but both and somewhere in between. Floodplain plants and animals in the Amazon are highly adapted to annual changes in water height.  For example, when water is low, fish can become concentrated in river channels and lakes. However, as waters rise (by more than 10 m in some areas), fish move into the floodplain and feast on newly available plants, fruits, and insects.

Many fish, including the arapaima, reproduce during the beginning of the high water season.  Arapaima breed along the edges of lakes and channels in flooded forests. These are no ordinary fish – they build nests by digging a hole using their mouths, sometimes brushing away nearby leaves and branches! What’s more, arapaima parents work together to protect their eggs and young throughout the flood season.

The perils of being a large, tasty fish

For people of the Amazon Arapaima are great eating. The meat has few bones, firm texture, large fillets and tastes delicious. Sometimes called the “cod-fish” of the Amazon, it can be cooked fresh or used later by freezing or salting and drying.

Traditionally, arapaima were captured by fishermen with a harpoon or a bow and arrow.  Skilled fishermen wait patiently, and strike quickly when the fish rises up to breathe. Commercial fishing for arapaima began in the early 1800’s, and since then, over-fishing, in combination with increasing habitat degradation, has caused sharp declines in arapaima populations across much of their range. The video below shows fishermen catching arapaima.

Fishermen catching arapaima in Brazil. Images: Rafael Sá Leitão Barboza.

Arapaima today

Today, arapaima are faced with continuing habitat loss and insufficient legislation for their protection. In the depths of the rainforest any regulations are tricky to enforce. As a result, arapaima are recognized on two international endangered species lists as Arapaima gigas: IUCN Red List as “data deficient” and CITES “Appendix II”. To ensure the diversity and uniqueness of this genus is preserved, much about arapaima biology and its ecological relations in the wild still needs to be discovered.

Arapaima research in action. Image: D J Stewart

Where to see arapaima

Aside from tropical lakes and rivers (and some restaurants or fish markets), arapaima can be found in public aquaria and even in some pet shops around the world.  Keep in mind they will outgrow the average aquarium and probably the average aquarium keeper within a couple of years. With enough space and food, they can grow to 1 m in just a year!

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Sea Lamprey – Petromyzon marinus

Sea lamprey. Petromyzon marinus. Photo from US EPA.

Parasitic, jawless ‘vampire’ fish with toothy suction mouth

The sea lamprey (Petromyzon marinus) is one of the most wonderfully weird freshwater creatures we’ve featured in the Cabinet in some time.  Translated from a mixture of Greek and Latin, lamprey means ‘stone licker‘, most likely due to the fish’s tendency to hug the bottom of the rivers of Atlantic Europe and America that it inhabits with its curious sucker-like mouth (see the amazing close up photo above).  Unlike many other lampreys, for the sea lamprey, this fearsomely toothy mouth serves a deadly purpose: to suck the blood of unwitting fish.

Sea lampreys attached to an American Lake Trout. Image: Wikipedia

Sea lampreys attach their wraith-like bodies to the skin of passing fish (such as the lake trout, above), gripping with rows of tiny sharp teeth embedded in their suction-cup mouth and probing with a keen tongue to drain blood from the host’s body.  The lamprey’s fantastically odd characteristics: its eel-like, jawless body, the suction-cup mouth and its anadromous behaviour (i.e. lives in both the sea and freshwater), have led some scientists to contest that it should even be described as a ‘fish’ at all!

Sea lampreys in an aquarium. Image: Wikipedia

Despite these fearsome, wonderfully odd adaptations, within a healthy ecosystem (for example in the River Usk in the U.K.), the sea lamprey is important to the complex web of life, both as a predator and as prey (e.g. to birds such as herons, and to people – indeed King Henry I of England (1068–1135) was said to be so fond of the taste of lamprey that he died after eating ‘a surfeit’ of them!).  However, accidental introductions of the sea lamprey to the Great Lakes of America through shipping canals and container ships in the 1800s have caused an ecosystem crisis as the lamprey feasts on native lake trout, as described in the video below.

Millions of dollars have been spent in trying to eliminate the sea lamprey from the Great Lakes.  In 2009, under the headline “Sex smell lures ‘vampire’ to doom” (!), the BBC reported on conservation managers in the Great Lakes trialling the use of a laboratory version of a male sea lamprey pheromone, released into waterways to trick ovulating females into swimming upstream into traps.  However, the picture is not so rosy for the sea lamprey on the other side of the Atlantic Ocean.  Over the last century, sea lamprey numbers have declined dramatically in northern Europe, with no current breeding sites known of in the Baltic region.   As a result, conservation efforts are aimed at increasing sea lamprey populations in the region.

Isn’t it fascinating how perceptions and management of this much-maligned, incredible fish can vary so widely on different sides of the Atlantic? A true freshwater curiosity!

More information on the sea lamprey:

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The Iberian Peninsula: a European hotspot for freshwater diversity and curiosity

Achondrostoma: four tiny, beautiful fish species endemic to Iberia. Image: Ana Maria Geraldes

This month’s guest curator is Dr Ana Maria Geraldes from the Instituto Politécnico de Bragança in Portugal, who supplied the information about an incredible range of freshwater species found in the Iberian Peninsula (Spain, Portugal, Andorra and Gibraltar).

The Iberian Peninsula. Image: Wikipedia

The Iberian Peninsula is a hotspot for freshwater fish biodiversity and curiosity in Europe. The existence of geographical barriers such as the Atlantic Ocean, Mediterranean Sea and Pyrenees Mountains prevents fish from moving to other regions, creating a variety of different, isolated habitats, each subject to a wide range in climate.  This peculiar combination of factors has combined to produce a high number of endemic species through evolution.

What are ‘endemic species’?

Endemism is the word used to describe species that are only found in a very small habitat and nowhere else.  For example, the Galapagos tortoise is endemic to the Galapagos islands, because it is only found there (except in zoos, which don’t necessarily count as “native” habitats!).  Species become endemic to a small area because of geographical isolation, causing them to evolve in different ways to similar species found in other habitats (see for example, the differences between the Galapagos tortoise, and other tortoises found elsewhere).

Endemic fish species on the Iberian Peninsula: a hotbed of diversity and curiosity

The Iberian peninsula is home to 36 endemic species of cyprinids: tiny, shimmering and beautiful fish. Some are restricted to very small areas, such as the Southwestern arched-mouth nase (only named in 2005!), Portuguese arched-mouth nase, Western ruivaco, Squalius aradensis and Squalius torgalensis….not all of these fish are well-known enough to have been named yet!). More

Red bellied piranha – Pygocentrus nattereri

Red-bellied piranha. Image: Wikipedia

Infamous predators reveal more bark to their bite

What do we talk about when we talk about piranhas?  Huge teeth? Voracious appetites?  Frenzied feeding shoals?  All fascinating, and all true.  However, a new study by Belgian scientists has shown that one species of piranha has a similarly curious (and previous undocumented) characteristic: the ability to create an amazing, complex range of sounds.

Sandie Millot and colleagues at the University of Liege in Belgium used underwater microphones to record a shoal of piranhas during a range of different behaviours such as aggression, intimidation, food competition and chasing (all characteristics we’ve come to expect from Hollywood depictions of the fish…!).  Fascinatingly, the researchers found that the piranhas produced different, distinct sounds depending on their behaviour (play the audio below!).

The clip features three sounds.  The first is a “bark” produced in what the researchers called a “frontal display”, meaning where two fish swam quickly towards each other and stayed still, aggressively intimidating and staring at each other.  The second is a “drum beat” produced by the largest fish in the group when circling the shoal, mostly when there was competition for food.  The third “croak” was generally associated with a piranha chasing and biting another fish.

How the sounds are produced

The sounds are produced by the piranhas using their swimbladder – an organ which helps keep the fish buoyant and stable in the water.  The piranhas vary the sounds produced by quickly contracting muscles leading the swimbladder.  The rate at which the muscles contract varies the sound produced.

A fearsome reputation. A close up of the piranha's impressive teeth. Image: ARKive

Why is this important?

The piranhas in the study were silent for most of the time, only producing these weird and wonderful sounds during (very aggressive…!) social situations such as group feeding.  What this research shows, that hadn’t been seen (or heard) before, was that sound is a key part of how shoals of piranhas interact with each other.  The fact that all the sounds produced were associated with aggression only serves to reinforce the reputation of this fearsome little freshwater predator!  In fact, one of the researchers suffered a serious bite to their finger from the piranha when carrying out the recording…

Where does the red-bellied piranha live in the wild?

In the wild, the red-bellied piranha is native to the freshwater rivers of South America, congregating in large shoals.  Whilst the fish has a reputation for its aggressive feeding, it rarely feeds in groups, instead individually preying on fish and molluscs.  However, under conditions of extreme stress, the piranha shoals will exhibit a spectacular ‘feeding frenzy’ if presented with suitable food, potentially stripping a large item of prey to the bone in minutes.

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Senegal Bichir – Polypterus senegalus

Senegal bichir (Image: Strange Waters)

Ancient fish defences used to inspire military armour of the future

The design of the protective scales of this primitive African curiosity is being used by the US military to help design the armour of the future.  The incredible Senegal bichir (Polypterus senegalus) – a species that has barely evolved in its 96 million year existence – developed a light, multilayered set of scales to fend off the crunching bites of ferocious prehistoric predators.

When cracked or penetrated, the scale design deflects cracks to run in a circle around the wound, rather than spreading through the scale (like a crack in a windowpane).  This strengthens the bichir to attacks from other predators (carnivorous attacks are not uncommon!) in the warm, shallow and muddy African river estuaries in which it lives.  In fact, whilst the Senegal bichir is a popular species with aquarium keepers (often sold as a “dinosaur eel”), it’s aggressive, predatory nature means it isn’t recommended to be kept with smaller fish, which are likely to be quickly eaten!

Following research by a team from Massachusetts Institute of Technology Institute for Soldier Nanotechnologies, the US military is actively looking to learn from the way the bichir’s scale design effectively deflects impacts.  Using nanotechnology, a team led by Christine Ortiz discovered multiple layers of material within each scale, which interlock to effectively protect the bichir’s soft tissues from attack.  Extraordinarily, this knowledge about the structure and geometry of the bichir’s scale will be used to help design improved military armour which can better resist the impacts of war.

Senegal bichir (image: Primitive Fishes)

Unlike most fish, the Sengal bichir doesn’t possess a swim bladder – the internal gas-filled sac which aids buoyancy – instead using a primitive set of lungs to gain oxygen.  This means it occasionally rises to the river surface to gulp air.  Amazingly, as long as the bichir’s skin remains moist, this armour-plated curiosity can remain alive out of water for long periods of time.  With poor eyesight, the fish relies on a keen sense of smell to hunt for prey in its native, muddy river habitats, aided by the large protruding nostrils you can see in the photos above.

Another fascinating curiosity for the cabinet, and one that certainly proves that biodiverse ecosystems don’t only have value in their own right, as resources, or as places for recreation. As the extraordinary case of this ‘living fossil’ shows, they may also provide unknown and unpredictable benefits for humans.

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Round goby – Neogobius melanostomus

Round goby (image: wikipedia)

Globetrotting bottom dweller hitches lifts in cargo ships

This entry into the Cabinet of Freshwater Curiosities isn’t endangered – in fact it is actively considered an invasive pest.  Nor is it what you might term particularly beautiful – a small, bottom feeding fish with big, protruding eyes and stubby fins.  However, despite this, the round goby is one of the world’s most curious, adaptable and plain weird fish.

Native to Eastern Europe and west Asia, the round goby is a remarkably adaptable little fish, colonising waterways in North America by hitching lifts in the ballast water of cargo ships.  Reaching a maximum size of around 25cm, and mottled in shades of brown, grey and green, round gobies are extremely successful spawners.  Females lay as many as 6 clutches of highly adhesive eggs during the summer breeding season, which stick to stones, shells and occasionally cargo ships!

It is through this transatlantic trade route that the hardy little goby has colonised North American waterways.  Adult gobies are astonishingly resilient – withstanding water with extremely low oxygen content where other fish cannot live.  This adaptation, coupled with the goby’s aggressive behaviour means that it can outcompete native fishes for food and nests.

In the American Great Lakes, the goby is proving extremely successful at outcompeting native fish like the sculpin and logperch.  Despite this, the round goby’s invasion has had unexpected positive consequences for its predators in the Great Lakes, with the largemouth bass, walleye and the endangered Lake Erie watersnake all benefitting from increased food supplies.

Round goby (image: wikipedia)

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The European Eel – Anguilla anguilla

European eel (image: BBC World on the Move)

Mysterious oceanic wriggler continues to defy science

Truly one of the world’s most curious fish, European eels are catadromous – which means they live in fresh water and breed in the sea.  However, their migration between freshwater and the sea remains one of the natural world’s great mysteries.

The only known eel spawning ground is in a peculiar area of the western Atlantic Ocean called the Sargasso Sea.  A “sea without a shoreline” the Sargasso Sea is a deep, calm and clear 2,000,000 square mile eddy, surrounded by Gulf Stream currents and festooned with seaweed.  Encountered by Christopher Columbus on his transatlantic voyage, and later featured in A Hundred Leagues under the Sea and as an inspiration for Jean Rhys’ Wide Sargasso Sea, the area has entered popular culture as a symbol of mystery and uncertainty.  The mysterious tale of the eel only heightens the intrigue.

Adult eels can grow to over a metre in length, maturing for up to 20 years in freshwater rivers and lakes.  Incredibly, eels have been known to leave the water to move across damp ground in search of watercourses in which to feed or migrate through.  This curious behaviour is thought to be especially common on stormy autumn nights under a waning half-moon.

Glass eels (image: wikipedia)

When adult eels leave freshwater to start their 5,500 km migration to the Sargasso Sea, a number of fascinating changes occur to their body.  They stop feeding – relying on energy reserves to fuel their journey; their bodies turn silver to aid concealment from ocean predators; and their eyes enlarge and optimise for sight in low oceanic light.  Once they reach the Sargasso Sea, they spawn and die.

The hatched eel larvae begin the journey towards freshwater, carried on the Gulf Stream current, taking around 3 years to reach European shores, by which time they measure around 8-10 cm.  Often known as ‘glass eels’, the tiny, translucent fish are widely considered a culinary delicacy, and have been historically harvested in their millions.  Moving upstream from the river estuary, the eels become known as ‘elvers’, developing a darker pigmentation as they feast on invertebrates and small fish.  Once the eel reaches a suitable habitat withsufficient food and shelter, it will settle in to mature and start the cycle again.

Eels were once incredibly common throughout Europe. The town of Ely in Cambridgeshire, UK was named after the rent of 100,000 eels that were paid annually to the lord of the manner. However, in recent years there has been a dramatic decline in eel numbers with some estimates suggesting that only 1% of the number of young eels are returning to freshwater than were counted 20 years ago.

Where to see: Whilst eel numbers have decreased dramatically, you may still be lucky enough to see elvers wriggling up weirs and waterfalls on European rivers throughout the summer months.  Herons often stand on these obstructions, waiting for the eels to pass by and provide a meal.  However, as this video shows, the eel is likely to put up a fight!

There are many aquariums throughout Europe containing European eels too!

Find out more:

Videos at ARKive:

ARKive video -  Common eel elvers migrating upstreamARKive video -  Common eel - overview

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