Sound Characteristics and Complex Sonic Apparatus Morphologies in Two Ophidiiformes: Ophidion rochei (Ophidiidae) and Onuxodon fowleri (Carapidae)

Ophidiiformes show complex and highly diverse sonic apparatus morphologies allowing them a great variety of calls. Some Ophidion (Ophidiidae) and all Onuxodon (Carapidae) species have in common, at the front of the swimbladder, a mineralized structure called rocker bone. According to morphological observations, it probably results from adaptive convergence. Its evolutionary advantage remains however to be determined. Sonic apparatus morphology and sound characteristics were examined in Ophidion rochei from Dùlce-Glàva (Croatia) and in Onuxodon fowleri from Makemo (French Polynesia). The rocker bone is only present in males in O. rochei but in both sexes in O. fowleri. Onuxodon fowleri and male O. rochei produce calls that often last more than 1 s. Calls are composed of 1 to 41 pulses lasting for 21±10 ms in O. fowleri and 1 to 55 pulses lasting for 16±13 ms in O. rochei. Mean pulse periods are also relatively long, ca. 95 ms and 125 ms, respectively. Females of O. rochei produce short (ca. 20 ms) hum-like sounds that are characterized by shorter pulses (mean duration: 0.7±0.2 ms) and higher pulse rates (mean pulse period: 4±1 ms). Differences in sound characteristics are likely due to the rocker bone that most probably evolved in response to mechanical constraints acting on the swimbladder in O. fowleri and male O. rochei. Its presence suggests a sustained sound production was crucial in their evolutionary success. However, the sexual dimorphism observed in O. rochei but not in O. fowleri suggests differences in way of life.