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 makes it highly unlikely that they see small plankton, especially in dark water. We can rule out smell, as well.
Further complicating the question is the fact that paddlefish less than 7 or 8 inches long do not have well developed gill rakers and cannot strain mass quantities of plankton from the water. Instead, small paddlefish feed by capturing individual plankton, one at a time.
To learn how small paddlefish detect plankton, we study feeding behavior by placing small paddlefish in an observation chamber supplied with circulating water. Under these `artificial stream' conditions, the fish swim in place, and we use remote video to monitor and record their movements. Using surveillance video cameras and infrared illumination, we can also monitor the fish in the dark, eliminating any possibility that plankton can be seen.
For plankton, we use water fleas or brine shrimp in a simulated "stream" environment. Examining the sometimes acrobatic motions the fish use to capture the plankton and analyzing the locations and rates of plankton capture, we see no differences between feeding behavior in the light or dark. Nor do we note any such differences under experimental conditions in which the fish's senses of smell, taste, and pressure are masked. In all controlled feeding situations, a paddlefish has no difficulty detecting and intercepting plankton approaching from above, below or the sides of its paddle.
Clearly, paddlefish have some sort of sensory mechanism to help them detect small objects in their environment.
Scientific literature provides clues as to how paddlefish might sense the presence of food and, ultimately, why they have such a long paddle. In 1972, Swedish scientists reported that the thousands of tiny pores that penetrate the skin on the upper and lower surfaces of the paddle are similar to the sensory organs in sharks and rays that function as electroreceptors. These receptors are amazingly sensitive and respond to electrical fields of less than 1/100th of one 1-millionth volt per centimeter that arise in conjunction with both feeding and mating behaviors.
In a classic experiment demonstrating the role of the electroreceptors in feeding, sharks trained to find and attack flatfish hidden beneath the surface of the sand also attacked buried electrodes that delivered weak electrical currents mimicking the presence of the flatfish.
In paddlefish, the sensory pores extend from the paddle to the top of the head, and also to the tips of the opercular flaps (gill covers). In all, they occupy nearly half the skin surface of the fish.
On the basis of these sensory structures, we predicted that paddlefish could sense the extremely small electrical fields emitted by plankton and would respond to artificial weak electrical signals just as a shark would.
Our laboratory experiments confirmed the prediction. We obtained the most dramatic demonstration of paddlefish electrosensitivity by placing a pair of thin silver wires into our paddlefish holding tanks. Through these wires we delivered weak oscillating currents on the order of 1/10th of 1-millionth ampere into the water.
In the dark, using remote infrared video, we observed paddlefish approach and strike at the electrodes as if they were trying to capture a water flea. The currents producing the most consistent strike response matched the strength and frequency of the electrical signals we measured from the plankton.
In other experiments in which we recorded nervous activity entering the paddlefish brain, we observed that electroreceptors on the paddle respond not only to the presence of individual plankton, but also to the feeding and swimming movements of an individual plankton's appendages, which arise from its appendage muscles. The signals are similar to the electrocardiograms and electromyograms with which we are more familiar, except they are many, many times weaker.
From these experiments, we concluded that the paddlefish paddle is actually a highly developed antenna. As an antenna, its primary function is to detect the tiny plankton on which the paddlefish feeds.
Without its antenna, the paddlefish would probably not survive as a planktivore in its murky aquatic environment. The paddle, with its electrosensory apparatus, allows the paddlefish to feed near the bottom of the food chain with little competition from other fish that feed primarily by sight.
Near exclusive access to this rich food explains how paddlefish can reach such large sizes.
Our antenna explanation has one loose end, however. Paddlefish that have lost their paddles, some as a result of being struck by boat propellers, not only survive, but become heavy. Their ability to produce eggs, in particular, indicates that they continue to feed successfully.
How do these apparently handicapped fish survive without their paddle/antenna? One possibility is that the sensory pores on the head and opercula are sufficient for detecting prey. It is also likely that large, filter-feeding paddlefish that capture quantities of plankton don't rely on the keen electrosense of the paddle as much as small fish that capture individual plankton.
Although our work suggests the paddlefish antenna is a prey-sensing organ, a study by Craig Gurgens, a former graduate student in our laboratory, suggests an additional function. By lowering obstacles into the swim path of a paddlefish, Gurgens demonstrated that paddlefish could detect and avoid a metal object without fail. However, they routinely collide with non-conductive, plastic obstacles. They also do not detect plastic-coated metal.
A metal bar produces a weak electrical field, and fish excitedly turn away at distances up to one foot. This may explain why paddlefish are reluctant to pass through partially open steel gates in dams, as has been documented. This, in turn, could interfere with their long-distance migrations.
Paddlefish rapidly learn to ignore steel rods placed in their tanks, and they never bump into them. Nevertheless, large metal structures in the aquatic environment may have a significant impact on paddlefish behavior, given the great sensitivity of their antenna.
The paddlefish is a recent addition to the catalogue of aquatic animals equipped with electrosense. The Australian duckbill platypus also has electroreceptors covering its broad, flat snout. Recent experiments have shown that the platypus bill also serves as a feeding antenna for finding crayfish and aquatic worms and insects.
Additional examples include several groups of primitive marine fish, including sharks, skates and rays, catfish, a diverse group of tropical freshwater fish that "talk" to each other by sending out weak electrical signals, and a few amphibians.
Among the various varieties and uses of electrosense, the paddlefish's electrosensory system is unique as an adaptation for locating plankton. Structurally, the paddle is unique because of its location in front of the mouth.
Exactly how a paddlefish translates the detection of weak electrical signals into accurate feeding strikes on tiny planktonic specks in the water will occupy our research activities for many years.
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