Sound-generating and -detecting motor system in catfish: design of swimbladder muscles in doradids and pimelodids

Catfishes have evolved a diversity of swimbladder muscles serving in the generation of different sounds and probably other acoustic functions. In order to find out if anatomical and acoustical differences are parallelled by fine structural differences, I examined the sonic muscles of the doradid Platydoras and the pimelodid Pimelodus by gross dissections and ultrastructural methods. In Platydoras, the sound-generating (drumming) muscle (DM) inserts on a dorsal bony plate that vibrates the swimbladder. In pimelodids, the large DM attaches directly on the ventral surface of the swimbladder, whereas the small tensor tripodis muscle (TT) inserts on the rostral surface near the tripus, the most caudal Weberian ossicle. Fibers of all three muscles possess an extensive development of sarcoplasmatic reticulum (SR) in association with very thin myofibrils (MF) but differed widely in their arrangement. In Platydoras, ribbons of MFs are arranged radially around a central core. Mitochondria were found within the core and the peripheral sarcoplasm. Pimelodus does not have a differentiated core and the cross-sectional area of DM-MFs is about 15% larger as determined by stereological measurements. The TT possesses shorter sarcomeres and more mitochondria than DMs, which were primarily found between MFs. This suggests faster contraction properties and greater resistance to fatigue compared with sonic muscles. Data indicate that the higher amount of DM-myofibrils in pimelodids might result in stronger muscle contractions and, presumably, in higher sound intensities. The fine structure of the TT reveals that contractions most likely prevent transmission of swimbladder vibrations to the inner ear via the Weberian ossicles during vocalization.