THE EFFECT OF DEPTH ON DEVELOPMENT AND SEXUAL DIMORPHISM OF THE SONIC SYSTEM IN DEEP SEA NEOBYTHITINE FISHES: THE UPPER CONTINENTAL SLOPE

Abstract Background: Cusk-eels from the subfamily Neobythiitinae are one of the major groups of sound-producing fishes on the continental slope. Sounds have never been recorded from a member of this subfamily, and sonic anatomy is considered a proxy for sound production. As the first part of a study on the effects of depth on sonic anatomy, we examined three relatively shallow species from the upper continental slope. Methods: Three species (Hoplobrotula armata, Neobythites longipes and Neobythites unimaculatus) were examined for sonic anatomy (skeleton, swimbladder and sonic muscles), and sonic and epaxial muscle fibers were measured for diameter. Regressions of dimensions and weights of sonic muscles and swimbladders against fish total length and weight were compared to determine sexual dimorphism and relative development between species. Results: Four pairs of sonic muscles (two medial and two lateral) originate on the skull and insert on the medial swimbladder or on modified epineural ribs that attach to the lateral swimbladder. The medial and medial intermediate muscles are generally larger in males than females and are made of relatively small fibers (ca 10 um in diameter), and lateral muscles are generally larger in females and consist of larger fibers as in epaxial muscles. Medial muscles are considerably larger in Hoplobrotula armata than in the Neobythites species. Conclusion: Neobythitines from the upper slope have relatively well-developed sexually-dimorphic sonic systems, suggesting that males produce advertisement calls for courtship. There are major quantitative differences between species. We suggest that sound production involves tonic contraction of the large-fibered lateral muscle pair and oscillatory contractions of the smaller medial fibers setting the swimbladder into vibration. Hoplobrotula armata is probably capable of making more intense sounds than the Neobythies spp