Jan Jeffrey Hoover and Edward J. Perkins
US Army Engineer Research and Development Center
The North American paddlefish (Polyodon spathula) is a primitive species with an unusually sophisticated multi-purpose tool: its snout. Common in the lower Mississippi River basin, it is harvested for its eggs which provide gourmets with caviar rivaling that of related sturgeon species. Ancestors of paddlefish, like those of modern sturgeon, were alive during the Cretaceous, the same period as Tyrannosaurus and Triceratops. Also, like sturgeon, the paddlefish has a skeleton made principally of cartilage (rather than bone), an internally corkscrew-shaped intestine (rather than a straight tube), and a tail-fin with an enlarged upper lobe (rather than equally sized upper and lower lobes). Unlike sturgeon, however, it is the sole surviving representative of its kind; a second species in China (Psephurus gladius) is believed to be extinct.
The most prominent feature of the paddlefish is its blade-shaped snout, or rostrum, which, depending on the size of the fish, may be anywhere from 30% to 85% as long as the body of the fish (Fig. 1). This structure was once believed to be a simple tool with which the paddlefish dug or stirred the river bottom. In fact, it is a complex device with remarkable properties of sense, strength, and hydrodynamics.
The rostrum of the paddlefish is an antennae for three kinds of signals: light, vibrations, and electromagnetism (Fig. 2). A large third-eye on the upper surface is a photoreceptor believed to track position of the sun and possibly detect shadows of objects above the fish and beyond the range of its vision. Canals extending from the lateral line on the body run the length of the rostrum and branch out laterally to detect movements and vibrations in the water. Clusters of pore-like ampullae stud the upper and lower surfaces and receive weak electrical signals from plankton, on which the paddlefish feeds, as well as from metal objects, which it avoids. Large nerves which receive and transmit information from these sensory systems, run the length of the paddle, branching out, and terminating in conspicuous rosettes.
The skeleton of the rostrum is a complex 3-dimensional network of stellate bones (Fig. 3). Like snowflakes, no two stellate bones are shaped exactly the same. They are the building blocks of a flattened rhomboid-shaped structure, surrounded by and partly filled with connective tissues, encasing a large core of fat, and covered by a thick, smooth, skin. Strong, but lightweight, it enables the rostrum to be readily maneuvered and provides a strong counter-force when the trapdoor-like mouth is open and the fish is filter-feeding.
The shape of the rostrum varies with size of the fish and among populations but is typically broader at the base than at the tip, thicker in the middle than at the edges, and gently curved along the perimeter. These characteristics and its size enable it to function like a man-made “hydroplane” which generates pressure fields causing lift for surface-cruising marine vessels. This lift, along with the paddlefish’s habit of swimming near the surface of the water, may explain why large paddlefish occasionally leap into the air.
Paddlefish have tested and refined the organic adaptations of their ancestors for millions of years. Because the rostrum has enabled paddlefish survival into contemporary times, it could also provide useful guidance for human technology. Studies by biologists and engineers at the US Army Engineer Research and Development Center are underway to see if features of the paddlefish rostrum can bio-inspire new technologies of use to the military. These could include innovations in metal detection, infrastructure protection, and vessel design.