Several species of Carapidae are known to have symbiotic relationships with marine invertebrates. The host reduces visual, chemical or tactile perception of both predators and congeners. The ability to communicate acoustically would be an advantage during host colonization and reproduction. Five Carapini species have been shown to produce species-specific sounds.
Brief Description of the Sonic mechanism
The sonic apparatus displays common characteristics in all known carapids. The first two vertebrae display epineural ribs that are movable in all directions, and attach to the swimbladder by a distal ligament. The third vertebra bears a broad, ossified swimbladder plate, which attaches to the swimbladder and is fixed to the fourth epineural rib. The almost cylindrical swimbladder may be divided into three regions.
The sonic muscles insert on the anterior region; the wall of the second region, the 'swimbladder fenestra', is situated just under the swim bladder plate and is thinner due to the lack of submucosa (part of the tunica externa); the posterior region is long and narrow and firmly attached to the abdominal vertebrae. Paired sonic muscles run from the upper wall of the orbit to the anterior face of the swimbladder forward of the swimbladder fenestra. These muscles present an unique helicoidal organization: central myofibrils are straight whereas peripheral ones are more and more twisted.
The study of the structures composing the sound-producing system seems to indicate that the action made by the primary sonic muscles is responsible for the sound emissions. The sonic muscles contract slowly, pulling the anterior bladder and thereby stretching the thin fenestra. Sound is generated when the tension trips a release system that causes the fenestra to snap back to its resting position. The sound frequency does not correspond to the calculated resonant frequency of the bladder, and we hypothesize that it is determined by the snapping fenestra interacting with an overlying bony swimbladder plate.
The anatomical structures of the sound-producing organ in some Ophidion males present an important panel of highly derived characters: three pairs of putatively slow sonic muscles; a neural arch that pivots; a rocker bone at the front pole of the swimbladder and a swimbladder osseous plate
Sounds were recorded in only two species. In each case, male courtship calls are produced nocturnally and consist of trains of pulses, looking like bursts of gunfire.
The anatomical structures of the sound-producing organ in Ophidion barbatum and Ophidion rochei males present an important panel of highly derived characters: three pairs of putatively slow sonic muscles; a neural arch that pivots; a rocker bone at the front pole of the swimbladder; a stretchable swimbladder fenestra; a swimbladder plate. Moreover, Ophidion rochei possess an internal cone that terminates in a pair of membranes in the caudal swimbladder.
In Ophidion rochei, sounds and morphology suggest two hypotheses on the sound-producing mechanism. The ‘pulley’ hypothesis would require an alternate contraction of the ventral and dorsal muscles to form the two parts of each pulse. The ‘bow’ hypothesis involves a release mechanism with the sustained contraction of the dorsal muscle during all of the call, and the rapid contraction/relaxation of the ventral muscle to form each pulse. Sounds were never recorded in Ophidion barbatum.