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Blue catfish, Ictalurus furcatus, the largest catfish in North America, produce pectoral stridulation sounds (distress calls) when attacked and held. They have both fish and bird predators, and the frequency spectrum of their sounds is better matched to the hearing of birds than to that of unspecialized fish predators with low frequency hearing. It is unclear whether their sounds evolved to function in air or water. We categorized the calls and how they change with fish size in air and water and compared developmental changes in call parameters with stridulation motions captured with a high-speed camera. Stridulation sounds consist of a variable series of pulses produced during abduction of the pectoral spine. Pulses are caused by quick rapid spine rotations (jerks) of the pectoral spine that do not change with fish size although larger individuals generate longer, higher amplitude pulses with lower peak frequencies. There are longer pauses between jerks, and therefore fewer jerks and fewer pulses, in larger fish, which take longer to abduct their spines and therefore produce a longer series of pulses per abduction sweep. Sounds couple more effectively to water (1400 times greater pressure in Pascals at 1 m), are more sharply tuned and have lower peak frequencies than in air. Blue catfish stridulation sounds appear to be specialized to produce underwater signals although most of the sound spectrum includes frequencies matched to catfish hearing but largely above the hearing range of unspecialized fishes.

Developmental variation in sound production in water and air in the blue catfish Ictalurus furcatus