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Consider: the American Paddlefish

Updated: 3 days ago

Note: Consider Nature contains some strong language and adult themes which may be inappropriate for younger readers. Reader discretion is advised.

I want to tell you about one of the weirdest fish in the world. This ancient fish’s lineage diverged from its closest relatives over 300 million years ago, in an environment radically different from our own. While the world around it has changed drastically, this pioneering fish has managed to mostly stay the same, leading it to barely resemble even its closest relatives in the tree of life. Its bizarre, yet effective set of adaptations has allowed it to persist through some of the most-challenging events in our planet’s history to remain with us today. And yes, as is on brand for Consider Nature, its face looks like it got caught in Willy Wonka’s taffy puller. Consider: the American Paddlefish. Polyodon spathula.

Image Credit: US Fish and Wildlife Service

The paddlefish is one of the largest freshwater fish species in North America, growing up to seven feet long (two meters, that’s right we’re in AMERICA so now the METRIC is in parentheses!) and weighing up to 200 pounds (90 kg). They inhabit deep, slow-moving rivers and lakes across the Mississippi River Basin, a gargantuan area that stretches from New York to Montana and all the way down south to Louisiana, covering a total of 31 states. Thanks to the primordial human desire to conquer fish in epic battle (i.e., sport fishing), they can also be found in fishing impoundments, man-made bodies of water constructed for the purpose of sport fishing or aquaculture.

That’s all well and good, but what the fuck is this thing? It looks like a salmon who wished to be a real boy, or perhaps a fraternity paddle from the University of Atlantis. Pddlefish belong to the taxonomic order Acipenseriformes, one of the oldest and most evolutionarily-isolated branches of the bony fish tree of life. Acipenseriformes first emerged in the late Carboniferous Period over 300 million years ago, only slightly after the evolution of the first reptiles. Earth had Acipenseriformes before it had flowering plants, dinosaurs, or even turtles. The only other Acipenseriformes besides paddlefish are the sturgeons, but paddlefish broke off from even these close relatives during the Jurassic, about 150 million years ago (the lack of a paddlefish tank in Jurassic Park was clearly a grave oversight). More or less this exact fish, with its beady eyes and goofy-looking schnoz, has swam Earth’s bodies of water since before Tyrannosaurus rex romped across North America, since before birds first began to fly.

Due to their long evolutionary isolation, paddlefish (and sturgeons) really don’t resemble any other bony fish on Earth. Going from the outside in, they have no scales, so their skin is smooth and covered in a thin layer of mucus. Above their eyes, paddlefish have a set of spiracles, openings kind of like nostrils that lead to their gill chambers and assist in breathing. While all cartilaginous fish have spiracles, very few bony fish do. They also are the only bony fish that have their jawbones detached from the rest of their skeletons, allowing them to open their mouths wide enough to deepthroat a fire hydrant. Seriously, you could store your bowling balls in those cheeks.

Most bizarrely, however, the paddlefish’s skeleton is made entirely of cartilage. While sharks and rays, the “cartilaginous” fish, all have skeletons made of cartilage, paddlefish and sturgeons are “bony” fish, meaning that they evolved from a common ancestor that had mineralized bones, not cartilaginous ones. Essentially, Acipenseriformes evolved skeletons made of cartilage completely independently from sharks and rays. From an evolutionary standpoint, I cannot even begin to explain how bananas this is. Paddlefish having a cartilaginous skeleton is kind of like if we found a species of bird that evolved to grow a turtle shell, or a human with cold blood and reptilian scales (though if you ask around the American South, some might insist that Nancy Pelosi fits that description.

So the natural next question is: why did paddlefish and sturgeons decide to go boneless? It may be that Acipenseriformes or their ancestors evolved cartilaginous skeletons for the same reasons that sharks and rays did: cartilage is lighter and more flexible than bone, so it offers the fish an increased range of motion. Contrarily, some researchers believe that Acipenseriformes developed this way for no reason at all. They may have just happened to lose the ability to mineralize their bones, and that change didn’t really affect their ability to reproduce, so it stuck around. We will almost certainly never know for sure: asking why a certain trait evolved is kind of like asking your cat why it pushed your favorite mug off of the counter: the process of evolution just does weird things sometimes, and all you can really do is shrug and grab the dustpan.

Externally, the paddlefish’s most striking feature is its bill, a tremendous, rounded appendage which can grow up to a third of the animal’s total body length. In addition to making the paddlefish look like Poseidon’s favorite BDSM toy, this nose is an extremely useful hunting tool.  The underside of the paddlefish's bill is covered in thousands of tiny electroreceptors called Ampullae of Lorenzini. These receptors consist of tiny tubes less than a millimeter across, each of which is filled with a highly-conductive electrolyte gel. When a nearby animal generates an electric field (such as when it moves a muscle), this gel transmits tiny electrical signals picked up in the water to sensory cells embedded in the walls of the tube, allowing paddlefish to detect them even if they are camouflaged or too small to see. Evolutionarily-speaking, electroreception is extremely old, and as such it pops up all over the animal kingdom, from cartilaginous fish to coelacanths to even some amphibians (like my son, the olm). Among bony fishes, however, electroreception has mostly been lost, making paddlefish and sturgeons members of a very selective club.

stellate bone, paddlefish skull, paddlefish bones
While on the outside the paddlefish's bill looks like a mermaid's BDSM paddle, on the inside it looks like an even kinkier mermaid's BDSM paddle. As opposed to having a single large bone, the paddlefish's rostrum is composed of many tiny stellate bones, which form a complex mesh-like supportive network. This web-like structure allows a paddlefish's rostrum to be strong, but flexible and lightweight. Image Credit: Lower Mississippi River Conservation Committee

Cartilaginous fish, like sharks and rays, tend to use electroreception to detect prey that is hidden beneath sediment or well-camouflaged. Paddlefish, though, have taken a different approach: why go through all the effort of chasing down or digging up your prey when you can just swim through it with your mouth open? Paddlefish are filter feeders, much like whales or basking sharks. The insides of their cavernous mouths are covered in this bristle-like structures called gill rakers that filter zooplankton (tiny animals like water fleas, copepods, or fish larvae) from out of the water, kind of like a whale's baleen. Paddlefish can detect high concentrations of zooplankton using those electroreceptive bills like a handheld metal detector, then yawn their way through the swarm, trapping thousands of microscopic animals on their gill rakers. Does that mean the paddlefish also eat all of the other random crap that happens to float into their mouths: mud? Used Band-aids? Fish pee? Almost certainly, but paddlefish are used to this. In nature, everybody pees in the pool.

Paddlefish eating, juvenile paddlefish, paddlefish conservation
A juvenile American paddlefish feeding.. Poor guy looks like he just told his first lie and his nose grew. Image Credit: Garrison Dam National Fish Hatchery.

Paddlefish mating season begins in March and continues through June. During this time of year, the fish travel hundreds of miles to reach specific spawning grounds that are conserved across generations. Scientists don’t completely understand what makes a certain area suitable as a paddlefish spawning habitat (one might imagine rose petals, an ample supply of red wine, perhaps underwater speakers playing the latest Hozier album. Such things have yet to be discovered in the wild), but we do know that they require slow-moving currents and gravelly bottoms. Once the paddlefish reach the spawning ground, it is time to let it all out. Unlike the more traditional insert-A-in-B approach we humans tend to enjoy, paddlefish perform a sort of mating called “broadcast spawning”, where they simply blast out their sperm and eggs in massive milky-colored clouds, allowing them to mingle and fertilize in the water (though I'm sure some depraved lunatic somewhere is into that). The freshly-fertilized eggs hatch around a week later, and the babies ride the currents downstream.

These paddlefish fry (who are presumably not thrilled about being in a dude’s hand out of the water) all look to be at different stages of life, but they are actually all the same age: 30 days old. Paddlefish fry grow as quickly as they can eat, so more aggressive fry who are willing to outcompete more passive individuals develop faster. Image credit: Garrison Dam National Fish Hatchery

Paddlefish fry look quite different from adults, lacking the oversized rostrums and having teeth. At this tiny size, the fry hunt individual zooplankton, such as water fleas in the genus Leptodora, as opposed to filter feeding (I imagine it like if human babies ate rice grains one at a time). The fry grow rapidly as long as food is abundant, and often reach ten to twelve inches in length within their first four months of life. Their goofy noses grow at an accelerated pace, and by the time a paddlefish fry reaches 8 inches long, its rostrum may make up half of its total body length. The rostrum and body grow at differing lengths throughout the rest of their adolescence, eventually settling on a roughly ⅓ ratio of nose to fish. In spite of this rapid growth, paddlefish mature slowly, with males reaching sexual maturity between the ages of nine and ten and females between sixteen and eighteen. Those same fish will then swim back upstream to where they were born, bust a nut all over the place, and continue the circle of life.

Protecting the Paddlefish

Despite having outlived the dinosaurs, American paddlefish populations have struggled in recent years due to human activity. In some regions, such as Canada and New York, paddlefish are locally-extinct, having not been seen in those areas since the early 1900s. The International Union for the Conservation of Nature (IUCN) has labeled paddlefish Vulnerable, though most populations are reasonably-stable due to a multitude of conservation efforts.

Modification of river systems for human use poses the biggest threat to paddlefish. Having adapted for a tremendous system of rivers and wetlands, paddlefish require access to free-flowing rivers abundant in zooplankton to survive and mate. Dam construction, river dredging, and diverting water for agricultural use all have the potential to sever the arteries of paddlefish habitats, causing them to die off. Since paddlefish take so long to reach sexual maturity, damage to their ecosystems one year can have lingering effects many years later. Some river habitats have been degraded so heavily that paddlefish can no longer spawn there, and the species must be artificially stocked for populations to remain at healthy levels.

An equally-insidious threat to paddlefish are invasive species, most-seriously Asian carp. Back in the 1970s, humans introduced bighead, black, grass, and silver carp (all of which are colloquially known as “Asian carp”) into waterways to help clean irrigation channels and fish farms. While they were initially confined to man-made bodies of water, Asian carp can jump very well, so they bypassed barriers meant to confined them and escaped into the Mississippi River Basin, spreading as far north as Minnesota. Asian carp are bastards. Growing up to 100 pounds (50 kg), all four species have voracious appetites and specialize on different prey (and none of them can even be bothered to evolve into Gyarados, sheesh). While all four species of Asian carp are bad news for the Mississippi, bighead and silver carp pose the biggest threats to paddlefish, as they eat the zooplankton that paddlefish depend on to survive. Researchers have shown that the presence of bighead carp slows the growth of juvenile paddlefish, as the carp outcompete them for food.

These carp pose direct threats to humans as well. Asian carp can jump really high, and they get easily-startled by boat motors, causing them to leap out of the water. Some areas of the Mississippi are so choked with these carp that when boats pass through, the water turns into a churning sea of jumping fish, all of which would love to body slam a human like Rob Van Dam coming off the top rope. These impacts have been known to break bones and even kill people, prompting small boat captains in some parts of the Mississippi to surround the bridges of their boats with steel caging for their own safety.

Asian carp, silver carp, Invasive carp
Members of the Invasive Carp Regional Coordinating Committee attempting in vain to navigate waters infested with Asian carp. I…think we’re going to need a bigger boat. Image Credit: Invasive Carp Regional Coordinating Committee

Finally, paddlefish eggs are harvested as part of the caviar trade. While we usually consider caviar to come from sturgeons, paddlefish are closely-related enough to them to produce the same kinds of eggs. Paddlefish eggs were initially stolen en masse across America to feed the massive demand for caviar, causing a population crash. Federal legislation has since regulated this harvesting to protect the species, but from time to time poachers still harvest paddlefish illegally.  In 2018, for example, a man in Indiana was arrested and sentenced to two years in prison for operating an illegal paddlefish caviar operation out of his trailer (he had an illegal sawn-off shotgun too, but it was mostly the fish).

In the wake of the environmental catastrophe caused by wild caviar harvesting, farm-raised paddlefish caviar has become incredibly popular worldwide, especially in Russia and China. Image Credit: Betty Willis under CC BY-SA 3.0.

Worldwide, the harvesting of caviar has caused tremendous environmental harm to sturgeons. All 27 sturgeon species are listed as "Endangered" by the IUCN, with 16 of those species listed as "Critically Endangered', meaning that they are in imminent risk of going extinct. Governing bodies like the IUCN and treaties like the Convention on International Trade in Endangered Species (CITES) regulate wild-caught caviar, but poaching has caused the development of a black market under the sway of organized crime. Cartels in Russia and other former Soviet Union states have in the past bribed police officers to allow them to harvest wild caviar illegally. In some cases, these poachers have even fired on law enforcement with automatic weapons. Despite sturgeon aquaculture producing 95% of the world’s caviar, wild-caught caviar still has a market due to its perceived superior taste.

While American paddlefish have so far survived the onslaught of irresponsible human behavior, its closest relative did not. The Chinese paddlefish, the only other modern member of the paddlefish family, once swam the waters of the Yangtze river in China. The species, which could grow to a whopping 23 feet long, had many of the same adaptations as the American paddlefish, though it is believed to have used its rostrum for more active hunting. When the Gezhouba Dam was constructed in the 1970s and 80s, the Chinese paddlefish were completely cut off from their upstream spawning grounds, and this, combined with unsustainable harvesting by fishermen, destroyed the species. The last Chinese paddlefish was spotted in 2003, and in 2020, the species was declared extinct.

This was the Chinese paddlefish, a close relative of the American paddlefish and one of the largest freshwater fish in the world. It was declared extinct in 2020, having not been seen since 2003. No brothers left, and there'll be no more after me. Image credit: Wei Qiwei

So what has been done to protect the paddlefish from extinction, and what do we still need to do? Farmers have been able to grow paddlefish via aquaculture since the 1960s, using a variety of methods to simulate the kinds of environmental changes the fish need to survive and reproduce. Since these discoveries, paddlefish aquaculture has taken off worldwide, especially in Russia, China, and the USA. Farmed paddlefish have greatly reduced (though not eliminated) the demand for wild-caught caviar, and has provided the additional benefit of creating a stock that can be used to supplement wild populations. Sport fishing and caviar harvesting of paddlefish are both strictly-regulated in the United States, with teams of researchers at wildlife institutes and universities across the country setting bagging limits to protect populations. Some states, like Texas, have illegalized catching or killing paddlefish in any way (look at that, Texas doing something right for once).

As far as invasive species are concerned, the US Department of the Interior, as well as several state-run agencies, are researching methods to contain the carp plague and remove the species from areas they’ve already colonized. For sport fishermen or boaters in the Mississippi watershed, the National Parks Service has a list of tips to help prevent the further spread of these bastard carp. You can see these tips for yourself and learn more about invasive carp at this link. Additionally, if you do catch a species that you know to be invasive, DO NOT return it to the water.

Finally, if you do choose to eat seafood, make sure that what you eat is harvested in an environmentally-friendly way. The Monterey Bay Aquarium’s Seafood Watch program monitors seafood harvesting around the world and recommends fisheries that harvest seafood in the most environmentally-sustainable ways. While the project isn’t perfect, it’s a good place to start for those wishing to lessen their impact on aquatic ecosystems. You can learn more about Seafood Watch at this link.

American Paddlefish are survivors. While they may look silly to us, their unique set of ancient traits have allowed them to survive some of the most difficult periods in the history of life, from the extinction event that ended the dinosaurs to the world-reshaping ice ages of the Pleistocene. Yet, within the course of the last two centuries, hardly a blink of an eye in the species' history, inconsiderate modification of river systems and the introduction of invasive species have decimated paddlefish populations. If a species that survived an apocalyptic asteroid impact is wiped out by human behavior, what does that say about us? Are humans just another ecological disaster, one of many that life on Earth has weathered, shrugged off, and forgotten about, or can we learn from our mistakes and become something more? Personally, I believe that even though humans have caused tremendous harm to the environment, we have the potential to atone for our actions and change our behavior to become more responsible citizens of Earth. I believe that sharing stories of the many incredible creatures we share this planet with is a step in the right direction, and I hope the knowledge I share will inspire you to take action in your life to lessen your impact on the environment. Let us do better by our neighbor the paddlefish, as well as the many other species, both odd and mundane, that we live alongside.

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Consider Nature is written and published by Stephen Goralski. Editors for this article included Benjamin Gamble. Our web designer is Oli Platt. Olmungandr, the Consider Nature logo, was made by Maxwell Schumacher.

I can't help but imagine what hitting someone with a paddlefish cricket bat-style would be like. I bet it would make a fantastic sound.

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