Tuning in to Paddlefish
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.