Tuning in to Paddlefish
Anyone who's ever seen a paddlefish can't help but wonder about the unique, paddle-shaped appendage on its forehead. Its big nose easily makes the paddlefish, or spoonbill, America's most recognizable freshwater fish, but what purpose does its humongous snout serve?
Other fish with big snouts, like the longnose gar, have long mouths with very long, toothy jaws. The gar's nose, however, is at the tip of its snout. In the paddlefish, neither the mouth nor nose contributes structurally to the paddle, which is known scientifically as the rostrum.
Several years ago, I, along with students and colleagues at the University of Missouri, St. Louis, initiated a study aimed at understanding the biological function of the paddlefish's paddle. We conducted our research on paddlefish supplied by the Missouri Department of Conservation from the Blind Pony Fish Hatchery at Sweet Springs. This contribution was crucial to the success of the study because collecting appropriate-size fish from the wild would have been next to impossible.
Many people believe paddlefish use their paddles to dislodge food from river bottoms and vegetation, a function implied by its commonly known names, "spoonbill" and "shovelnose". However, we found no evidence of this in any scientific literature.
On the contrary, studies show that paddlefish feed almost exclusively on tiny crustaceans and insect larvae that drift as plankton in the water. The common water flea, Daphnia, and closely related plankton species are reported to comprise more than 75 percent of the paddlefish's diet.
This raises two very interesting questions. First, how do paddlefish find near-microscopic plankton in the murky waters of their native habitat, which includes the Missouri and Mississippi rivers and their tributaries? Second, how does a fish capable of reaching weights heavier than 150 pounds and lengths of six feet capture enough of these small prey to thrive?
Regarding the first question, paddlefish are filter feeders, similar to the giant baleen whales of the world's oceans. Paddlefish gills are equipped with comb-like rakers that strain plankton from the water, much like the curtains of baleen that suspend from the roof of a whale's mouth. Like a whale, a paddlefish swims with its mouth open wide so it can filter plankton from large quantities of water.
Both paddlefish and whales grow large by consuming large quantities of small zooplankton. Using this analogy, the water flea becomes "krill" for paddlefish.
How paddlefish detect plankton is the key to answering the first question. Paddlefish eyes are poorly developed, which