Exchanges involving chemicals primarily involve the release and reception of pheromones, which are chemicals secreted by one fish and detected by conspecifics and that produce a particular behavioral or developmental response in the receiving individual (Hara 1982, 1993; Liley 1982). Chemicals are sensed by both gustation (taste) and olfaction (smell) in fishes. Sensory receptors are often located not only in the mouth and nostrils, but also on the barbels or even the body surface in many fishes (see Chemoreception), or on filamentous, muscularized pelvic fins (e.g., gouramis, Osphronemidae).
Chemicals play an important role in food finding and predator avoidance , mating, migration, parental care, species and individual recognition, aggregation, and aggression in fishes. Parents of several cichlid species recognize their young via chemical signals, and young recognize their own parents in the same way (Myrberg 1975). In salmonids, skin mucus contains species-specific amino acids that are used for individual and sexual recognition. Species recognition in other species is also mediated by chemicals in skin mucus (Hara 1993). Bullhead catfishes (Ictaluridae) and European Minnows (Phoxinus phoxinus) can recognize individual conspecifics based on odor. Several schooling species (herring, minnows, plotosid catfish, young salmonids) show an attraction response to water that has contained conspecifics (Pfeiffer 1982).
Chemically mediated agonistic interactions include scent marking of territories or shelters, which takes advantage of the persistence of chemical signals relative to other sensory modes (Hara 1993). Members of sexual pairs of Blind Gobies, Typhlogobius californiensis, defend a burrow against individuals of the same sex. Recognition of burrow mates and of intruders is based on chemical cues. Yellow Bullhead Catfishes develop dominant–subordinate relationships that are mediated by chemical secretions. Experimen tally blinded fish can discriminate between odors produced by different individuals. If a dominant fish is removed and then returned to a tank the next day, a blinded subordinate still treats it as dominant. If the dominant is returned after losing an agonistic encounter with a third fish, the previous subordinate will attack it, again based on chemicals apparently produced in the skin mucus. Bullheads also produce an aggression-inhibiting pheromone when living in groups. Fighting by aggressive individuals even decreases when they are exposed to water in which a communal group was living. In Siamese Fighting Fishes, Betta splendens (Osphronemidae), males display more actively in front of mirrors when placed in water that had contained another male (Todd et al. 1967; Hara 1993).
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