Ostariophysan fishes

The chemicals that indicate an increased risk of predation can come from several sources. Perhaps the most widespread are chemicals released by physical damage to a conspecific crustacean. Almost every species tested has shown an increase in behavior patterns that can be related to predation avoidance when odors of crushed conspecifics are presented. There is no indication of crustaceans having specialized cells such as the epidermal club cells (the previously purported source of alarm odor in the narrow sense) found in ostariophysan fish (Smith 1989,1992). But rather some chemical or chemicals, very probably including peptides, found in the hemolymph seem implicated as the cue in crustaceans (Rittschof et al. 1992 Rittschof 1993 Rittschof and Cohen 2004 Acquistapace et al. 2005), as well as in... 

Brown, G. E., and Cowan, J., 2000, Foraging trade-offs and predator inspection in an Ostariophysan fish switching from chemical to visual cues. Behaviour 137 181-196. 

In most families of the Superorder Ostariophysi (e.g., minnows, suckers, catfish, characins and loaches), individuals show antipredator responses when they detect chemicals released from injured conspecifics. This response is absent in some armoured catfish, pencilfishes and ostariophysans with electric organs (Pfeiffer 1977). In ostariophysan fishes the alarm pheromone is contained in epidermal club cells, or alarm substance cells, that have no pores to the exterior, and have no proven function except the production and storage of the alarm substance that triggers antipredator behavior in conspecific receivers. 

Two analogous chemical alarm systems have been described in fishes and there is evidence that other comparable systems exist. I will deal primarily with the best known of these systems, the alarm substance or Schreckstoff system found in the ostariophysan fishes (e.g. minnows, suckers, catfish), first described by von Frisch (1938) and recently reviewed by Pfeiffer (1977, 1982) and Smith (1977, 1982a). The terms "alarm substance" and "alarm substance cells" (ASCs) will be used only for the ostariophysan system. The second established alarm system is found in darters of the Family Percidae (Smith, 1979, 1982b). The darter system will be specifically designated when it is described. 

The skin of two groups of fish that show alarm responses to skin extract. (A) The skin of an ostariophysan fish, the bluehead chub (Nocomis leptocephalus), illustrating the alarm substance cells (ASCs) in the epidermis (b)... 

Why should ostariophysan fishes differentiate and maintain specialized cells that may constitute over 30% of their epidermis when these cells have no known function other than warning other individuals Explanations for this situation fall into two main categories. What I will term alarm theories are based on some benefit of the alarm signal redounding to improve the fitness of the sender. Non-alarm theories are based on some primary function for the ASCs and their contents that benefits the possessor directly without reference to the alarm response of other individuals. 

Both Reed et al. (1972) and Ruddy and Baeder (1973) elicited fright reactions with histamine but histamine is present in all vertebrates while the alarm pheromone is specific to ostariophysans. Histamine "Occurs widely in nature as a result of putrefactive processes" (Stecher et al., p. 533, 1969). So the response reported by Reed et al. (1972) and Ruddy and Baeder (1973) may be an adaptive avoidance response to odours of putrefaction rather than a specific response to conspecific alarm pheromone. Neither group of researchers tested the response of non-ostariophysan fish to histamine. 

Ostariophysan fish that do not have ASCs. There are two situations in which ostariophysan fishes do not possess ASCs. Some taxonomic subgroups within the superorder lack ASCs throughout life (Pfeiffer, 1977). In some other species, males or both sexes lose the ASCs during the breeding season and regain them after breeding is over (Smith, 1976). 

Table 1. Species of ostariophysan fishes examined for seasonal loss or retention of ASCs with information on endocrine control where available, and an assessment of the degree of abrasion during reproductive behavior, + indicates some evidence of abrasive behavior, - indicates that reproduction is not known to be abrasive. 

Brown, G. E., Adrian, J. C. Jr., Kaufinan, L H., Erickson, J. L., and Gershaneck, D, 2001b, Responses to nitrogen-oxides by Characiforme fishes suggest evolutionary conservation in Ostariophysan alarm pheromones, in Chemical Signals in Vertebrates, Vol. 9, A. Marchlewska-Koj, J.J. Lepri, and D. MOller-Schwarze, eds.. Plenum Press, New York, pp. 305-312. 

Trap experiments have also been used to determine the chemical composition of Ostariophysan alarm cues. In a study by Brown et al. (2000), traps labeled with fathead minnow extract, hypoxanthine-3-N-oxide or pyridine-N-oxide caught significantly fewer fishes (including dace and minnow) than those labeled with distilled water. These field results validate the outcome of laboratory studies in which significant increases in antipredator behavior in both fathead minnows and finescale dace were observed when they... 

Taxonomic Comparisons. Obviously there are many variations on the basic injury released alarm pheromone theme in fishes and other organisms. A more comprehensive examination of the distribution and variations of this phenomenon might reveal patterns. For example, are there alarm substances that are predator attractants and others that are predator repellants Are some ecological niches more amenable to chemical alarm signaling than others Electrical ostariophysans, for example, seem to lack the alarm pheromone system (Pfeiffer 1977) but may use their electrical signals in some comparable manner. 

The attributes of the ostariophysan alarm system make it unlikely that it functions to warn offspring or to draw parental assistance. Schools of fish will seldom contain both parents and offspring because fish growth is indeterminate and fish school with individuals of similar size, in response... 

Fish restrict their defensive response to a limited number of chemicals out of the large number of molecules that are released from damaged tissue. In the ostariophysans the response is restricted to some component or... 

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