Meet the Spatula-Nosed Fish Whose Anatomy Has Hardly Changed Since the Cretaceous

The National Museum of Natural History’s new exhibition “From These Lands” features an American paddlefish, the last surviving species of an ancient lineage of fishes that predates dinosaurs

The American paddlefish (Polyodon spathula) looks nothing like how you might picture a typical fish. With a long, flat nose making up much of its up to seven-foot body length, a gaping, gill-filled mouth and a skeleton made of cartilage, the enormous fish is reminiscent of something out of a sci-fi movie. But its unusual anatomy evolved to help it feed and survive, and if it ain’t broke, don’t fix it — the freshwater fish has hardly changed in the last 125 million years.

“Paddlefish have this long, paddle-like rostrum that is packed with ampullary organs that give them electrosensory capabilities,” said Kate Bemis, a research zoologist at NOAA Fisheries and curator of fishes at the National Museum of Natural History. “They use it like a big metal detector to find schools of small zooplankton in the water column to feed on.”

Two American paddlefish specimens are now on display alongside other ancient fishes in the “From These Lands” exhibition at the museum. One of the paddlefish, a juvenile preserved in alcohol, was collected from Boone County, Missouri in 1992 and is shorter than your forearm. The other, an adult skeleton, was collected from the Ohio River, transferred from the Army Medical Museum to the Smithsonian and catalogued in 1885.

As the official aquatic animal of Missouri, the American paddlefish was chosen to represent the state in the exhibition, which features specimens and objects from all fifty states, D.C. and U.S. territories where people live. The species is also the last of the paddlefish family, with the Chinese paddlefish going extinct in the early 2000s. Both specimens on display provide a clear look into fascinating aspects of the paddlefish’s anatomy that have persisted for millions of years.

What looks like a flat, elongated nose extending from the fish’s skull is actually called a rostrum. This snout-like structure holds more than 70,000 ampullae, a type of electroreceptive organ. The paddlefish uses its rostrum as an antenna to detect electrical fields generated by tiny crustaceans and other zooplankton, which make up most of its diet. The distribution of ampullae throughout the rostrum creates a striking array of interlocking star-shaped bones called stellate bone.

“Because of its electroreception capabilities, the rostrum has a crisscrossed, starry pattern that’s really quite beautiful,” Bemis said.

Once a paddlefish finds a school of zooplankton, another unique part of its body kicks in. Paddlefish are filter feeders, and they open their mouths wide while swimming to capture microscopic organisms to eat. One side of a paddlefish’s gills contain filaments that absorb oxygen from water being pumped through, which allows them to breathe. But the other side of the paddlefish’s gills hold structures called gill rakers, cartilaginous projections like the teeth of a comb that help trap food.

“You can tell a lot about how a fish makes its living by looking at its gill rakers,” Bemis said. Most fish that eat other fish only have a handful of short, stubby gill rakers. But paddlefish have numerous long, thin and closely packed gill rakers that help them sieve out small zooplankton from the water.

Based on their consistent anatomy, paddlefish have led a similar lifestyle and maintained their feeding habits for hundreds of millions of years. Paddlefish fossils collected from the Hell Creek Formation, a famous division of North American rocks dating back to the Late Cretaceous period around 68 million years ago, have very similar filter-feeding anatomy and gill rakers to the specimens on display at the museum. And fossil records of the paddlefish lineage date back more than 300 million years — well before the first dinosaurs came onto the evolutionary scene.

Despite its persistence throughout history, habitat loss and overfishing have caused the American paddlefish to decline throughout much of its historical range. Once found in the Great Lakes, the species is now restricted to the Mississippi River watershed. Paddlefish are migratory and face ongoing threats from dam and reservoir creation, factors that also contributed to the demise of the Chinese paddlefish by disrupting its reproductive cycle. The American species is not listed as federally endangered in the United States, but it is listed as globally vulnerable by the International Union for Conservation of Nature (IUCN).

Because paddlefish eggs are a popular food, commercially farmed paddlefish are helping bolster the overall population. However, the paddlefish’s closest living relative, the sturgeon, has been overharvested for its caviar, and most living sturgeon species are at risk of extinction. (An Atlantic sturgeon specimen from Massachusetts is displayed alongside the paddlefish specimens in “From These Lands.”)

American paddlefish may be in the same boat. “American paddlefish remain vulnerable to overharvesting because of aspects of their life history, such as late maturity, and large scale migrations among rivers and across state boundaries that necessitate coordinated management,” Bemis said.

Paddlefish and sturgeons have survived for millions of years, through mass extinctions that wiped out many of the other species swimming alongside them. But these prehistoric fishes are now under threat, thanks to an evolutionary upstart: us.

Ashley D'Souza | Read More

Ashley D’Souza is a science writing intern at the Smithsonian’s National Museum of Natural History, where they cover natural history research and collections news for the museum’s blog, Smithsonian Voices. Ashley is currently completing their master’s degree in science writing at MIT, where they write about any and all things wildlife. They also run the blog and YouTube channel, Confessions of a Bird Freak. In their free time, Ashley can be found wandering through the woods with their camera and binoculars and talking to trees and birds. See more of Ashley’s work at https://ashleydsouza.contently.com/ and https://confessionsofabirdfreak.com/.