The sonic midshipman fish, Porichthys notatus, is a bottom-dwelling species whose swim bladder has evolved into a highly specialized, sound-producing organ. The males of this species exist in two distinct morphs with different physical characteristics and sexual strategies. The Type I males have a much larger sound organ and are capable of generating a loud ∼100 Hz tone continuously for over an hour to attract females. This sound is produced by sonic muscle and represents one of the most superfast and super-enduring striated muscles found in nature. Each fiber contains a hollow, tubular contractile apparatus composed of radially arranged myofibrils with extremely broad Z-bands that are supported by a desmin-rich cytoskeleton. We have used micro computed tomography (CT) imaging and magnetic resonance (MR) imaging to visualize the location of the sonic organ in an intact male fish. We have also obtained high-resolution MR images of the excised swim bladders from both male types. The images of the Type I sonic organ are strikingly detailed and high-contrast, revealing both the internal organization of the bladder and the crisscrossing muscle fibers and their mode of attachment to the underlying bladder. The high-contrast variation in these images is due to different T 2 values for fiber bundles and the spaces between the bundles. Direct MR imaging of intact Type I sonic organ in Type I midshipman fish is a powerful approach to understanding the contraction of this superfast muscle and the oscillation of its bladder to produce mating calls, and how placement of the sonic organ in the body of the fish sheds light on its prodigious ability to produce and transmit its loud mating call.
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