What Good is Smelly Stuff in the Skin? Cross Function and Cross Taxa Effects in Fish “Alarm Substances”

Abstract

In fishes of the Superorder Ostariophysi (minnows, suckers, catfish, characins and loaches), and in at least seven non-ostariophysan families, the smell of injured conspeci-fics triggers antipredator behavior. Many thousands of species of fishes may possess such "injury-released alarm substances". These alarm substances often reinforce the learning of predator characteristics by prey that smell the substance at the same time as they smell or see the predator. In some species, the alarm substances retain their activity after passage through the digestive system of a predator and label the predator, triggering prey defense and learning after the death of the original sender. In ostariophysans, alarm substances not only warn conspecifics but also attract secondary predators, increasing the chance of escape by the injured prey. Predatory pike (Esox lucius) counter the predator labeling effect by defecating away from their foraging area, when they are eating prey with alarm substances. Some minnows lose their ability to produce alarm substances during the breeding season, perhaps to counter predator attraction when the minnows are rubbing their spawning surfaces clean. The variety of "functional niches" occupied by these injury-released compounds suggests conflicts between the requirements of various "functions". Selection on senders and receivers may each exert pressure on the system. For example, predator attraction as a defense mechanism may conflict with abrasive spawning behavior and resistance to digestive processes may conflict with optimal breakdown rates for alarm signaling.