(A) Representative oscillogram of three sounds in air by a male. Gray area indicates duration of a sound. (B) Expanded oscillogram of a sound (middle one in A). (C) Frequency distribution analysis on the sound shown in (B).

Diagrammatic drawings of the sound-producing system in M. japonicus. (A) Lateral cutaway view showing the position of the sonic muscle and swimbladder. The pectoral fin is omitted from the drawing to show better the sonic structures. Modified from Onuki and Somiya (2007). (B) Dorsocaudal view of the sonic muscle and swimbladder. The angle of observation is indicated by an arrow in (A). Posterior end of the sonic muscles continues into tendons attaching to the vertebrae. As  =  anterior chamber of the swimbladder; Cr  =  cranium; Ms  =  middle chamber of the swimbladder; Pel  =  pelvic fin; Ps  =  posterior chamber of the swimbladder; Sc  =  supracleithrum; Sm  =  sonic muscle; T  =  tendon; V  =  vertebrae.

Dorsolateral view of the sonic muscle and occipital nerve (SL  =  96 mm; left sonic muscle). Photograph (A) and diagrammatic drawing (B), modified from Onuki and Somiya [2007]) showing the lateral rami of the occipital nerve. The sonic nerve branches from two lateral rami and innervates the sonic muscle. Sensory roots of the occipital nerve are omitted from the drawing for simplicity. Scale bar  =  5 mm. C  =  caudal; Ocla  =  anterior lateral ramus of occipital nerve; Oclp  =  posterior lateral ramus of occipital nerve; R  =  rostral; Sm  =  sonic muscle.

Transverse semi-thin sections of the ventral roots of occipital nerve and sonic nerve branches (SL  =  96 mm, male, left side). (A, B, C) Ventral roots of the occipital nerve. (D, E) Sonic nerve branches. Area enclosed by dashed line in (E): a bundle of presumed sensory fibers. (F) Schematic drawing of the lateral view of the left sonic muscle and occipital nerve. Lines with letters indicate the approximate positions of the transverse sections shown in panels (A–E). Sensory roots of the occipital nerve are omitted from the drawing for simplicity. Scale bar  =  50 µm.

Photomicrographs of the sonic motor neurons labeled with WGA-HRP (SL  =  84 mm; immature). (A, B, C) Low magnification views of representative transverse sections of the caudal medulla oblongata. The level of section (A) corresponds to the vagal nerve entrance. Section (B) is 300 µm caudal to the vagal nerve entrance. The section shown in (C) is 100 µm rostral to the first ventral root of the occipital nerve. (D, E, F) Higher magnifications of sections shown in A–C, respectively. (G, H, I) Further higher magnifications of sections shown in D–F, respectively. Arrowheads indicate dendrites of sonic motor neurons. Scale bar  =  500 µm (A–C); 50 µm (D–I). mlf  =  medial longitudinal fasciculus; SMN  =  sonic motor nucleus; V  =  fourth ventricle.

Photomicrographs of WGA-HRP-labeled sonic motor neurons and a charting showing the distribution of sonic motor neurons (SL  =  98 mm; immature), as observed with horizontal sections. (A) Representative horizontal section of the caudal medulla oblongata. The panel shows ventral medulla oblongata rostral to the first ventral root of the occipital nerve (arrow). (B) Higher magnification of (A). Arrowheads indicate dendrites of sonic motor neurons. (C) Charting of the labeled cell bodies of the sonic motor neurons projected onto a dorsal view of the caudal medulla oblongata and spinal cord. Generally, each dot indicates a cell body of a sonic motor neuron. Some dots indicate multiple cell bodies that overlap as observed dorsally. Gray area indicates the extent of sonic motor nucleus (SMN). Scale bar  =  100 µm (A, B); 500 µm (C). Oc  =  occipital nerve; S1  =  first spinal nerve; S2  =  second spinal nerve; X  =  vagal nerve.

Frequency distribution of the soma sizes of the labeled sonic motor neurons (SL  =  84 mm; immature).