Sound-producing system (sonic muscle and swimbladder) and its innervation in the Walleye Pollack. (A) Lateral cutaway view showing sonic muscle (male, 36.4 cm SL), (B) ventral view of sonic muscle (female, 40.5 cm SL), (C) lateral view of sonic muscle (male, 37.5 cm SL). Arrowheads in C indicate the sites where the sonic muscle is attached to the ribs. Arrows in C indicate running directions of the sonic muscle fibers. (D) Schematic drawing showing the branching of ventral bundle of the spinal nerve. Gray colored nerve indicates the sonic branch (sob), and dark gray (dashed) and black nerves indicate superficial branch (sub) and deep branch (deb), respectively. (E)–(H) Photographs and diagrammatic drawings (male, SL = 46.0 cm, left sonic muscle) showing the innervation pattern of the sonic branches of the spinal nerves. Gray colored nerves in F indicate the spinal nerves containing the sonic branches (S3 to S8). Gray colored nerves in H indicate distributions of the sonic branches (S3so–S8so). SM, sonic muscle; SB, swimbladder; LH, lateral horns; deb, deep branch; sob, sonic branch; sub, superficial branch; S3–S10, third to tenth spinal nerves; S3so–S8so, sonic branches of third to eighth spinal nerves; Op, opercle; R, rostral side. Scale bar = 1 cm (A, B, E, F); 5 mm (C, G, H)