We have been examining proximate (functional morphology) and ultimate factors (survival from predation) relating to the pectoral spine in channel catfish Ictalurus punctatus. We briefly review functional morphology of the spine in channel catfish andthen add new data on morphology in other ictalurids, including interpretation of the relatively smooth anterior and armored posterior profiles of the spine, functions of the various pectoral spine muscles, and new details about the mechanism of stridulatory sound production. Binding, locking, and production of stridulation sounds by the spine utilize derived processes on the spine base (dorsal, anterior, and ventral processes) that mate with counterparts within the pectoral girdle. Stridulation sounds are produced as a series of individual pulses during a forward pectoral fin sweep (abduction), and we provide evidence that each pulse is produced by a small amplitude forward movement, a slip-stick movement similar to bowing a violin. An anti-predator adaptation can work as a deterrent that reduces the probability of attack and as a defense that reduces the risk of mortality. Previous work indicated that the spine functions in defense. Intact channel catfish have a threefold greater chance of survivinga session with a largemouth bass Micropterus salmoides predator as comparably sized individuals with clipped spines. The question of whether the spine reduces the probability of attack had not been investigated. We presented bluegills Lepomis macrochirusand channel catfish to largemouth bass and found that largemouth bass consume more bluegills than channel catfish in a choice situation. Thus, experience with the pectoral spines likely deters attack, particularly if other species are available, and thespine increases the probability of surviving attack by complicating ingestion and possibly by the production of distress calls. Survival advantages accruing from the spine appear to have lead to the evolution of complex adaptations that permit locking and sound production.
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