Different sound characteristics produced by the left and right pectoral fins constitute a new form of lateralization in a vocal fish.

Songbirds and toothed whales are able to produce different sounds with the left and right part of their sonic organs, a phenomenon termed lateralized sound production. In fishes this phenomenon is poorly known, with lateralization having been observed solely in the channel catfish (Ictalurus punctatus). They produce more sounds with their right pectoral fins. Croaking gouramis Trichopsis vittata beat their pectoral fins alternately, resulting in a series of two-pulsed sound bursts termed croaking sounds. This study investigates lateralized sound production by comparing temporal and amplitude characteristics of sound bursts generated by pectoral fins in T. vittata. Croaking sounds, produced during dyadic contests, were analyzed in 19 females. We investigated the following characteristics of sound bursts: burst period, pulse period within bursts, the relative peak-to-peak amplitudes of bursts, and the ratio of peak-to-peak amplitudes of the first and second pulse within bursts. Sound bursts produced by the right and left sonic organ differed in 17 out of 19 females in at least one to four measured sound characteristics. The number of females whose temporal characteristics differed between pectoral fins was significantly higher than the number of females lacking such differences (16 out of 19). This was not the case for amplitude characteristics. Our data demonstrated that the sound characteristics produced by the left and right sonic organ in T. vittata differed significantly in most specimens. These differences in sound properties may constitute a new form of lateralized sound production in vocal fishes.

Revealing sound-induced motion patterns in fish hearing structures in 4D: a standing wave tube-like setup designed for high-resolution time-resolved tomography.

Modern bony fishes possess a high morphological diversity in their auditory structures and auditory capabilities. Yet, how auditory structures such as the otoliths in the inner ears and the swim bladder work together remains elusive. Gathering experimental evidence on the in situ motion of fish auditory structures while avoiding artifacts caused by surgical exposure of the structures has been challenging for decades. Synchrotron radiation-based tomography with high spatio-temporal resolution allows the study of morphofunctional issues non-invasively in an unprecedented way. We therefore aimed to develop an approach that characterizes the moving structures in 4D (=three spatial dimensions+time). We designed a miniature standing wave tube-like setup to meet both the requirements of tomography and those of tank acoustics. With this new setup, we successfully visualized the motion of isolated otoliths and the auditory structures in zebrafish (Danio rerio) and glass catfish (Kryptopterus vitreolus).

Auditory chain reaction: Effects of sound pressure and particle motion on auditory structures in fishes.

Despite the diversity in fish auditory structures, it remains elusive how otolith morphology and swim bladder-inner ear (= otophysic) connections affect otolith motion and inner ear stimulation. A recent study visualized sound-induced otolith motion; but tank acoustics revealed a complex mixture of sound pressure and particle motion. To separate sound pressure and sound-induced particle motion, we constructed a transparent standing wave tube-like tank equipped with an inertial shaker at each end while using X-ray phase contrast imaging. Driving the shakers in phase resulted in maximised sound pressure at the tank centre, whereas particle motion was maximised when shakers were driven out of phase (180°). We studied the effects of two types of otophysic connections-i.e. the Weberian apparatus (Carassius auratus) and anterior swim bladder extensions contacting the inner ears (Etroplus canarensis)-on otolith motion when fish were subjected to a 200 Hz stimulus. Saccular otolith motion was more pronounced when the swim bladder walls oscillated under the maximised sound pressure condition. The otolith motion patterns mainly matched the orientation patterns of ciliary bundles on the sensory epithelia. Our setup enabled the characterization of the interplay between the auditory structures and provided first experimental evidence of how different types of otophysic connections affect otolith motion.