The Mechanics of Penetration by the Fruit of Bidens cernua L (Compositae) into the Cephalic Region of Cyprinids

Abstract

AbstractThe dry, indehiscent fruit of Bidens cernua L. occasionally becomes deeply embedded in the head and jaws of small cyprinids. The tips of the fruit spines are equipped with a set of three barbs. If a fish comes into contact with one of the many such fruits that fall into the water during autumn, one of the barbs can easily penetrate the skin. When the tip of the barb becomes anchored beneath a scale or small bone, movements of the fish will tend to cause the fruit to rotate around that point, forcing the tip of the spine into the flesh by a lever action with a mechanical advantage of about 50. Barbs on the shaft allow the spine to work deeper but prevent its extraction, even when the fruit becomes entangled among submerged plants. Such attachments, which lead to the death of the fish, occur regularly, but not enough fishes are affected to cause serious problems for aquaculturists. The possibility that the decomposing fishes provide nitrogen sources for the sprouting plants is discussed.