#Fissue 6. FOTW- Pirarucu (Arapaima gigas), the largest 'obligate air-breathing freshwater fish'

You heard it, we've really pushed the canoe out this week, all the way to the Amazon to get our Fish of the Week, the arapaima, pirarucu, paiche- nominated by NatureMetrics ' very own Natalie Swan (PhD) .

Yes, if variety is the spice of life, the >34,800 species of fish that we share our planet with are sizzling. Half of all vertebrate life. This week's Fish of the Week (FOTW) is a real scorcher. Coming hot out of the Amazonian rainforest- we like a forest that does exactly what it says on the tin- comes our #FOTW: the pirarucu or paiche (Arapaima gigas). Nominated by friend and favourite of the author, Natalie Swan (PhD) by name, swan by nature.

Pirarucu comes from the Tupi language words “pira,” meaning fish, and “urucum,” meaning red.

Behold:

Pirarucu (Arapaima gigas) Tom Dolan/Encyclop?dia Britannica, Inc., painted under the supervision of Loren P. Woods, Chicago Natural History Museum

I know what you're thinking, this fish has driiiip. There is so much to talk about, where do we start? Let's begin with taxonomy, as this is a truly remarkable line of fishes.

Class: Actinopterygii - all of the ray-finned fishes, notable exceptions being sharks and rays (Chondrichthyes) and coelacanths

From here things get odd.

Order: Osteoglossiformes - a proper bunch of weird fishes, including elephant fish- Notopteroidei, butterfly fish and African knife fishes- you couldn't make these up.

Family: Osteoglossidae- even nuttier group of primitive fish called the 'boney tongues' which includes other fruitcakes like the Arowanas

Genus: Arapaima- once thought to be only one species, apparently, further research has discovered there are four species in the Amazon basin with more waiting to be discovered, but the most well-known is:

Arapaima gigas the pirarucu

These fish have remained almost unchanged since their first fossils which date back about 20 million years ago.

The armoured head of a pirarucu- see it in action Man Gets Knocked Out By Fish (youtube.com)

Morphology

On encountering a pirarucu, what would we first notice? Namely, its enormous size. These fish easily grow to over 2 meters in length and over 100kg in weight, quickly. They're often recorded as reaching 3 meters in length and over 200kg. Without intensive fishing pressure and given this fishes' capacity for growth, it is likely they could achieve even greater sizes- anecdotal records put them at 4 meters.

The gigantic size of A. gigas should be a challenge; they live in very hot Amazonian flooded forests where water temperatures regularly exceed 27 degrees celsius- for you luddites the wrong side of the pond- that's 80 Fahrenheit.

The dense accumulations of anaerobically (oxygen-consuming) rotting vegetation and high temperatures make this very oxygen-depleted. That is, good old-fashioned eutrophication. Less dissolved oxygen concentrations in water means it is harder for fish to obtain oxygen in sufficient quantities to service a large body mass. Cell sizes can also be a factor too. See the scale below

There is a general shift in size where warmer temperatures limits body size, pirarucus buck this trend

So what does our guy do? He's an obligate air-breather, coming to the surface to gulp air using a modified swim bladder as a lung that can extract oxygen from these breathes. Surfacing every 20 minutes, this way they can get enough oxygen to service that enormous body. Excellent adaptation. The crazy thing is that when they are small- maybe the first 8 days- they are not air-breathing but fish with gills. Imagine what that first taste of air must feel like. More importantly, this makes it the 'largest obligate air-breathing freshwater fish'.

Next, while this enormous fish is truly off the scale, the scales themselves are tough. We have to remember it lives in the Amazon alongside another famous resident, the Piranhas.

While public opinion has these savage little bastards ripping anything to shreds, a large proportion of the diet of juveniles consists of taking bites off larger fishes. These opportunistic attacks are rarely fatal- more parasitic- to the larger fish hosts. Our pirarucu would be a sitting duck, were it not for its incredibly tough scales, designed to withstand such attacks. Yes, these leviathans are armoured. More on this here: Amazon fish wears nature's 'bullet-proof vest' to thwart piranhas | Reuters who knew Reuters were so sensationalist?

You've got mail - chain mail. Sadly this claim to be the largest freshwater fish is false and is more likely a toss-up between the Mekong catfish, Giant stingray or Beluga sturgeon- still, good effort

Ecology:

Unlike many large freshwater fish, such as salmon and the Amazonian fish with the longest migration in the world- the Dorado, arapaima do not migrate 'longitudinally' up and down rivers.

Their migration is much more localised and 'horizontal', these fish migrate out of the permanent river channel into the surrounding seasonally flooded rainforest where they feed on anything that fits in their mouths. This ability to exploit the incredibly productive but oxygen-depleted flooded forest enables them to rapidly reach epic sizes.

Spawning takes place during low-water dry seasons, with both parents creating and protecting the nest. Quite different from many other large fish, the male will stick behind to protect the young for up to three months- this kind of parental care is very rare in fish, with a few exceptions like the peacock bass.

Approximate distribution of A. gigas range - Hrbek et al., 2005

Over fishing and conservation

The arapaima has been hunted extensively throughout its range, typically indigenous groups use a harpoon attached to a long line. The harpoon is fired using a bow and arrow (typically from a canoe) to pierce the tough scales as the fish rises to gulp air. The fish is then tired out before being reeled in.

Extensive over fishing resulted in a catastrophic population crash to 2,500 individuals in Brazil. A subsequent outright ban from 1996 to 1999 has seen populations recover to over 170,000 in 2017. Populations continue to be carefully managed throughout A. gigas' range.

This fish is now extensively reared in community aquaculture projects throughout the Amazon. However, this is not all rosy; due to the complexity of A. gigas spawning, the aquaculture stock is still dependent on the capture of wild juveniles.

When we were last in the Colombian Amazon, in Leticia, we were given exclusive access to a warehouse of frozen arapaima ready to be transported down river to Brazil, were there is a highly lucrative arapaima meat industry.

"The paiche has delicious, white, semi-fatty meat with few bones and therefore [is] very versatile for cooking,"

More on arapaima cuisine here: The Amazon's colossal prehistoric fish - BBC Travel

Arapaima gigas ready for market, Leticia, Colombia, 2022. Photo credit Henry B.

TBC

Fish on - a week in clickbait

• A lovely little article on the diversity of fish: How Many Species Of Fish Are There? - WorldAtlas
• Marine fish in a warming world: Can the fish in the ocean handle marine heatwaves? | The Independent Barents Observer (thebarentsobserver.com)
• 'An alive fish is more important than a dead fish': on location with Tui Care (ttgmedia.com) we wholly endorse this statement
• We are yet to even begin discussing eDNA, but its coming, still- food for thought- Harnessing eDNA metabarcoding to investigate fish community composition and its seasonal changes in the Oslo fjord | Scientific Reports (nature.com)
• DNV takes ownership of Ocean Ecology | The Fish Site

Until next Friday: Tight Lines