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BackgroundMany Ophidiidae are active in dark environments and display complex sonic apparatus morphologies. However, sound recordings are scarce and little is known about acoustic communication in this family. This paper focuses on Ophidion rochei which is known to display an important sexual dimorphism in swimbladder and anterior skeleton. The aims of this study were to compare the sound producing morphology, and the resulting sounds in juveniles, females and males of O. rochei.ResultsMales, females, and juveniles possessed different morphotypes. Females and juveniles contrasted with males because they possessed dramatic differences in morphology of their sonic muscles, swimbladder, supraoccipital crest, and first vertebrae and associated ribs. Further, they lacked the ‘rocker bone’ typically found in males. Sounds from each morphotype were highly divergent. Males generally produced non harmonic, multiple-pulsed sounds that lasted for several seconds (3.5 ± 1.3 s) with a pulse period of ca. 100 ms. Juvenile and female sounds were recorded for the first time in ophidiids. Female sounds were harmonic, had shorter pulse period (±3.7 ms), and never exceeded a few dozen milliseconds (18 ± 11 ms). Moreover, unlike male sounds, female sounds did not have alternating long and short pulse periods. Juvenile sounds were weaker but appear to be similar to female sounds.ConclusionsAlthough it is not possible to distinguish externally male from female in O. rochei, they show a sonic apparatus and sounds that are dramatically different. This difference is likely due to their nocturnal habits that may have favored the evolution of internal secondary sexual characters that help to distinguish males from females and that could facilitate mate choice by females. Moreover, the comparison of different morphotypes in this study shows that these morphological differences result from a peramorphosis that takes place during the development of the gonads.

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Acoustic signals are critical for inter-and intraspecific communication in many animals. In bony fishes, sound production is widespread and is used in a variety of speciesspecific behaviors such as agonistic interactions and reproduction (see Lobel, 1992;Zelick et al., 1999;Myrberg and Fuiman, 2002). Acoustic cues can provide information on mate location, reproductive readiness to synchronize gamete release, size, aggression level, territory quality, fitness, and species or individual identity (Myrberg and Riggio, 1985;Myrberg et al., 1986;Myrberg et al., 1993;Kenyon, 1994;Lobel and Mann, 1995;Myrberg, 1997;Lobel, 2001;Myrberg and Stadler, 2002). Thus, identification and characterization of both biologically relevant acoustic signals and the response properties of the auditory system are important to understand the function and evolution of acoustic communication in fishes.Acoustic communication involves the transmission of information by a sender to a receiver with potential benefits to both individuals (Bradbury and Vehrencamp, 1998). In this case, natural selection should favor the production of sounds with frequency spectra and energy content that match the hearing abilities of the intended receiver, and vice versa. This sensory drive model of signal evolution assumes that sender and receiver coevolve within specific constraints of the environment (e.g. background noise and transmission properties) (Endler, 1992). However, few studies on fishes examine both sound production and hearing ability within a single species to test for adaptive coupling of sender signal production and receiver signal reception. Hearing sensitivity was found to match the characteristics of sounds produced in the frequency domain in some species, but a mismatch was observed in others (Cohen and Winn, 1967;Myrberg and Spires, 1980;Fine, 1981;Schellart and Popper, 1992;Ladich and Yan, 1998;Yan et al., 2000;Ladich, 2000). As a result, enhanced intraspecific acoustic communication may not be the main driving force for the evolution of accessory hearing structures because there is no clear relationship between frequency spectra of fish sounds and auditory sensitivity among species with Weberian ossicles, suprabranchial chambers and auditory bullae that serve to increase or modify hearing ability (Ladich, 1999; Ladich, 2000).Sounds provide important signals for inter-and intraspecific communication in fishes, but few studies examine fish acoustic behavior in the context of coevolution of sound production and hearing ability within a species. This study characterizes the acoustic behavior in a reproductive population of the Hawaiian sergeant fish, Abudefduf abdominalis, and compares acoustic features to hearing ability, measured by the auditory evoked potential (AEP) technique. Sergeant fish produce sounds at close distances to the intended receiver (р1-2 body lengths), with different pulse characteristics that are associated primarily with aggression, nest preparation and courtship-female-visit behaviors. Energy peaks of all sounds were be...