Tuning in to Paddlefish
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