Sonic/vocal motor pathways in catfishes: comparisons with other teleosts

Among teleost fishes, representatives of several distantly related groups have sound-producing (sonic/vocal) muscles associated with the swimbladder or pectoral girdle/fin. Here, the diversity of vocal organs and central motor pathways in four families of catfish, order Siluriformes, is compared to that in families from two distantly related orders, the Scorpaeniformes and Batrachoidiformes. Several catfish families have two sonic mechanisms--a swimbladder vibration established by 'drumming muscles' that differ in origin and insertion between families, and a pectoral spine stridulatory apparatus. In ariids, mochokids and doradids, sonic swimbladder muscles originate at various cranial or postcranial elements and insert onto an 'elastic spring' that vibrates the swimbladder, while in pimelodids the muscles insert ventrally at the swimbladder. Sonic motoneurons are located along the midline, ventral to the fourth ventricle/central canal in doradids and mochokids but lateral to the medial longitudinal fasciculus in ariids; pimelodids have motoneurons in both locations. The axonal trajectory for the lateral motoneurons in pimelodids and ariids implies that they are a migrated, midline population of sonic motoneurons. Pectoral spine-associated motoneurons are located in the ventral motor column. Unlike catfishes, a diversity of sonic mechanisms in Scorpaeniformes is not associated with different positions for sonic motoneurons. Cottids (sculpin) lack a swimbladder but have sonic muscles that originate at the occipital cranium and insert at the pectoral girdle; sonic motoneurons are located within the ventral motor column. Some triglids have intrinsic swimbladder muscles, although ontogenetic data indicate a transient association with the pectoral girdle; sonic motoneurons are in the same location as in cottids. Among Batrachoidiformes, all known representatives have intrinsic swimbladder muscles that are never associated with the pectoral girdle and are innervated by midline sonic motoneurons. The results suggest two patterns of organization for sound-producing systems in teleost fishes: pectoral fin/girdle-associated muscles are innervated by sonic motoneurons positioned within the ventral motor column, adjacent to the ventral fasciculus; non-pectoral associated muscles are innervated by sonic motoneurons located on or close to the midline, adjacent to the medial longitudinal fasciculus.