The oar-like rostrum of the Paddlefish has evolved to aid plankton feeding, not for excavation as previously believed.
Paddlefish, Polyodon spathula, were once believed to use their rostrums to dig up the substrate. But a new study has found that the rostrum actually functions as an antenna for detecting electric fields of plankton, the primary food source of the species.
The coldwater fish, which are found in large rivers in North America, can reach nearly two metres in length and feeds upon tiny crustaceans that live in the water column. Like pelagic sharks, Paddlefish are ram ventilators and swim continuously throughout their life.
Lon Wilkens of the University of Missouri and Michael Hofmann of the University of Bonn, who have just published their findings in BioScience, conducted sophisticated feeding experiments with Paddlefish to determine how they detected prey.
They found that the fish had a highly sophisticated electrosensory system which allows them to detect and capture individual Daphnia, even when all other senses apart from the electrosensory system had been blocked.
"The paddle provides space for an extravagant array of ampullary electroreceptors that are found in common with elasmobranchs and primitive bony fish", said Wilkens and Hofmann.
Feeding behaviourThe authors believe that the highly sensitive rostrum allows the fish to track down plankton to feed upon, even when they live in very muddy rivers, such as the Mississippi.
To test their theory, they placed young Paddlefish in a recirculating flume tank and added plankton that drifted past the move as they swam against the current:
"Small paddlefish, available from nearby state fish hatcheries in Missouri
and well suited for the small-scale laboratory stream, lack the comblike gill rakers that develop in larger fish as they switch to straining plankton in large quantities from the water.
"Accordingly, small paddlefish feed by selective prey capture, sensing individual plankton and adjusting their swimming direction to gulp in the small prey, a ram-feeding motion that frequently involves acrobatic maneuvers of yaw and roll.
"These movements are necessary to keep the paddle from interfering with the path of the mouth toward the prey, and to minimize the resistance that would otherwise be encountered by the large surface of the paddle in rapid vertical movements."
Metal sensitivityThe authors said that the the electrosensory system of Paddlefish is so sensitive that it may be used in migration, and could be affected by electric signals, such as corrosion potentials, from metal structures in the water.
In the lab, Paddlefish are sensitive to metal and avoid metal objects, even when it is too dark for them to see. When an aluminium rod, which produces just 2-3 millivolts, was placed in the water it repelled the Paddlefish:
"Paddlefish turned abrubtly away from the rod at an average distance of 22cm, with maximum avoidance distance of 38cm", said the authors. "In hundreds of approaches, the Paddlefish never bumped into the rod, the closest approach being 10cm. When a plastic rod or plastic-coated aluminium rod of equal size was lowered, fish either passed close to the obstacle or frequently bumped into the rods."
For more information see the paper: Wilkens LA and MH Hofmann (2007) - The paddlefish rostrum as an electrosensory organ: a novel adaptation for plankton feeding. BioScience, May 2007, Vol. 57, No. 5.